3.1 Basic measurementsMeasurements that are carried out in forest inventories are usually of one of the following types:
- Measurement of distance/lengths.
- Measurement of angles.
- Measurement of areas (in maps or remote sensing imagery).
- Measurement of position (satellite navigation).
- ¿Measurement¿ of a condition class = classification = assigning an object to one of a defined set of condition classes.
3.1.1 Measurement of distance/lengthLength measurements are done for individual tree characteristics (actually, also diameter and height are distances/lengths), for plot establishment (e.g. measuring the radius of a circlular plot) and when navigating to the selected plot location. Length/distance measurements can be made directly or indirectly. Direct length measurement is accomplished by placing a prototype standard of a defined unit beside the object to be measured. Indirect linear measurement is accomplished by employing other techniques (geometry, trigonometry) that allow to calculate the length value. Many measurement techniques and devices are available, with a broad range of cost and of accuracy. Different options are available for the measurement of distance such as estimation by ¿calibrated¿ pace (pacing), measurement by tape (horizontal measurement) or by mechanical-optical devices (Blume-Leiss, Suunto). Higher speed and accuracy (at higher cost) is obtained with instruments that use laser and ultrasonic technology; there, the distance is calculated from the time that a sound or light impulse needs to travel the distance to the target object.
The most commonly used instruments for measuring dbh are caliper and diameter tape Tree height is indirectly measured by a simple trigonometric or geometric approach with mechanical-optical devices like the Spiegel Relascope, the Suunto altimeter or the Blume-Leiss altimeter. (See Annex Brief description of measuring instruments ).
In all cases, length measurements need to follow clear definitions: distances measured in the field are usually meant to be horizontal distances and need to be corrected in sloped terrain; tree height is the vertical distance between highest an lowest point of the tree - and not necessarily the length of the oblique/curved stem.
3.1.2 Measurement of anglesSlope angle and bearing are basic components of various measurements. Slope angle measurement is needed for the measurement of tree height, to determine a factor for slope correction of plot area and to reduce slope distances to horizontal distances.
There are various instruments used for measuring the vertical angles such as Suunto clinometer, Haga, Blume Leiss and Relascope (See Annex Brief description of measuring instruments ).
3.1.3 Measurement of areas (in maps or remote sensing imagery)Direct measurements (as opposed to sample based estimations) of areas are only carried out on maps and remote sensing imagery, usually after having digitized the corresponding polygons. In NFAs, it is rare that area measurements need to be done in the field, although possible by GPS or with electronic distance meters in combination with a digital compass.
If, for example, the plot area needs to be determined which is outside the present condition class, then usually not a direct measurement of area is made, but distance measurements are made which are later used to calculate the area.
3.1.4 Measurement of position (Satellite navigation)Satellite Navigation allows via distance measurements to a set of satellites to determine three dimensional positions. The GPS (Global Positioning System, USA) was the first system in place and some times the term GPS is used in more general terms for satellite navigation systems. The Russian system GLONASS is fully operational since 1995, and the European system GALILEO is planned to be operation in 2008. GPS receivers are now available at low cost (standard receivers of various brands starting from under $200) and support NFA planning and implementation. Three basic functions are important for NFAs: (1) navigation = ¿finding the way to the sample plot in the field¿, and (2) position = ¿determining the position of sample points or other reference points in the field¿. Also (3) the tracking function = ¿monitoring the movement of people¿ can be of interest to document the access path to a plot and to monitor that field crews did really reach the target sample plot location.
3.1.5 ¿Measurement¿ of a condition class = ClassificationFor categorical variables the measurement is about assigning the response to one of a set of defined classes. Here, the complete set of possible ¿values¿ (classes) needs to be defined. Typical examples are the variables ¿forest type¿ or ¿tree species¿.
3.1.6 Subjective estimations of values / guessesNot all measurements follow objectively reproducible observation procedures. Some times, guesses are a quick method for data provision. However, for obvious reasons their use should be restricted to those variables where more objective and reproducible data acquisition techniques are not feasible.
3.2 Measurements of some NFA relevant variablesThe concrete set of variables to be covered by a specific NFA depends on the specific set of objectives. The variables can be grouped into major subject areas. As an example, in a comprehensive comparative study of European National Forest Inventory Systems (Anon. 1996) the following grouping of variables had been used.
- - Geographic and topographic variables.
- - Ownership variables.
- - Variables on wood production.
- - Variables of site and soil.
- - Variables concerning forest structure.
- - Variables concerning regeneration.
- - Variables concerning forest condition.
- - Variables concerning accessibility and harvesting.
- - Variables describing forest ecosystems.
- - Variables concerning non-wood forest products.
- - Ownership variables.