Updated February 1998
From "AFRICOVER land cover classification" (FAO, 1997).
1.1 Origin and objectives
For the last two years, FAO has been involved in the organization of the AFRICOVER project, whose goal is to establish for the whole of Africa, a digital geo-referenced database on land cover and a geographic reference (geodesy, toponomy, roads, hydrography) at a 1:200,000 to 1:250,000 scale (1:100,000 for small countries and specific areas). This base will also be generalized at a 1:1,000,000 scale, updated, made homogeneous and comparable/compatible from thematic and geographic points of view, for the whole African continent.
A further objective of AFRICOVER is to reinforce and to build up the national and sub-regional capacities for the establishment, update and operational use of geographic reference and land cover maps and geo-databases.
This project has been prepared in response to a number of national requests for assistance in the implementation of reliable and geo-referenced information on natural resources at sub-national, national and regional levels.
The analysis of national needs proved that no information system on resources, covering or supporting early warning, forest and rangeland monitoring, land use planning, catchment management, production of statistics, biodiversity or climate change, can do without reliable and homogeneous basic geographic information, showing both usual landmarks (infrastructure, settlement, hydrography) and land cover.
Hence, the purpose of AFRICOVER is to prepare this basic geographic information common to the information components of actual and future programs on natural resources in African countries.
1.2 Implementation of AFRICOVER
From a technical point of view, the preparation of AFRICOVER products depends essentially on remote sensing data and geographic information systems (GIS).
The land cover will be mainly derived from visual interpretation of recent high resolution satellite images digitally enhanced. This will be carried out according to a homogenized and hierarchical classification system. The geographic reference will be derived from existing topographic maps and updated from remote sensing documents and ground surveys geo-referenced with GPS points. The geometrical base, which will be used as a reference, will depend on the quality of the geodetic network and on the topographic maps. It may be either the existing topographic maps themselves, or the satellite images geo-coded with GPS measurements and using spatiotriangulation techniques, when possible. In order to maximize the synergy and the scale savings, the cartographic methods will use techniques optimizing the teamwork and allowing task division, such as a multiphase approach.
From an operational point of view, the cartography will be carried out by a national team, with the assistance of specialized international experts, originating, when possible, from other African countries. Several fully compatible approaches could be considered for the implementation of the project in the different countries, depending on countries and sub-regions:
The final users of AFRICOVER will be technicians and decision-makers involved in natural resource management and monitoring at regional, national and sub-national levels, e.g. ministries (Planning, Rural Development, Environment, Statistics, Agriculture, Forest, Fisheries, Water, etc.), international organizations of development (United Nations, World Bank, European Union, FIDA, etc.), intergovernmental organizations (IGAD, SADC, CILSS, OACT, etc.), bilateral cooperating agencies, non-governmental organizations and national private operators.
The FAO objective is to assist in the preparation (technical, institutional and financial arrangements) and the technical monitoring of AFRICOVER.
Beyond this technical assistance function, FAO offers to act as a normalization and labeling agency for AFRICOVER: this role will consist in defining in detail, in the framework of international working groups, the standards to be applied in all African countries in terms of information, tools, analysis methods and utilization procedures. This standardization will greatly integrate the national and sub-regional specific needs and it is absolutely necessary, from a technical point of view, in order to ensure homogenization and a better distribution of final products (databases and maps), as well as important scale savings for production, update and use of geo-referenced data on resources. FAO will guarantee that these standards are respected and will deliver an AFRICOVER label. FAO could eventually be involved as executing (or co-executing) agency for the project, on request of individual countries and sub-regions.
The budget of each national component of the project ranges between 1 and 2.5 US$/km2 depending on the countries and approaches to be used.
The time is estimated between three and five years.
1.3 Present state of AFRICOVER
AFRICOVER proposal was approved in principle in July 1994 by an international consultation at the ECA headquarters, Addis Ababa, which was attended by representatives of eight African countries, 8 sub-regional organizations (IGADD, SADC, CILSS, RCSSMRS, OACT, CRTO, CRTEAN, RECTAS), 4 United Nations organizations (UNEP, UNDP, FAO, UNITAR) and 19 international and national organizations.
In parallel, detailed inquiries and studies were conducted within each African country, in order to assess national and regional capacities, the available maps and data, the future and in progress projects/programs which could be linked with AFRICOVER, as well as final users needs.
Today, the East-African sub-region (11 countries of the Nile Basin: Burundi, Egypt, Eritrea, Ethiopia, Kenya, Rwanda, Somalia, Sudan, Tanzania, Uganda, Zaire) is just starting the implementation of the project, due to a financing of 5.4M US$ provided by the Italian government. This execution will follow mainly a regional approach. For this sub-region, FAO will also act as executing agency of the project.
For the other sub-regions and countries, negotiations are in progress between the different African partners and many sponsors or collaborating agencies (French Cooperation, European Union, GTZ, CIDA, World Bank). All approved the principle of AFRICOVER, and intend to participate at the level of the different countries. FAO is also assisting the World Bank with the technical preparation of important environmental information projects, in particular in Ivory Coast and in the Central Africa sub-region (the REIMP project), which would include the AFRICOVER products.
A number of African countries (Côte d'Ivoire, Equatorial Guinea, Guinea, Mali, Mauritania, Mozambique, Namibia, Nigeria, Senegal, Togo, Zimbabwe) have already sent official requests; while other countries have requests under way within the different ministries. Several countries such as Senegal and Tunisia have built-up national AFRICOVER Working Groups which started with the formulation of project documents according to the national approach.
Finally, FAO has established three international working groups (1. Legend and Classification, 2. Geometry and Topography, and 3. Technical Methods). The first one of these will be responsible for developing the standardized methodology for definition and classification of land cover for the entire AFRICOVER project and for drawing up specifications for the database to be used. The AFRICOVER project will, therefore, be the first project to use a standardized land cover classification system at continental scale.
1.4 Context of technical study on AFRICOVER land cover classification
The AFRICOVER initiative on the definition of a Land Cover Classification was initiated during the expert consultation held in Addis Ababa, in July 1994.
The Working Group on Classification and Legend which was set up has the task to define a standardized classification which could be used for mapping land cover in all African countries. This classification should meet the following requirements:
Since this expert consultation, a user definition study has been held in 27 African countries to evaluate the information requirements and the priority classes to be mapped within the project. The latter survey clearly indicated the importance of cultivated cover types. This survey also included an extensive review of existing maps and databases on land cover and land use existing in Africa. This data is now stored in the geo-metadatabase.
Several papers and preliminary reports (Nègre, Barisano) were prepared in which the existing classification/legends were analyzed, drafts of nomenclatures were submitted and proposals made for the standardized classification.
Many discussion were held between members of the working group to discuss and review the main land cover features which can be seen on high resolution satellite imagery.
Finally, the cooperation between the working group and the FAO/UNEP/ITE/ITC /WCMC initiative should be mentioned. At a meeting in Rome early last year, the approaches of AFRICOVER and FAO/UNEP/ITE/ITC/WCMC, which were developed in parallel, merged into one common approach.
2.1 Land cover and land use
The distinction between land cover and land use is fundamental. In previous classifications and legends the two were often confused. They are now defined as follow:
Land cover is the observed physical cover, as seen from the ground or through remote sensing, including the vegetation (natural or planted) and human constructions (buildings, roads, etc.) which cover the earth's surface. Water, ice, bare rock or sand surfaces count as land cover.
Land use is based upon function, the purpose for which the land is being used. Thus, a land use can be defined as a series of activities undertaken to produce one or more goods or services. A given land use may take place on one, or more than one, piece of land and several land uses may occur on the same piece of land. Definition of land use in this way provides a basis for precise and quantitative economic and environmental impact analysis and permits precise distinctions between land uses, if required.
2.2 Classification and legend
Classification is an abstract representation of the situation in the field using well-defined diagnostic criteria, the so-called classifiers. SOKAL (1974) defined it as the ordering or arrangement of objects into groups or sets on the basis of their relationships. A classification system describes the names of the classes and the criteria used to distinguish them.
From the above, it follows that a classification should be:
2.2.1 Classification structure
22.214.171.124 Hierarchical system
Classification systems come in two basic formats, i.e., hierarchical or non-hierarchical. A hierarchical classification offers more flexibility with the ability to accommodate different levels of information, starting with broad-level classes which are structured to allow further sub-division into more detailed sub-classes.
126.96.36.199 A-priori and a-posteriori systems
Classification can be done in two ways, i.e. a-priori and a-posteriori. An a-priori classification is based on classes defined before actual data collection takes place. This means that all possible combinations of classifiers must be dealt with in the classification system. Basically, in the field each sample plot is identified and labeled according to the classification system adopted using a kind of dichotomous key approach. This method is used extensively in plant taxonomy (flora) and soil science. The main advantage of a-priori classification is that classes are standardized independent of the area to be described and the means used.
A-posteriori classification means that classes are defined after clustering similarity or dissimilarity of samples. The Braun-Blanquet method is an example of this approach. The advantage of this type of classification is its flexibility compared to the implicit rigidity of the a-priori classification. On the other hand, because a-posteriori classification depends on the area described it is unable to define standardized classes.
A legend is the application of a classification in a particular area using a defined mapping scale and a particular data set. Therefore, a legend may contain only a proportion, or sub-set, of all possible classes of the classification. Thus, a legend has the following characteristics:
3.1 General criteria for a standard land cover classification
A standard classification should meet the following general criteria:
Many classification systems are being used throughout the world. However, there is no single internationally accepted land cover classification system. With reference to the factors listed above, the following points can be made:
3.3 The AFRICOVER land cover classification scheme
3.3.1 Conceptual basis
The general criteria as explained in paragraph 3.1 are the conceptual basis for the proposed classification system. The main conditions to fulfill are:
To fulfill these requirements two problems arise:
In addition, the free combination of the whole set of classifiers involved would lead to an enormous number of combinations (which are, in most cases, not present in the field).
The fulfillment of all these requirements, without incurring in the problems mentioned above, is the objective of the proposed method.
3.3.2 Design criteria
The classification is designed according to two main phases:
A dichotomous phase where a simple sub-division is made in order to define eight major land cover types (see paragraph 188.8.131.52), from which point:
a so-called modular-hierarchical phase starts. In this phase the creation of a land cover class is based on the combination of a set of pre-defined classifiers. These classifiers are tailored to each of the eight major land cover types (see paragraph 184.108.40.206).
The tailoring 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 total number of impractical combinations of classifiers.
This results in a class defined by a Boolean formula showing each classifier used, a unique number for the geographical database, and a name which can be both the provided standard name or user-defined.
220.127.116.11 Dichotomous phase
As stated above a dichotomous key is used at the main level of classification to define the major land cover classes.
At the main level a distinction is made between:
- A. Vegetated areas: areas which have a vegetative cover (which may consist of woody, herbaceous, trees, shrubs, forbs, graminoids, mosses/lichens) of at least 4 percent during at least two months of the year. This class is determined by the presence of vegetation and the time factor of vegetation present.
- B. Non-Vegetated areas: areas which have a total vegetative cover of less than 4 percent during at least 10 months of the year. This class is determined by the time factor of absence of vegetation.
At the second level a distinction is made in:
- A1. Vegetated Terrestrial: vegetation is influenced by the edaphic substratum which is terrestrial. This class is determined by the vegetation, cover, time factor of vegetation present and edaphic condition.
- A2. Aquatic or Regularly Flooded Vegetated Land: the environment is significantly influenced by the presence of water over extensive periods of time, i.e., water is present for more than three months a year and when water is present less than three months a year, it is present 75 percent of the flooding time. This class is determined by vegetation, cover, time factor of vegetation present and edaphic condition.
- B1. Terrestrial Non-Vegetated: the cover is influenced by the edaphic condition which is terrestrial. This class is determined by the absence of vegetation, cover, time factor of absence of vegetation and edaphic condition.
- B2. Aquatic or Regularly Flooded Non-Vegetated Land: the environment is significantly influenced by the presence of water over extensive periods of time, i.e., water is present for more than three months a year. This class is determined by absence of vegetation, cover, time factor of absence of vegetation and edaphic condition.
These distinctions result at the third level in eight major Land Cover Categories:
18.104.22.168 Modular-hierarchical phase
- A11. Cultivated Terrestrial: areas where the natural vegetation has been removed or modified and replaced by different types of vegetative cover resulting from anthropic activities. This vegetation is artificial and requires human activities to be maintained over the long-term. In between the human activities, the surface can be temporarily without vegetative cover. Its seasonal phenological appearance can be regularly modified by humans (e.g., irrigation). All vegetation that is planted or cultivated with the intent to harvest is included in this class (e.g. wheat fields, orchards, rubber and teak plantations). This class is determined by vegetation, cover, time factor of vegetation present, edaphic condition and artificiality of vegetative cover.
- A12. Natural and Semi-Natural Vegetation: natural vegetated areas are defined as areas where the vegetative cover is in balance with abiotic and biotic forces of its biotope. Semi-natural vegetation is defined as vegetation not planted by humans but influenced by human actions. These may result from grazing, possibly overgrazing, the natural phytocenoses, or else from practices such as selective logging in a natural forest whereby the floristic composition has been changed, also previously cultivated areas which have been abandoned and where vegetation is regenerating are included. The human disturbance may be deliberate or inadvertent. Semi-natural vegetation thus includes vegetation due to human influences, but which has recovered to such an extent that species composition and environmental and ecological processes are indistinguishable from, or in a process of, achieving, its undisturbed state. The vegetative cover is not artificial in contrast classes A11 and A24 and it does not require human activities to be maintained over the long term. This class is determined by vegetation, cover, time factor of vegetation present, edaphic condition and natural cover.
- A23. Cultivated Aquatic: areas where an aquatic crop is purposely planted, cultivated and harvested which is standing in water over extensive periods during its cultivation period (e.g. paddy rice, tidal rice and deep-water rice). In general, it is the emerging part of the plant which is fully or partly harvested. Other plants (e.g., for purification of water) are free-floating. They are not harvested but they are maintained. This class excludes irrigated cultivated areas. This class is determined by vegetation, cover, time factor of vegetation present, edaphic condition and artificiality of vegetative cover.
- A24. Natural and Semi-Natural Aquatic Vegetation: areas where the vegetative cover is significantly influenced by water and dependent on flooding (e.g., mangroves, marches, swamps and aquatic bed). Occasionally flooded vegetation within a terrestrial environment is not included in this class. Natural Vegetated Aquatic habitats are defined as biotopes where the vegetative cover is in balance with the influence of biotic and abiotic forces. Semi-Natural Aquatic vegetation is defined as a vegetation which is not planted by humans, but which is influenced directly by human activities which are undertaken for other unrelated purposes. Human activities (e.g., urbanization, mining, agriculture) may influence abiotic factors (e.g. water quality), which influence the species composition of the vegetation. Further, vegetation is included in this class which developed due to human activities, but which recovered to such an extent that it is indistinguishable from the former state, or, which build up a new biotope which is in balance with the present environmental conditions. A distinction between Natural and Semi-Natural Aquatic Vegetation is not always possible because human activities far away from the habitat may create chain reactions which ultimately disturb the aquatic vegetative cover. Human activities may also take place voluntarily to compensate reactions as described above with the aim of keeping a "natural" state. This class is determined by vegetation, cover, time factor of vegetation present, edaphic condition and natural cover.
- B15. Built-up and Associated Areas: areas which have an artificial cover which is the result of human activities such as construction (cities, towns, transportation), extraction sites (open mines and quarries) and waste disposal sites. This class is determined by absence of vegetation, cover, time factor of absence of vegetation, edaphic condition and artificiality of cover.
- B16. Bare Areas: areas which do not have an artificial cover resulting from human activities. These areas include areas with less than 4 percent vegetative cover. Included in this class are bare rock areas and deserts. This class is determined by cover, time factor of absence of vegetation and edaphic condition.
- B27. Artificial Water Bodies: areas which are covered by water due to the construction of artifacts such as reservoirs, canals and artificial lakes. Without these the area would not be covered by water. This class is determined by absence of vegetation, cover, time factor of absence of vegetation and edaphic condition.
- B28. Inland Water: in the case of rivers, the lack of vegetation cover is often due to high flow rates and/or steep shores. In the case of lakes, their geological origin affects the life conditions for aquatic vegetation. The following circumstances might cause water surfaces to be without vegetation cover: depth, rocky basins, rocky and/or steep shorelines, infertile washed-in material, hard and coarse substrates. This class is determined by cover, time factor of absence of vegetation and edaphic condition.
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 and this difference is due to the tailoring of the classifiers to their respective type. This type of classifier can be combined with attributes. Two types of attributes are considered:
- environmental and/or other types of attributes; and
- specific technical attributes, e.g., floristic, crop type and soil type.
The user is obliged to start with the pure land cover classifiers. However, the user can stop at any time depending on the level of detail required and he/she can derive a land cover class. Further definition of this class can be achieved by adding a combination of any of the other types of attributes.
Due to the fact that the classification is designed for mapping purposes (the system gives high priority to mapability), the user needs to follow specific rules:
- a higher level must be fulfilled before going to a lower level (as the mapability is high at high levels and decreases at lower levels);
- within each level there are essential pure land cover classifiers (in the figures in bright green), plus a further subdivision (in the figures of the modular-hierarchical phase in light blue) which defines in greater detail the class. The latter type of classifier is optional and does not need to be fulfilled;
- all essential classifiers within one level should be determined before going to a lower level;
- at any time within a level the user can stop and a mutually exclusive class is defined;
- all land cover classes defined in such way are hierarchically arranged;
- at any time the user can further define the land cover class using environmental and/or specific technical attributes;
- each land cover class is defined by a Boolean formula (combination of classifiers used), a unique code (numerical) and a name (nomenclature);
- the land cover class can be combined with environmental and/or specific technical attributes at any level. The attributes will add a second, separate, code to the land cover class.
3.3.4 Advantages of the adopted method
From a conceptual point of view, the advantages of the proposed classification are:
From a practical point of view:
Parallel to the development of the classification scheme a prototype classification software has been developed. This software is intended, at this stage, to show the advantages of the conceptual basis adopted, as well as to show it as a useful tool combined with on-screen image interpretation. The purpose of this software is also to demonstrate that the adopted system is flexible and allows the user, at almost any level of the system, to ask for the class name, its code and string of classifiers. It also enables the user to add attributes to the land cover code.
The software begins at the dichotomous phase where one derives the main land cover type. At this phase no classifiers are used. Dependent on the land cover type selected, the user enters the modular-hierarchical phase where the classifiers are defined.
In this phase the selection of one classifier at a high level may have consequences for the options available at a lower level. The system is built up in such a way that automatically choices which are no longer valid in connection with a chosen classifier at a high level become inactive. As the system will be used in several countries by different teams, data consistency needs to be guaranteed. The user can ask for the land cover class at any level and store its Boolean formula, its code and class name in a file named "legend". Any user having derived a certain class will store the same Boolean formula and the same code. The class name can be the given class name or may be user-defined.
For every mapping unit the user goes again through the same sequence of screens to derive the land cover class which is subsequently stored in the "legend". To facilitate the interpretation process, selection of the land cover class for a repeatedly occurring mapping unit can be done directly from the "legend" file.
Together with the Boolean formula and class code, the interpretation level is also stored in the "legend". This has the advantage that the field surveyor can directly comprehend where more field information is needed. This will assist the selection of field sampling. As already explained, the field survey concentrates on defining the class classifier per classifier.
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