2. THE CONCEPTUAL BASIS

2.1 PROBLEMS WITH CURRENT CLASSIFICATION SYSTEMS

Despite the necessity for a standard classification system, none of the current classifications has been internationally accepted (Danserau, 1961; Fosberg, 1961; Eiten, 1968; UNESCO, 1973; Mueller-Dombois and Ellenberg, 1974; Kuechler and Zonneveld, 1988; CEC, 1993; Duhamel, 1995). Often, the land cover classes are inappropriate for particular purposes (e.g., statistical or rural development needs), the scale is related to a specific purpose and the information is mostly obsolete. Furthermore, factors are often used in the classification system which result in a undesirable mixture of potential and actual land cover (e.g., including climate as a parameter). The reasons why none of the current classifications could serve as a reference system are manifold, as will be explained below.

2.1.1 Purpose

A proportion of the existing classifications are either vegetation classifications (e.g., Danserau, 1961; Fosberg, 1961; Eiten, 1968; UNESCO 1973; Mueller-Dombois and Ellenberg, 1974; Anderson et al., 1976; Kuechler and Zonneveld, 1988), broad land cover classifications, or systems related to the description of a specific feature (e.g., agricultural areas). Thus, they are limited in their capacity to define the whole range of possible land cover classes. An illustration is the UNESCO Vegetation Classification (designed to serve primarily for vegetation maps at a scale of 1:1 000 000), which considers only natural vegetation, while all other vegetated areas, such as cultivated areas and urban vegetated areas, are ignored. Other vegetation classifications, even if they consider agricultural areas, do not describe these classes with the same level of detail as used for the natural vegetation ones. In contrast, systems used to describe agricultural areas give very few details in their description of natural vegetation.

Many systems have been developed for a certain purpose, at a certain scale, and using a certain data type (e.g., the IGBP-DISCover global 1 km data set based on the National Oceanic and Atmospheric Administration - Advanced Very High Resolution Radiometer (NOAA-AVHRR)). Hence the derived classes are strictly dependent on the means used (e.g., in the previous example the classes will be only those that can be detected using NOAA).

Many current classification systems are not suitable for mapping, and subsequent monitoring purposes. The use of the type of diagnostic criteria and their hierarchical arrangement to form a class is very often in conflict with the ability to define a clear boundary between two classes. For monitoring, land cover changes take two forms: conversion from one category to another (e.g., from forest to grassland), and modification of condition within one category (e.g., from natural vegetation to cultivated ). The broader and fewer the categories used to describe land cover, the fewer the instances of conversion from one to another. If land cover classes are as broad as "forest and woodland", "arable land" and "permanent meadows and pastures" (from the FAO Production Yearbook) then forest fragmentation, a shift from rainfed to irrigated cultivated areas and less dense grass cover due to overgrazing will not register as conversion nor as modification. A multi-user-oriented classification system should capture both.

2.1.2 Consistency

In most current classifications, the criteria used to derive classes are not systematically applied. Often, the use of different ranges of values depends on the importance given by the user to a particular feature (e.g., in many systems the cover ranges to distinguish tree-dominated areas are many, whereas only one single cover range is used to define shrub- or grass- dominated areas).

In some classifications the class definition is imprecise, ambiguous or absent. This means that these systems fail to provide internal consistency (e.g., the frequency with which classes in the CORINE (Co-ordination of Information on the Environment) Land Cover system overlap with other classes elsewhere in the same classification (CEC, 1993)).

In most systems, the full combination of diagnostic elements describing a class is not considered (e.g., a system which describes vegetation with the diagnostic criteria of three ranges of cover matched with three ranges of height must consistently apply these ranges for all life forms considered). The reason why most systems fail in application of this basic classification rule is that the entire set of permutations of the possible classifiers would lead to a vast number of classes which cannot be handled with the current methods of class description (e.g., in the example above, if there were 10 classes of each, the result would be 100 combinations). Therefore, the current systems often leave gaps in the systematic application of the used diagnostic criteria.

Very often the systems contain a number of classes, which due to their interrelation and hierarchical structure, appear to be a proportion of a broader set of classes. Thus, these types of systems are mere legends. The characteristic of legends is that only a proportion or subset of the entire range of possible classes is described. Such legends have the disadvantage that the user cannot refer back to a classification system, which precludes comparisons with other systems.

Threshold values are very often derived from knowledge of a specific geographic area, so that elsewhere the class boundary definition between two classes may become unclear, that is with overlaps or gaps. In these cases any comparisons will be impossible or inaccurate.

2.1.3 The Underlying Common Principle

An underlying common principle has often not been defined in land cover classification. A mixture of different features is used to define a class, especially features such as climate, geology, soil type and landform (thus, in "tropical rain forest" the term "tropical", which is usually climate related, is used to describe a certain floristic composition). Features such as climate, geology and landform influence land cover but are not inherent features of it. This type of combination is frequently found and is often applied in an irregular way without any hierarchy. This may lead to confusion in the definition of the class.

Classification of vegetation using the diagnostic criteria of "height" and "cover" will lead to a different perspective of the same feature in comparison with the use of "leaf phenology" and "leaf type" (Figure 5). It is therefore important to come to a basic understanding of the criteria to be used as underlying principles for land cover description.

FIGURE 5.
Example of description of a land cover using a different underlying principle.

2.1.4 A priori classification systems

Often an a priori classification system is used in which classes are arranged. However, the use of such a classification assumes that all possible classes any user may derive, independent of scale and tools used, are included in the system. Having all classes pre-defined in the system is the intrinsic rigidity of an a priori classification system. The advantage of such a system is mainly that it is the most effective way to produce standardization of classification results between user communities. The disadvantage is that to be able to describe consistently any land cover occurring anywhere in the world, one needs an enormous amount of pre-defined classes. Such a system should be flexible in the sense that any occurring land cover can be accommodated. How can one introduce this type of flexibility while using the "classical" approach of class names and descriptions?

By increasing the number of classes in an a priori system, the problem arises of how the users will find their way through a "jungle" of class names (Figure 6). Furthermore, this situation aggravates standardization, namely that every user may have a slightly different opinion on how to interpret some classes because the class boundary definitions between classes will be based on very slight differences. The wrong, or different, designation of the same land cover feature in different classes will affect this standardization process that is one of the chief objectives of the classification system. Ultimately, the attempt to harmonize will fail. The a priori classification approach appears to be a vicious circle: the attempt to create this type of classification as a tool for standardization obliges one to fit the enormous variety of occurring land cover in a limited number of more generic classes, while the endeavour to create more classes increases the danger of having a lack of standardization, the very basic principle used as starting point.

FIGURE 6.
Problem of the current a priori classifications in relation to their flexibility.

The above illustrates that there is not as much compatibility between classification systems, or between classification and legend, as may be desired. There are numerous inconsistencies in definition of classes, class boundaries, in the use of threshold values, etc. However useful the current classifications may be, these factors limit the possibility of the use of such classification results by a large audience for a broad range of applications.

In the context of developing a new system, it is fundamental to identify the criteria to which any reference classification, to the extent possible, should adhere (Box 1).

2.2 THE BASIS FOR A NEW APPROACH

2.2.1 Adopted Definition of Land Cover

The common integrated approach adopted here defines land cover as the observed (bio)physical cover on the earth's surface (see Section 1.1, above), but, in addition, it is emphasized that land cover must be considered a geographically explicit feature which other disciplines may use as a geographical reference (e.g., for land use, climatic and ecological studies).

Land is a basic source of mass and energy throughput in all terrestrial ecosystems, and land cover and land use represent the integrating elements of the resource base. Land cover, being the expression of human activities, changes with modifications in these activities. Therefore, land cover as a geographically explicit feature can form a reference basis for other disciplines.

2.2.2 A New Approach to Classification

Increasing flexibility while maintaining mapability

To create a standardized, hierarchical, consistent, a priori classification system containing systematic and strict class boundary definitions implies the basic requirement of having to build flexibility into the classification system. In this context, flexibility has different meanings. First of all, flexibility should address the potential for the classification system to describe enough classes to cope with the real world. At the same time, however, flexibility should adhere to strict class boundary definitions that should be unambiguous and clear. In addition, the classes in such a system should be as neutral as possible in the description of a land cover feature in order to answer to the needs of a wide variety of end-users and disciplines.

Many current classification systems are not generally suitable for mapping, and subsequent monitoring, purposes. The integrated approach requires clear distinction of class boundaries. Furthermore, the use of diagnostic criteria and their hierarchical arrangement to form a class should be a function of the mapability, that is the ability to define a clear boundary between two classes. Hence, diagnostic criteria should be hierarchically arranged in order to assure at the highest levels of the classification a high degree of geographical accuracy.

How does one increase the classification system's flexibility while maintaining the principle of mapability and aiming at standardization? These prerequisites can only be accomplished if the classification has the possibility of generating a high number of classes with clear boundary definitions. In other words, it should be possible to delineate a large number of classes in order to suit the enormous variation of land cover features, while maintaining the clear distinction of class boundaries. In current classification systems this possibility is hampered by the manner in which these classifications are set up. Differences between classes can only be derived from class descriptions. Therefore, it would be very difficult for the user to distinguish between such classes just based upon class names or unsystematic descriptions, as is the case with most of the current classification systems.

Basic principle

One of the basic principles adopted in the new approach is that a given land cover class is defined by the combination of a set of independent diagnostic attributes, the so-called classifiers. The increase of detail in the description of a land cover feature is linked to the increase in the number of classifiers used. In other words, the more classifiers added, the more detailed the class. The class boundary is then defined either by the different amount of classifiers, or by the presence of one or more different types of classifiers. Thus, emphasis is no longer on the class name, but on the set of classifiers used to define this class.

Issues impeding application of the new approach

The straightforward application of this condition is hampered by two main factors. First, land cover should describe the whole observable (bio)physical environment and therefore deals with a heterogeneous set of classes. Obviously, a forest is best defined using a set of classifiers which differ from those to describe snow-covered areas. Instead of using the same set of classifiers to describe such heterogeneous features, in the new approach the classifiers are tailored to major land cover features. According to the general concept of an a priori classification, it is fundamental to the system that all the combinations of the classifiers must be created in the system. By tailoring the set of classifiers to the major land cover features, all combinations can be made without having a tremendous number of theoretical but redundant combinations of classifiers. Secondly, two distinct land cover features, having the same set of classifiers to describe them, may differ in the hierarchical arrangement of these classifiers in order to ensure a high mapability.

2.3 LAND COVER CLASSIFICATION SYSTEM: DESIGN CRITERIA

Land cover classes are defined by a string of classifiers, but due to the heterogeneity of land cover, and with the aim of achieving a logical and functional hierarchical arrangement of the classifiers, certain design criteria have been applied.

The Land Cover Classification System (LCCS) has been designed with two main phases (Figure 7):

• An initial Dichotomous Phase, in which eight major land cover types are defined:
• Cultivated and Managed Terrestrial Areas
• Natural and Semi-Natural Terrestrial Vegetation
• Cultivated Aquatic or Regularly Flooded Areas
• Natural and Semi-Natural Aquatic or Regularly Flooded Vegetation
• Artificial Surfaces and Associated Areas
• Bare Areas
• Artificial Waterbodies, Snow and Ice, and
• Natural Waterbodies, Snow and Ice.

This is followed by a subsequent so-called Modular-Hierarchical Phase, in which land cover classes are created by the combination of sets of pre-defined classifiers. These classifiers are tailored to each of the eight major land cover types.

The tailoring of classifiers in the second Phase allows the use of most appropriate classifiers to define land cover classes derived from the major land cover types and at the same time reduces the likelihood of impractical combinations of classifiers. This results in a land cover class defined by:

• a Boolean formula showing each classifier used (all classifiers are coded);
• a unique number for use in Geographical Information Systems (GIS); and
• a name, which can be the standard name as supplied or a user-defined name.

TABLE 1.
Distinction at the main Dichotomous level and the second level.

TABLE 2.
Distinction at the third level of the Dichotomous Phase into eight major land cover categories.

2.3.1 Dichotomous Phase

As stated above, a dichotomous key is used at the main level of classification to define the major land cover classes (Figure 7). Each major land cover type is defined as shown in Tables 1 and 2.

Three classifiers are used in the Dichotomous Phase, namely Presence of Vegetation, Edaphic Condition and Artificiality of Cover. These three classifiers have been hierarchically arranged, although independent of this arrangement the same eight major land cover types would be keyed out. The hierarchical arrangement is thus not important in this Phase, but is a guiding principle in the subsequent Modular-Hierarchical Phase.

FIGURE 7. Overview of the Land Cover Classification System, its two phases and the classifiers.

2.3.2 Modular-Hierarchical Phase

In this phase the creation of the land cover class is given by the combination of a set of pre-defined pure land cover classifiers. This set of classifiers is different for each of the eight main land cover types (Appendix B and C). This difference is due to the tailoring of the classifiers to their respective type (see Figure 8).

These pure land cover classifiers can be combined with so-called attributes for further definition. Two types of attributes, which form separate levels in the classification, are distinguished (Figure 8 for two examples):

• Environmental Attributes: these attributes (e.g., climate, landform, altitude, soils, lithology and erosion) influence land cover but are not inherent features of it and should not be confused with "pure" land cover classifiers. These attributes can be combined in any user-defined order; and
• Specific Technical Attributes: these attributes refer to the technical discipline. For (Semi-)Natural Vegetation, the Floristic Aspect can be added (the method how this information was collected as well as a list of species); for Cultivated Areas, the Crop Type can be added either according to broad categories commonly used in statistics or by crop species; and for Bare Soil, the Soil Type according to the FAO/UNESCO Revised Soil Legend can be added). These attributes can be added freely to the pure land cover class without any conditions.

FIGURE 8.
The Modular-Hierarchical Phase: example of tailoring of the classifiers and attributes for the "Cultivated and Managed Terrestrial Areas" (left) and "Natural and Semi-Natural Aquatic or Regularly Flooded Vegetation" (to the right).

The user is obliged to start with the pure land cover classifiers. However, at any time the user can stop - dependent upon the level of detail required - and derive a land cover class (Table 3). Further definition of this class can be achieved by adding a single or a combination of any of the other types of attributes. These attributes are not hierarchically ordered and selection of them will generate a separate coded string.

Because the classification is suitable for mapping purposes, the system gives high priority to "mapability." and therefore the user needs to follow specific rules:

• A higher level of land cover classifier must be used before going to a lower level (because mapability is high at higher levels and decreases with lower levels).
• Within certain levels of land cover classifier there are pure land cover classifiers, and a further subdivision of them, the modifiers, which refine the classifier further but are optional and do not necessarily need to be determined.
• All land cover classifiers at one level of the classification have to determined before the system allows one to go to the next level.
• At any time inside a land cover classifier level the user can stop, and a mutually exclusive class is defined.
• All land cover classes defined in such way are hierarchically arranged in the Legend (see Legend Module).
• At any time the user can further define the land cover class using environmental or specific technical attributes, alone or in combination. These attributes will add a second, separate, code to the land cover class because they are not inherent features of land cover.
• A Boolean formula (i.e., a combination of the classifiers used), a unique code (numerical) and a name (nomenclature) define each land cover class.

TABLE 3.
Example of the formation of land cover classes.

2.3.3. Concepts for the Primarily Vegetated Areas

There are different ways of making an orderly arrangement of the Primarily Vegetated Areas, with varying success according to region or purpose. Vegetation has a multitude of properties and features, and a certain degree of abstraction is required when classifying. However, agreement could be reached on selection of a relatively small number of diagnostic criteria to identify plant communities.

Plant communities, or phytocenoses, are characterized by two important features:

• all plant communities consist of growth forms; and
• all plant communities consist of plant species.

This applies to all phytocenoses on earth (Kuechler and Zonneveld, 1988). Growth forms (e.g., trees, shrubs, herbaceous, etc.) are so important that various vegetation scientists have used them as criteria for classification (Danserau, 1961; Mueller-Dombois and Ellenberg, 1974). The growth forms are distributed within the plant community in layers or strata. This stratification is common and the distinction of the individual strata is of fundamental importance when analysing the plant community. Plant communities are not limited to vertical arrangement into layers: they are also arranged horizontally (i.e., the horizontal spatial distribution).

Thus, when observing plant communities and considering their growth forms, two factors are fundamental:

• physiognomy, the overall appearance of the vegetation; and
• vegetation structure which is defined as "the spatial distribution pattern of growth forms in a plant community" (Kuechler and Zonneveld, 1988). The structure, then, describes the individual strata, usually characterized by height and density or coverage of the respective growth forms.

At the same time, a plant community consists of taxa (botanical species) that are usually unevenly distributed insofar as some may be common, or dominant, while others are less conspicuous. The component taxa can be used to describe the plant community as well as the structure. A description using taxa is called the floristic composition of the plant community. The floristic composition usually contains all species, though it is unusual to include the rare or incidental ones.

The various existing classification systems have emphasized one or other of the above (e.g., physiognomic-structural systems; floristic systems; physiognomic-floristic systems). There is no doubt that a full description of a plant community must consider both physiognomic-structural and floristic aspects. A phytocenose can have the same structural aspect but different floristic composition, as well as the same floristic composition but a different structural aspect. However, problems arise when attempting to incorporate both types of information in a single classification system.

In the Land Cover Classification System, Natural and Semi-Natural Vegetation, in both the Terrestrial Areas (A12) and Aquatic or Regularly Flooded Areas (A24), are classified using a pure physiognomic-structural method. The aspects considered are, thus: (1) physiognomy; (2) vertical and horizontal arrangement; (3) leaf type; and (4) leaf phenology of plants. This concept has been adopted with the conviction that only a pure structural representation of vegetation is able to incorporate, without any confusion of terms, floristic aspects of vegetation as well as environmental attributes (e.g., landform, climate, altitude, etc.). The proposed classification allows the user to add freely these attributes at any level of the created structural land cover class (Appendix B and C).

Users not familiar with classical vegetation classification and mapping (Eiten, 1968; UNESCO, 1973; White, 1983; Kuechler and Zonneveld, 1988) or ecological studies should be able to build up a scientifically sound vegetation classification by following the Land Cover Classification System. This will avoid the separation between classical vegetation classification and land cover classification. A variety of users should be able to apply the results of the classification, even those who are not specialized in vegetation mapping.

The physiognomic-structural approach selected for classification of vegetated areas in a land cover classification system poses a challenge with regard to classification of vegetated areas other than (semi-)natural vegetated areas, namely cultivated and urban vegetated areas. These managed vegetated areas are also characterized by plant communities having growth forms and taxa, a structure and a floristic composition. Therefore, the physiognomic-structural approach adopted is equally applicable to such areas. Using the same approach to describe and classify this type of area at a certain level of detail has the advantage that all Primarily Vegetated Areas can be compared.

2.3.3.1 Natural and Semi-Natural Vegetation (A12 and A24)

General rules for classification

Before starting to use the classifiers, the user has to take into account some basic rules governing the concepts of classification of (Semi-)Natural Vegetation, namely:

• the definition of Life Form; and
• the definition of dominance

These two main aspects are very important and must be carefully determined because in the software the determination of Main Life Form has consequences for the selections available at subsequent levels. Certain choices at a high level of the system may disable choices at lower levels.

• Life Form of a plant is defined by its physiognomic aspect. This is the case when Woody plants, subdivided into Trees and Shrubs, are distinguished from Herbaceous plants, subdivided into Forbs and Graminoids, and Lichens/Mosses.
• A condition of Height is applied to separate Trees from Shrubs: woody plants higher than 5 m are classified as Trees. In contrast, woody plants lower than 5 m are classified as Shrubs. This general rule is subject to the following exception: a woody plant with a clear physiognomic aspect of trees can be classified as Trees even if the Height is lower than 5 m but more than 3 m. In this case, a subcondition of physiognomic aspect is added to the Height condition.

These are the limits recommended for Life Form distinction, but exceptions are allowed:

• Plants essentially herbaceous but with a woody appearance (e.g., bamboos and ferns) are classified as Trees if the height is more than 5 m, and as Shrubs if the height is less than 5 m.
• These are the recommended thresholds for Life Form characterization.

Concerning the concept of dominance, two criteria need to be considered:

• The main criteria is the uppermost canopy layer. This means that the dominant layer goes from Tree canopy to Shrub to Herbaceous/Forbs/Graminoids.
• This general rule is subject to a sub-condition of Cover: it is only valid if the dominant Life Form has a Cover either Closed or Open. If the Life Form is Sparse, the dominance goes to another Life Form that has a Closed or Open cover (Figure 9).

When the user has decided these two main aspects, the building of classes can start. The rules explained above show that in order to determine a (Semi-)Natural Vegetation class, a minimum of three classifiers need to be selected:

• Life Form
• Cover
• Height

These are the minimum elements required to form a Natural or Semi-Natural Vegetated land cover class, for both Terrestrial and Aquatic or Regularly Flooded Areas. Because Height (in its standard denotation) is automatically linked to the Life Form chosen, the classifiers needing to be determined are actually two: Life Form and Cover.

FIGURE 9.
Main Structural Vegetation Domains (Di Gregorio and Jansen, 1996a).

A. Life Form and Cover

A Life Form is a group of plants having certain morphological features in common (Kuechler and Zonneveld, 1988). According to the quality of the main axis or shoots, a further distinction is made into Woody or Herbaceous. For further subdivision, the following growth form criteria can be applied:

• Branching symmetry, subdividing Trees and Shrubs; and
• Herbaceous plant physiognomy, subdividing Forbs from Graminoids (Strasburger et al., 1983; Kuechler and Zonneveld, 1988) and from Lichens/Mosses.

The full definitions and guidelines for application in the system are found in the Help of the software application, and as Appendix A here.

Cover can be considered as the presence of a particular area of the ground, substrate or water surface covered by a layer of plants considered at the greatest horizontal perimeter level of each plant in the layer (according to Eiten, 1968). A distinction is made between Closed (>(60 - 70) percent), Open (between (60 - 70) and (10 - 20) percent) and Sparse (below (10 - 20) percent but >1 percent). As herbaceous plants are seasonal in character, it is always assessed in terms of fullest development.

The reason for expressing cover in terms of ranges instead of absolute values is discussed in the relevant guidelines of the software application and in Appendix A here.

B. Height

The Height of a certain layer is measured from the ground to the average top of the life form that is being examined (Kuechler and Zonneveld, 1988). The fact that single plants of one synusia differ from the average height can be ignored, apart from the fact that they can form their own layer (e.g., the emergents of a rainforest that tower above the rest). The Height is classed as: Trees >30 - 3 m; Shrubs 5 - 0.3 m; and Herbaceous 3 m - 0.03 m. Each class can be further subdivided.

The major Height classes are linked to the Life Form selected. These classes provide general information regarding height because, in the concept of the classification, this criterion has not been given a prevalent importance. The user can choose to remain at this generic level, or to go to the modifiers, whereupon the importance of height increases.

In the case of Shrubs or Herbaceous (Forbs or Graminoids) life forms, it is strongly recommended not to remain at the level of the standard definition of Height, if this is possible, but instead to select one of the modifiers. The ecological significance of these life forms can be strongly correlated with height (e.g., separation between low and tall herbs or between dwarf and high shrubs, especially concerning potential for grazing/rangeland).

C. Spatial Distribution or Macropattern

The next classifier that can be applied is the Macropattern. It is defined as the horizontal spatial distribution of vegetation in a certain area. It should not be confused with Cover because that defines the spatial arrangement of Life Forms (e.g., trees, shrubs, etc.). Macropattern describes the spatial arrangement of specific structural vegetation types (e.g., Closed Forest, Closed Shrubs). This classifier may seem unusual, but there are good reasons:

• Often the Macropattern reflects an ecological or an evolutionary aspect of vegetation (e.g., scattered vegetation in arid areas; agricultural encroachment inside forest areas; degradation due to overgrazing; etc.). In many classifications, one find terms that are extremely subjective, like "Degraded Forest" or similar. The present classification aims to be neutral in its land cover description without including ambivalent terminology. Therefore Macropattern is used as a neutral classifier to describe vegetation status;
• this classification has been built up for mapping purposes, therefore spatial distribution of land cover is an important aspect; and
• macropattern is easy detectable from remote sensing data (photographs and imagery), i.e., it has good "mapability."

Macropattern should thus be used to give supplementary ecological information (or to show a human-induced degradational aspect of natural vegetation). The user has the possibility of skipping this classifier if it is felt to be irrelevant.

The combinations between Cover and Macropattern are unrestricted (this is nevertheless only valid for Closed and/or Open Cover, as will be explained later) which means that, for instance, a Closed Tree formation (Closed Forest) can be either Continuous or Fragmented depending on its spatial distribution in the mapping unit.

Because of this dimensional aspect, Macropattern is linked with the mapping scale. This may seem a contradiction with the main classification concept explained above, namely that the elements of a classification system must be scale-independent. To determine Macropattern, one should refer to the overall appearance of a vegetation formation in a certain area in a homogeneous landscape. However, if one wants to be more precise or objective in the application of this classifier, some specific rules are given below to help the user who is not familiar with this concept in order to standardize the interpretation. Because we are dealing with the practical application of this concept in a cartographic context, the concepts of mixed units and minimum mappable areas will be used. These concepts are further described in Section 2.5.

A certain structural vegetation type has a continuous Macropattern if, inside the minimum mappable area, it covers more than 80 percent of the area.

A particular structural vegetation type would be considered a Fragmented Macropattern if inside the minimum mappable area it covers more than 20 percent but less than 80 percent. This situation is linked with the concept of mixed unit. Three cases are possible:

• The structural vegetation type (e.g., dense forest) covers more than 50 percent of the area and the other element (e.g., agricultural fields) covers less than 50 percent but more than 20 percent. In this case the resulting unit will be a mixed unit with the fragmented dense forest as the dominant one (e.g., fragmented dense forest/agricultural fields).
• The structural vegetation type (e.g., dense forest) covers less than 50 percent but more than 20 percent of the area. The other element (e.g., agricultural fields) covers more than 50 percent. In this case the class is also mixed, but the dominant class will be the agricultural fields (i.e., agricultural fields/fragmented dense forest).
• When a unit contains three elements (e.g., fragmented dense forest, agricultural fields and bare areas) the rules for mixed units should be applied (see Section 2.5). In this case it could be possible to have a structural vegetation type with a Fragmented Macropattern as single unit (e.g., fragmented dense forest, 70 percent; agricultural fields, 15 percent; and bare areas, 15 percent. As neither of the subsidiary elements reach a cover exceeding 20 percent, the unit must be considered a single mapping unit of fragmented dense forest). This is the only case when a structural vegetation type with Fragmented Macropattern must be considered as a single mapping unit. Even if theoretically possible, this case must be considered a very unusual one, and therefore should be avoided.

The Continuous or Fragmented classifiers are linked with the Cover, Closed or Open (e.g., Closed Continuous Forest, Closed Fragmented Forest, Continuous Woodland and Fragmented Woodland). Fragmentation can be further subdivided into Striped or Cellular (e.g., the tiger bush in the Sahel, where Closed Shrubs are present in the interdunal areas, which can be represented as Fragmented (Striped) Closed Shrubs).

The Parklike Patches Macropattern is directly linked with the cover category Sparse. Basically, this is simply redundant information. When the user defines the cover of a certain life form to be Sparse, the only Macropattern available for this structural vegetation type is Parklike Patches.

The Macropattern concept is preferentially used for Woody Life Forms (Trees, Shrubs). Herbaceous Life Forms (Graminoids, Forbs) can have a Macropattern, but this is subordinated to the absence of Woody Life Forms. When linear patches of dense shrubs (typical of tiger bush) are present together with dense herbaceous vegetation filling the space between patches, one could have two different perspectives of this situation, either Fragmented Shrubs/Herbaceous or Fragmented Herbaceous/Shrubs. In the application of the Macropattern, the rule obliges the user to always give preference, to the Woody component. Macropattern can be applied to Herbaceous Life Forms only when there is no significant presence of Woody Life Forms (Trees, Shrubs). For instance, patches of dense herbaceous vegetation in sandy areas can be called fragmented herbaceous/sand.

A structural vegetation type is Fragmented when the size of the patches of the vegetation are between 1/15 and 1/2 of the minimum mappable unit. This rule is a very artificial one and should not be rigidly applied. Nevertheless, the rule assists the user by providing some reference indicator of what a Fragmented Macropattern should look like. If the patches become too small, at a certain level they could coincide with the life form itself, thus contradicting the basic rule explained above, namely that Macropattern describes the specific arrangement of structural vegetation types and must not be confused with the cover of the life form.

If all the above mentioned classifiers are determined, the user can enter the next level and add a new set of information.

C. Water Seasonality

For Aquatic or Regularly Flooded Natural and Semi-Natural Vegetation (A24), the second level classifier consist of Water Seasonality. This classifier can be considered as the type of persistence of the water at or near the surface. There are three subdivisions:

• (Semi-)Permanent (more than four months a year or more than a specific season);
• Temporary or Seasonal (less than four but more than two months a year or during a specific season); and
• Waterlogged.

D. Leaf Type and E. Leaf Phenology

This level consists of the classifiers Leaf Type and Leaf Phenology. It can be entirely skipped. This option is included to allow the user to opt for a basic physiognomic-structural vegetation classification. The choice of the dominant Life Form will disactivate a number of choices at this level as a consequence of the conditions of the classification.

The classifier Leaf Type is subdivided into:

• Broadleaved: referring to trees and shrubs of the botanical group Angiospermae, with the exception of ginkgo (Ginkgo biloba), which belongs to the Gymnospermae taxonomically.
• Needleleaved: referring to trees and shrubs of the botanical group Gymnospermae (Ford-Robertson, 1971) carrying typical needle-shaped leaves.
• Aphyllous: this category encompasses plants without any leaves and plants which apparently do not have leaves in the common sense. In the first case, photosynthesis takes place through other organs, such as stems, branches and twigs; in the latter case the leaves are very short-lived or extremely reduced, to scales and thorns. Characteristic genera are Casuarina, Euphorbia, Tamarix and many others mostly found in arid and semi-arid regions (Kuechler and Zonneveld, 1988).

Leaf Phenology is determined from the general behaviour of woody plants through the year. A distinction is made between evergreen and deciduous:

• Evergreen: perennial plants that are never entirely without green foliage (Ford-Robertson, 1971).
• Deciduous: perennial plants that are leafless for a certain period during the year (Ford-Robertson, 1971). Leaf shedding usually takes place simultaneously and in connection with the unfavourable season (UNESCO, 1973).

The modifiers Semi-Deciduous, Semi-Evergreen and Mixed, as well as Perennial and Annual, are explained in Appendix A.

F. Stratification or Layering

The user can describe up to three layers of stratification (including the main layer) for Terrestrial Vegetation (A12) and up to two layers in Aquatic or Regularly Flooded Vegetation (A24) (see Appendix B). The users may be disappointed by the limited number of layers at their disposal, but the classifier Stratification should contribute to the structural definition of a vegetation class. This means that this classifier must cover all the possible combinations with the main Life Form selected and its Cover (e.g., if we can have layering for Closed Trees, the same must be valid for Closed or Open Shrubs or Closed Graminoids, etc.). The layering is an active component of the class set-up; it is not a mere descriptive (optional and unsystematic) item of the class. The proposed classification allows the user to first build up a land cover class with the use of the classifier Stratification and, where more detail is wanted, add a users' description to the standard one, which may contain information on any additional layers/strata.

Some limitations in the use of the classifier Stratification have been introduced in order to avoid irrelevant (from the structural point of view) class combinations. The following examples will further clarify this concept:

• "Tree Savannah" is clearly defined by two main elements: a Herbaceous vegetation layer and a Sparse Trees layer. Thus, the Stratification of the two elements Herbaceous and Tree layer is crucial for the definition of this class.
• "Closed Forest" is clearly defined by the element of a Closed Trees layer. Limitations have been introduced (as will be explained below) for this class in the use of Stratification. It is not possible, in this case, to determine the presence of a Herbaceous layer because the classification rules set up for the Layering allow the user only to determine sub-layers of Trees and/or Shrubs. The determination of a Herbaceous layer would not contribute to the main structural meaning of the class as defined at the first level. The element Herbaceous layer can be added as part of the user-defined description of the class (see Legend - Edit).

The limitations introduced, as shown in the two examples above, are to avoid introducing elements not crucial for the determination of the structural aspects of a land cover class. These elements can be added in the class description in the Legend (see Legend - Edit). These limitations have the practical purpose of reducing the number of possible combinations of classifiers, which otherwise could lead to the creation of an even larger number of classes that yet would have the same structural meaning. All limitations in use of Stratification are built into the software application.

From the practical point of view in the use of the Stratification concept, it is important to recognize that two possible types of Stratification exist:

1. where the second stratum consists of the same Life Form as the main stratum (e.g., trees-trees and shrubs-shrubs); and
2. where the second stratum consists of a different Life Form (e.g., trees-shrubs).

The second case is quite straightforward and does not present any difficulty in the selection of classifiers. The first case needs additional explanation. In the case of a dominant Life Form of Trees with a second stratum of Trees, it is important that these layers are clearly distinguishable from one another (e.g., a second strata of Trees Emergent over a Closed Tree canopy; where these emergents must not be part of the discontinuity of the Closed Tree canopy but clearly a distinct layer). The sub-condition of Height will pre-set the available choices of Height for second and/or third layers/strata (e.g., main stratum of Closed Low Trees (3-7 m), the emergents to be defined in the second stratum cannot have the same height (option 3-7 m therefore not available) because the Sparse Trees of the second layer have to be taller).

The Height parameter explained above depends on the Height value chosen for the main stratum; it is not applied if the general Height class is selected. If the user selects the general Height class for the main stratum, then for subsequent strata the general Height classes are the only options available.

The main conditions applied for Stratification/Layering are the following:

(a1) Forbs and Graminoids are considered always together as Herbaceous.

(a2) For Trees, three strata including the main, can be considered (e.g., a main Closed Tree layer with a second lower Closed to Open Tree layer and a third Sparse Tree layer of emergents is called a "Multi-Layered Forest With Emergents").

(a3) When the main stratum is Closed Trees or Open Trees and there is a second layer of Sparse Trees then the Height of the second layer must be higher, i.e. emergent. If they are lower they are not considered as an independent stratum.

(a4) For Shrubs, the number of strata with the same Life Form is two, including the main stratum.

(a5) For Herbaceous, only one stratum is possible.

(a6) Lichens/Mosses are not described in the layering.

(a7) If the main stratum is Trees and the Cover is Open, then it is impossible to have the same Life Form with Cover Open To Closed with a different height as a second stratum (e.g., Open High Trees with Open Low Trees is impossible).

(a8) If the main stratum is Shrubs and the Cover is Closed or Open with the general option of Height, then it is impossible to have the same Life Form with Cover Open to Closed with a different height as a second stratum (e.g., Open High Shrubs with Closed to Open Low Shrubs is impossible). The only exception to this rule is when the second stratum consists of Dwarf Shrubs.

and

(b1) If the cover of the main stratum is Closed Trees or Closed Shrubs, then any Herbaceous layer is not considered or described (this can be added as a user-defined description).

(b2) Sparse Herbaceous is never considered as second layer except when the main layer is Sparse Trees or Sparse Shrubs (but it can be added as user-defined description).

(b3) If the main stratum is Shrubs or Herbaceous, only one layer of trees can be considered. This is linked with the criterion of dominance, as described earlier, because the Trees or Shrubs can be only Sparse.

(b4) Only two layers other than the main layer are considered for Terrestrial Vegetation (A12), and only one additional stratum for Aquatic or Regularly Flooded Vegetation (A24).

2.3.3.2 Cultivated and Managed Terrestrial Areas (A11 and A23)

Cultivated areas are often only described and classified by determining the crop species, the cultural practices and in some case land tenure information. This may result in descriptions like "rainfed agricultural area" or "state-owned rubber plantation." These descriptions are highly sectoral and do not address the needs of a wide variety of end-users. Another important aspect is that in the sectoral approaches the principle of having a high level of geographical accuracy is frequently lacking.

Description of agricultural areas in land cover terms should be exhaustive and neutral in the sense that the results may be used by many. Furthermore, these areas are primarily vegetated land cover types; thus their description should have a link to (semi-)natural vegetated land cover types at a certain level of detail (e.g., a user interested in trees because of the nesting prospects of a certain bird may not be directly interested in knowing if these trees are part of a crop or (semi-)natural vegetation). Furthermore, the focus should be on the definition of geographically well-defined classes, i.e., classes having a high mapability.

Therefore, the approach taken in order to enable a wide variety of users to employ the descriptions of cultivated areas is that of a basically physiognomic-structural classification. This means that at a high level of classification the cultivated area description is based on the structure of the vegetation, whereas at lower levels, with lower mapability, the focus is on description of the spatial and temporal dimensions. This type of description should, however, assure a high degree of compatibility with existing agricultural classification systems. This means that not only should the classes be compatible but also the method of deriving classes and their spatial and temporal dimensions (Duhamel, 1995). The spatial and temporal dimensions for cultivated areas clearly differ from (semi-)natural vegetation, as in most cases there is a constant flux in the observable cover.

Owing to this flux, the moment of observation of the land cover is very important, as the land might be ploughed, sown or harvested (with no crop actually visible), or a crop is clearly visible and different crop growth stages can be identified. These temporal dimensions influence the land cover but should not influence its description, because the area should be classified independent of the time of observation. It is for this reason that in the definition of Cultivated Areas provision is made for the fact that vegetative cover is not always present.

In the structural approach, physiognomy or Life Form is the principal classification criterion, followed by the vertical structure, the crop layering and horizontal structure, i.e., the Field Macropattern, of the area. This will result in detailed cover information that can be optionally combined with Crop Type as a specific technical attribute to establish a link with many current classification systems (Appendix B and C).

In the major land cover type of Terrestrial Cultivated Areas and Managed Lands (A11), Managed Lands form a separate category. They comprise land cover classes that are clearly vegetated and managed, though not with the intent of harvesting, as is the case for Cultivated Areas. The structural description of their cover in this classification may appear simplistic, but a further description in land use terms would render much more information. The description in cover terms will assure a high level of mapability, which can be freely combined with user-defined land use descriptors.

A. Life Form - Managed Lands

Managed Lands form a separate category inside the Cultivated Terrestrial Areas and Managed Lands (A11) and consist of one single classifier: Life Form. The Managed Land Areas are described by the Life Form composition rather than description of the individual Life Forms of the vegetation. They are defined by specifying the occurrence of trees, shrubs and/or herbaceous life forms. Three options are available: Parklands, Parks or Lawns.

Managed Lands may comprise private gardens, public green areas, sport fields, etc. They are usually found in the (peri-)urban environment. This category may be further elaborated in future to include a wider range of classifiers for more detailed descriptions.

A. Life Form - Cultivated Areas

Two main aspects of the classifier Life Form should be taken into account:

1. the concept of Life Form in this classification; and
2. the determination of the dominant Life Form.

Careful determination of these two main aspects is important because the classification is set up in such a way that the choice of the main Life Form has consequences for the choices available at lower levels due to certain built-in conditions.

Life Form is defined by the physiognomy of the plants. Under Cultivated Terrestrial Areas, Trees and Shrubs are distinguished from Herbaceous plants, subdivided into Forbs or Graminoids. Under Cultivated Aquatic or Regularly Flooded Areas, only Graminoid and Non-Graminoid crops are distinguished. The following rule applies: those plants that belong to the Graminae family but have a woody appearance (e.g., bamboos) are classified as Herbaceous plants. This rule differs from the rules applied in Natural and Semi-Natural Vegetation (major land cover types A12 and A24).

For determination of dominance the following rules apply:

• The main criterion is the uppermost canopy layer. This means that the cover goes from Trees to Shrubs to Herbaceous/Forbs/Graminoids.
• This general condition is subject to a sub-condition of "marginality", i.e. the crop should cover at least 15 percent of the area and/or should return the highest economic revenue.

These two rules are the main criteria for determining the main crop. There are no restrictions to possible crop Life Form combinations (in contrast to the description of (Semi-)Natural Vegetation, as explained in the next paragraph).

The Trees and Shrubs Life Forms can have two additional modifiers: Leaf Type (Evergreen or Deciduous), in combination with Leaf Phenology (Broadleaved or Needleleaved). The introduction of this modifier for these two Life Forms assures a link with the description of the natural vegetated areas.

B. Spatial Aspect - Field Size and Spatial Distribution

The second classifier that can be applied is Spatial Aspect - Size. This classifier often implies other aspects (e.g., land tenure, mechanization, land reclamation, etc.). In many classifications one find terms like "large-scale irrigated agriculture" or similar. This classification needs to be neutral in its land cover description without including ambivalent terminology. Therefore, Spatial Aspect has been selected as a neutral classifier. For mapping exercises, Spatial Aspect is an important aspect at the meso- or macro-level. Furthermore, it is an easily detectable characteristic (e.g., on aerial photographs and satellite imagery), i.e., it has good "mapability."

Field Size may differ according to biophysical conditions. Therefore, the quantitative values are indicative. The classifier is applicable at the level of the individual field and three categories are distinguished: 

• less than 2 ha;
• 2 to 5 ha; and
• more than 5 ha.

This classifier can be skipped because size is a very subjective element.

Spatial Distribution is the horizontal pattern of cultivated fields in a certain area. It can be easily measured, taking the distance between one field and the next. A distinction has been made into three classes:

• Continuous describes a continuum of more than 50 percent of cultivated fields. In this case the land cover mapping unit may be single (inside the mapping unit the fields take up more than 80 percent) or mixed (the fields occupy 51-80 percent of the mapping unit) (see also Section 2.4.2). Generally, when the fields occupy 51-80 percent of the mapping unit, the area in between the fields can be considered by the user as part of the cultivated area, or the user can decide to make a mixed mapping unit, depending which land cover features the user wants to highlight.
• The Spatial Distribution is Scattered Clustered or Scattered Isolated when, within the cultivated field area, other land cover types are present. They are defined as follows:
  1. Percentage of fields is more than 20 percent but less than 50 percent, it is Scattered Clustered: this means that the resulting mapping unit is a mixed land cover class of a cultivated area with another land cover class. If the unit is composed of two land cover classes the cultivated area class is always the secondary class in the mixed unit, and both components need to be defined in the legend (e.g., 60 percent of semi-natural vegetation and 40 percent of fields).
  2. Percentage of fields is more than 10 percent but less than 20 percent it is considered Scattered Isolated: this means that the resulting mapping unit is a mixed land cover class where the dominant class is not this one. It is the only case where a class comprising less than 20 percent is present in a mixed mapping unit (see Section 2.5).

C. Crop Combination (A11) and E. Crop Combination (A23)

At the second level the Crop Combination is specified for the Cultivated Terrestrial Areas and at the fourth level for Aquatic or Regularly Flooded Cultivated Areas. If there is more than one crop, the crops present can be specified together with details of the possible overlap in growing period between the main and secondary crops (A11) or the type of setting the crops are growing in (A23)¹. The order in which an additional crop is specified, follows the same condition as stated above.

• The dominance is determined by the main criterion of the second-uppermost canopy layer. This means that the cover goes from Trees to Shrubs to Herbaceous/Forbs/ Graminoids.
• This general condition is subject to a sub-condition of "marginality," i.e. the crop should cover at least 15 percent of the area (but less than the main crop) and/or should return the second highest economic revenue.

It is important to note that the classifier Crop Combination can also be skipped by the user because of the apparent difficulty in determining the classifiers correctly. This skip function will then permit the user to continue the description of the main crop at the third level. This function is unavailable in A23 because it is the final classifier to be determined.

C.  Water Seasonality (only for A23)

The second level classifier Water Seasonality of Aquatic or Regularly Flooded Cultivated Areas describes the duration of water on or near the surface during the main crop cultivation period. If any additional crops are cultivated after, or in overlap with, the main crop the period of water at or near the surface for these crops should be neglected.

D. Cover-related Cultural Practices - Water Supply and Cultivation Time Factor ( A11)

At the third level of classification the classifier Cover-Related Cultural Practices - Water Supply is determined. The options Rainfed Agriculture, Post Flooding and Irrigated Agriculture for Cultivated Terrestrial Areas have implications for the options available under Cultivation Time Factor. Post Flooding cultural practices are not possible in a Permanent Cultivation system. It is also obvious that the dominant crop determined will have implications for other classifiers (e.g., a Tree Crop will result in a Permanent Cultivation system).

A Permanent Cultivation system in combination with either a Trees or Shrubs Life Form designates what is commonly known as plantations and orchards (e.g., a forest plantation or a coffee plantation). However, these names do not occur per se in this classification system. In combination with Crop Type, a link to current systems can be made and to commonly used names such as "plantation" (e.g., the combination of Shrub Crop and Crop Type: Tea covers "Tea Plantation," while Tree Crop and Crop Type: Hevea spp. refers to "Rubber Plantation").

D. Cover-related Cultural Practices - Fallow Period (only for A23)

Cover-Related Cultural Practices - Fallow Period is the third level classifier for Aquatic or Regularly Flooded Cultivated Areas. It has two subdivisions: Relay Intercropping, and Sequential. They refer to the practices that occur after harvest of the main aquatic crop (see also Appendix A). These practices may not relate to the same Aquatic or Regularly Flooded environment of the main crop.

2.3.4. Concepts for Primarily Non-Vegetated Areas

Areas primarily characterized by a cover other than vegetation fall into two categories: those with a non-vegetal cover and those with no cover at all. The latter is a category that describes the land surface rather than any cover of the land but which has been included here, as explained earlier (see Section 1.1).

The approach adopted for describing Primarily Non-Vegetated Areas is, as for Vegetated Classes, a "structural-physiognomic" approach, that is the physiognomy, the cover (i.e., density) and structure are used as parameters. The classifiers Surface Aspect (Artificial Surfaces and Bare Areas) and Physical Status (Artificial and Natural Waterbodies, Snow and Ice) can be regarded as descriptors of the physiognomy of the materials, like Life Form for vegetation. The further classifiers and modifiers of Bare Areas and Artificial Surfaces contain elements of Cover, as for Terrestrial Vegetation, whereas the Water Persistence classifier is similar to Water Seasonality in Aquatic Vegetation (Appendixes B and C).

2.3.4.1 Artificial Surfaces and Associated Areas (B15)

Areas with an artificial cover resulting from human activities are described in most classification systems in terms of use, whereas the description of cover is equally important. An example is urban areas where the surface generally consists of impervious materials. This type of impervious surface greatly influences run-off and the peak flow characteristics of water. Another example is tarmac roads in hilly terrain, where road constructors need to carefully plan for the discharge of excess water that, in poor designs, may lead to disastrous forms of erosion.

The Associated Areas are mainly domains where the original surface is removed, such as extraction sites, or where materials have been deposited on top of the original surface, such as waste dumps and other type of deposits.

The characteristics of the cover of the surface are crucial in the land cover description and therefore embody the main classification concept. This major land cover type is classified depending upon the Surface Aspect. A category for the Built-Up Object can be specified using the scroll list (e.g., cities and towns, roads, open mines, official waste dump sites, etc.).

A. Surface Aspect

The Surface Aspect distinguishes two main classes, with one class having two levels with an increase in detail. A much more detailed class description can be made using the modifier options. These modifiers are explained in terms of cover rather than land use terminology.

The Artificial Surface areas can be further defined according to the shape and density of the artefacts.

2.3.4.2 Bare Areas (B16)

Areas which are primarily bare are usually described by geologists, soil scientists or geomorphologists (using technical terms like granite rock, rendzhina, sand dunes, inselberg, tor, etc.). This type of description is highly technical, and may be difficult to understand for users with a different background. An approach is therefore needed which describes the type of material on the surface, with additional options to go into more detail, in combination with elements describing either some specific properties (physical or chemical) of the surface material, or describing some specific forms. Specific forms implies that the surface may consist of shapes that form a pattern at the macro-level. The focus of the cover description is on the surface and not on the subsoil.

The major land cover type Bare Areas is, therefore, described mainly by the appearance of the surface. The concept adopted describes the aspects of the cover: whether it is consolidated or not, and of what kind of material it comprises (e.g., rock, sand, etc.), and which may be combined with Macropattern. The more discipline-related descriptors for geology, landform and soil are available as attributes and can be used to link the land cover description to the technical disciplines.

A. Surface Aspect

The Surface Aspect describes the surface of the Bare Area at two levels, with an increase in detail. A further specification can be made by using one of the modifiers. These modifiers specify some physical or chemical properties.

B. Macropattern

The Macropattern describes the pattern of the surface. This classifier is linked to the Surface Aspect because a Macropattern can only be of the same material as the surface described. Hence the choice made under Surface Aspect may disable certain choices in this classifier. Two types are distinguished, namely Bare Soil and Loose and/or Shifting Sands.

2.3.4.3 Artificial and Natural Waterbodies, Snow and Ice (B27 and B28)

The two major land cover types describing water surfaces or other physical appearances of water, Artificial Waterbodies, Snow and Ice (B27) and Natural Waterbodies, Snow and Ice (B28) are described by taking into account their temporal aspect. Water, snow and ice may not be present all year round and therefore it is also important to know what the cover is when they are absent. This temporal aspect should not influence the classification results because classification by default is independent of temporal change.

In most existing classification systems these land cover types are only briefly described in terms of cover, with no additional information. The concept adopted by this classification puts more emphasis on the temporal aspect.

The major difference between these two major classes is that Artificial Waterbodies, Snow and Ice are surfaces in places where, under natural circumstances, no water, snow or ice surface would exist. Therefore these surfaces are the result of an artefact, such as the construction of a dam, the making of artificial ice or snow, etc.

A. Physical Status

The Physical Status describes in which form water is found. Three options are available: Water, Snow or Ice. Depending on the choice made here, other classifiers at lower levels may be disabled. For water and ice a further specification can be made into Flowing or Standing Water and Moving or Stationary Ice.

B. Persistence

Persistence, i.e., the duration that Water, Snow or Ice covers the surface, is described. If Water, Snow or Ice is present for nine months or less per year, the surface then exposed can be further specified.

C. Depth

The Depth can be described because this is directly related to cover aspects. The proposed classifier has not been given a lot of detail because the most important feature to be determined is whether it is deep or not, i.e., whether it is shallower or deeper than 2 m. This limit has an ecological meaning as it is the maximum rooting depth for the great majority of aquatic plants (Cowardin et al., 1979).

D. Sediment Load

The suspended Sediment Load in the water influences the cover and implies other environmental aspects, such as upstream erosion and downstream sedimentation. It also influences the aquatic fauna and flora. It is a relatively easily observed characteristic of the water, but difficult to measure as it fluctuates. Therefore the subdivision has not been given great detail.

2.3.5 Environmental and Specific Technical Attributes

The pure land cover classifiers can be combined with so-called attributes for further definition (see also Section 2.3.2.) of the land cover class. These attributes are intended to be used as a further characterization of the land cover itself rather than to add a new data layer. Application of the attributes in the full legend, when all classes would have the same set of attributes, they could be used as a separate layer in the database. Two types of attributes, which form distinct levels in the classification, are distinguished:

• Environmental Attributes: attributes that are not inherent features of land cover but may influence the land cover.
• Specific Technical Attributes: attributes referring to the technical discipline of the major land cover type.

2.3.5.1  Environmental Attributes

L.  Landform

Land forms are described first and foremost by their morphology, and not by their genetic origin or the processes responsible for their shape. The dominant slope is the most important differentiating criterion, followed by relief intensity (Appendix A).

This attribute can be applied to all classes except Artificial Surfaces and Artificial and Natural Waterbodies, Snow and Ice. The attribute consists of two different levels, that is major land form and slope class according to the Soils and Terrain (SOTER) methodology (UNEP/ISSS/ISRIC/FAO, 1995).

M.  Lithology

The lithology can be described based on the geological parent material and the age of it. The options were devised by Dr S.B. Kroonenberg, Delft University of Technology (pers. comm., 1998). Three major groupings are distinguished and further subdivided (see Appendix A).

N.  Soils

For the Primarily Vegetated Areas, the user can describe first the soil's Surface Aspect, followed by a detailed description of the soil profile according to the Revised Soil Legend (FAO, 1988). For Bare Areas (B16) only the soil profile description is applicable because the soil surface aspect is a classifier of this major land cover type.

O.  Climate

The concept adopted to add climatic parameters to the land cover classes is from De Pauw et al. (1995). The revised Length of Growing Period (LGP) approach gives recognition to the relevant climatic constraints in any major region of the world. The combination of Thermal Classes and Moisture Classes gives the climate. No conditions have been pre-set.

P.  Altitude

This attribute can be used in all major land cover types. The classes of this attribute are a proposal and can be further subdivided by using the possibility available in the Legend Module to create a user-defined attribute (see Section 5.2.5).

Q.  Erosion

In the description of Erosion in the land cover, emphasis is given to accelerated or human-induced erosion. Human-induced erosion is often the result of irrational use and poor management, such as incorrect agricultural practices, overgrazing or overexploitation of the (semi-)natural vegetation. These practices result in a cover type with specific features. Most of the erosion can be classified as either Water or Wind erosion and deposition, with the Mass Movements as a third major category. Further subdivision can be made by using the User-defined Attribute option in the Legend Module.

This attribute is applicable in all Primarily Vegetated Areas and Bare Areas (B16).

R.  Water Quality (only for A24)

This attribute is only applicable in (Semi-)Natural Aquatic or Regularly Flooded Terrestrial Areas (A24). It can be used to specify the salinity of the water (measured in ppm of TDS) according to Cowardin et al. (1979).

U.  Scattered Vegetation (only for B16, B27 and B28)

This attribute is applicable for Bare Areas and Artificial and Natural Waterbodies, Snow and Ice (e.g., sandy riverbed with scattered vegetation) to indicate that less than 4 percent of vegetation is present. In the case of the presence of Lichens and/or Mosses, they should be less than 20 percent (see Appendix A).

W.  Crop Cover/Crop Density (only for A11 and A23)

This attribute is only applicable for the Cultivated Areas, both Terrestrial and Aquatic or Regularly Flooded. This attribute gives information on the density of the permanent crops, (e.g., Trees and Shrubs), or the cover of the temporary life forms (e.g., Herbaceous, Forbs and Graminoids). This information is an indicator of the success of crop establishment and hence its possible yield.

The density has not been used as a land cover classifier, as for (semi-)natural vegetated areas, because it normally would not add any useful information to the land cover class. The density is related to the planting distance of the crop, which differs according to crop (e.g., olive trees versus maize). However, it is a useful attribute when describing a cultivated area which does not have the expected density of the crop (e.g., in marginal areas).

2.3.5.2  Specific Technical Attributes

These attributes are related to the technical discipline associated with the major land cover type: thus, for (Semi-)Natural Vegetated, areas the Floristic Aspect can be described; for Bare Areas, the Soil Type (as discussed under N. Soils); for Cultivated Areas, the Crop Types; and for Artificial and Natural Waterbodies, Snow and Ice, the Salinity.

S.  Crop Type (only for A11 and A23)

The Crop Type can be specified according to the major groupings used for the FAO Production Yearbooks. If a Crop Type is not present, it can be defined and added under the header Other in the boxes which open upon clicking. Furthermore, the name of the crop has to be linked to the dominant, second or third crop choices, if not the entry is not saved. Thus, a maximum of three names can be specified.

T.  Floristic Aspect (only for A12 and A24)

This attribute has two major divisions: if the name is derived from a single plant species or a group of plants. In the first option, a further subdivision is possible into Dominant Species (Height, Cover or combination of both) and Most Frequent Species. The second option is subdivided into: Plant Groups (e.g., Braun-Blanquet) and Plant Groups Derived Without Statistical Methods (e.g., same ecological significance, same geographic distribution, same dynamic significance, etc.). The specific name of the Floristic Aspect can be added with the User-Defined Attribute option in the Legend Module.

V.  Salinity (only for B27 and B28)

The Salinity of the water can be specified for Artificial and Natural Waterbodies. Three main classes are distinguished, based upon Cowardin et al. (1979).

2.4  THE ADVANTAGES OF THE ADOPTED METHOD

2.4.1  Advantages from the conceptual point of view

It is a real a priori classification system in the sense that, for the classifiers considered, it covers all their possible combinations. Some particular combinations are excluded, due to conditions that are elements of the classification system. In this case the type of combinations and the conditions, i.e., the reasons, for this "exclusion" are clearly listed and explained.

A given land cover class is clearly and systematically defined making a clear and unambiguous differentiation by use of the classifiers as follows:

• pure land cover classifiers (each one ordered from general to more specific level);
• environmental attributes (e.g., Climate, Landform, Geology, etc.); and
• specific technical attributes (e.g., Floristic Aspect for (Semi-)Natural Vegetation).

This system avoids unclear definitions (e.g., "tropical rain forest" where a climatic attribute is used for a floristic description).

The classification is truly hierarchical. The class' hierarchical arrangement is a basic component of the mechanism of the class formation. The difference between a land cover class (at a more general level) and a further subdivision of it is given through the addition of new classifiers (or a more detailed level of the one forming the previous class). The more classifiers used, the greater the detail of the land cover class defined.

The classes derived from the proposed classification system are all unique and unambiguous, due to the internal consistency and systematic description of the class as a basis for objective and repeatable classification. Correlation studies between classifications show that in many cases definitions of the class names are often either unclear or unsystematic, or both, due to the fact that in traditional classification and legends the "meaning" of a class is derived only from its general description. Such a descriptive text is very often unsystematic, and as a result in many cases there are insufficient details to define strict boundary conditions. The classes are therefore open to misinterpretation and lack internal consistency. With the present classification the user's primary descriptive tool is the Boolean Formula of all classifiers used to build the class; this cannot be anything other than a systematic description of the class. In addition to this, the traditional class description is used. A strict class boundary definition and internal class consistency are inherent to the method.

LCCS is designed to map at a variety of scales, from small to large (see Section 3).

For two main reasons, the classification can be used as reference classification:

• the classification contains a large number of classes (the classes of the existing classifications and legends can always be accommodated); and
• emphasis is on a set of classifiers rather than just a name, which allows easy correlation even when a range of values, such as the percent of cover of a given life form, does not fit with the proposed value; the dissimilarity is clear and remains limited to only a portion of the elements forming the class. This event however should be extremely rare due to the different levels, from more general to more specific, forming a single type of classifier.

2.4.2 Advantages from the practical point of view

The specific design of the classification allows easy incorporation and integration into GIS and databases. The mechanisms of how the classes are built up (see Section 2) facilitate overlay procedures.

It will produce a real multi-user database. Despite the high demand for natural resources information, many databases are not developed to meet multi-user requirements. This is shown by the fact that very often the number of real users is often a small portion of the potential ones. An important cause is the inherent rigidity of the natural resources information (i.e., land cover) of the databases. Two cases are typical:

• the original project is very specialized (e.g., vegetation ecology), and hence the class name and description of the resulting legend are difficult to understand by other users (such as rural planners, statisticians, etc.); or
• the original project is not specialized, and so the classes or the class descriptions are too generic to be used by specialized disciplines.

The ways in which current classifications determine the classes (names and generally a broad description) do not allow a great deal of flexibility of use by the final user. The present classification system assumes two types of final users:

• the one that uses the classification to built up the database (the user basically doing the interpretation activity); and
• the one that is the final user of the database created.

The system obliges the first user (the database builder) to follow specific rules in the combination of classifiers (to assure standardization and comparability of the data set) but allows the database user (see Section 3) to define freely the set of classifiers with which they wish to re-aggregate the original polygons of the database. Because the class definition is linked with the classifiers' Boolean Formula, this is a straightforward process. Of course, the number of potential recombination of classifiers is extremely large, and some combinations may be illogical, but this respects the concept of multiple users, each with their very specific needs.

For interpretation purposes, the advantages are:

• It is highly flexible, responding not only to the information available or gathered in a given area, but also to the time and budgetary constraints of a project. This means that within one land cover map, mapping units will contain the maximum available information, but that the quantity of information may differ between the mapping units. This will not affect the homogeneity of the resulting map. It will be possible, for instance, to have within the same map, in a certain geographical area polygons of a class formed with a certain number of classifiers (a high number as here more ancillary information is available), while, in another part, polygons where the same type of class will have fewer classifiers. It will always be possible to compare the two classes.
• It rationalizes the field data collection. Because the classes are defined by a combination of classifiers, field surveyors should detect the single classifiers and not deal with the final class name. This means that the field survey can be done independent of, or parallel to, the interpretation process.
• It facilitates standardization of the interpretation process, contributing to its homogeneity. Despite the huge number of classes the interpreter can generate to fit the land cover variations, one is dealing only with a limited number of classifiers. So one does not need to scroll inside a big, obscure list of class names but must simply aggregate a limited number of well-defined classifiers. This will also reduce heterogeneity between interpreters and between interpretations over time.
• It allows the building up of a new procedure of accuracy analysis of the result. Until now, accuracy analysis was done for single classes; henceforth it will be possible to assess the accuracy not only for the entire class but also for each of the classifiers forming the specific class. This will give a high flexibility to finalization of the classes. If, for instance, a class formed by five classifiers shows an accuracy of 60 percent, which is too low according to the established standard, then by looking at the individual classifiers forming this class the user can analyse the contribution of each individual classifier to the overall class accuracy. If, in the example, the first four classifiers have an accuracy of 90 percent while the fifth classifier only 60 percent, the user may decide to eliminate this last and less accurate classifier in order to have a final class with less detail but with a higher accuracy.

2.5 FROM CLASSIFICATION TO LEGEND

Classification is an abstract representation of the situation in the field using a particular set of diagnostic criteria, whereas a legend is the application of the classification's abstract design in a particular area using a defined mapping scale and a particular data set. This transition implies establishment of specific conditions not present in the classification concept (e.g., Minimum Mappable Area and Mixed Mapping Units). Because one of the ultimate goals of this classification is to provide a useful tool for mapping exercises, these conditions will be discussed here even if they are not strictly appropriate to the main subject of this chapter.

2.5.1  The Minimal Mappable Area concept

The Minimal Mapping Area is a concept applied by cartographers when addressing the smallest area that can be shown on a map. This concept is therefore scale-dependent and not related to classification. However, the issue is addressed here as it usually poses problems.

The concept of one single mappable area is generally applied. Historically, the cartographer determined one particular minimum size of area to be represented on the map. This was applied to all classes contained in the legend. The disadvantage of this method is that classes with a difference in importance would follow the same rules. It would have been more logical to define a set of different sizes for the various features with differing importance (Di Gregorio, 1991).

The flexibility of this current classification allows the introduction of the concept of a variable minimal mappable area. Thus, the user can relate the size of the minimal mappable area to the eight major land cover types from which the classes are derived (Figure 10).

FIGURE 10.

Example from the East Africa Project, with variable minimal mappable areas (not at original scale).

2.5.2 The Occurrence of Mixed Mapping Units

In the classification system, all classes are unique and no Mixed Mapping Units are considered. Mixed Mapping Units are cartography related. However, the user can go from a more general to a more detailed level of definition of a class. If for instance the classifier Woody is used, this implies that an intricate mixture of trees and shrubs is present in which neither is clearly dominant. This results in a mixture of two life forms but not in a mixed land cover class. A Mixed Mapping Unit in the legend is always characterized by two, or a maximum of three, separate single land cover classes as defined in the classification system. The conditions governing the utilization of mixed mapping units are that within the minimal mappable area, two or more land cover classes are present, which can be:

1. in a spatially separate entity (e.g., patches of agriculture fields inside a forest); or
2. in an intricate mixture (e.g., rainfed cultivated fields with Baobab woodland).

The latter is applicable only if a more general definition of the class (as explained above) is not appropriate.

In the case of spatially separate entities of two or more classes, the general criteria proposed is that the cover of each one of the class considered must be more than 20 percent (and consequently less than 80 percent) of the mapping unit. The limit of 20 percent is thus the threshold of "visibility" of a class in a Mixed Unit. The only exception to this rule is in the major land cover type of Cultivated Areas, where the use of the option Scattered Isolated of the classifier Spatial Distribution goes from 10 to 20 percent (see Section 2.4.2).

The sequence of the class names in a mixed mapping unit represents the dominance (e.g., for Forest/Cultivated Areas, Forest is more than 50 percent and less than 80 percent, whereas Cultivated Areas is less than 50 percent but more than 20 percent). A Mixed Mapping Unit can contain a maximum of three classes.

In the particular case of classes belonging to the major land cover categories Cultivated and Managed Terrestrial Area(s) (A11) or Cultivated Aquatic or Regularly Flooded Area(s) (A23), the user has an additional means to create a Mixed Mapping Unit. The classification system offers the possibility to create a so-called "Temporal Mixed Unit". Such a unit is used to describe the situation were in different years, different types of cultivation are executed in the same field (i.e. the mapping unit). This is the case when the user has, for example, a situation with cultivated fields of paddy rice in one year (e.g., when there is sufficient rainfall) followed by a terrestrial crop in the subsequent year(s) (e.g., when rainfall is poor). This particular type of Temporal Mixed Unit shows often a cyclic, almost customary, alternation of different crops in subsequent years (e.g., generally an Aquatic crop followed by Terrestrial crops, or an Irrigated crop followed by Rainfed crops, etc.). It is important to note that the alternation of crops should be considered only when this occurs on an annual basis. The combination of different crops in the same growing period is an option already considered in LCCS' class creation (see the classifiers related to Crop Combination in A11 and A23). However, because of the specific nature of this type of Mixed Unit, that occurs only where crops are growing, the classes composing such a mixed unit can only be those of Cultivated Area(s).

¹ If there is more than one growing season in the area and multiple crops are grown in the major season, the user is advised to limit the description of additional crops to one instead of two and to use the second additional crop for description of the main crop in the second season.