Estimating forest area
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General observations
Area estimation is among the very principal results of large area forest inventory. Estimation can be done for different categories such as forest and non-forest or different forest types. Forest area estimation is also a crucial input to all quantifications or estimations of “deforestation”. A clear definition of “forest” is required in any case. For the determination of forest area there are at least two basic approaches:
- delineation (mapping) from remote sensing imagery, or
- sampling (either in the field or from remote sensing imagery).
Mapping can either been done automatically (in the presence of suitable algorithms) or manually. Various sources of error are associated with this exercise but the result is a map that shows the spatial arrangement of the forest patches.
Sampling approach is a different approach that produces an overall estimation – but not a map. It has the advantage that the standard error is easily quantified (if statistical sampling has been used) and gives information about the reliability of the estimation.
Results of area estimations are given either in absolute (\(km^2\) or \(ha\)) or in relative term (%). Also, precision statements can be given either in absolute terms (standard error, confidence interval) or in relative terms (relative standard error, percent confidence interval). If a percent forest cover estimation is accompanied by a precision statement (for example 40% ± 5%) – then it is important to clearly state whether the precision statement is meant in absolute terms (in this case (35% ≤ cover ≤ 45%) or in relative terms (38% ≤ cover ≤ 42%); otherwise, confusions may easily occur.
Area estimation can be based on different plot designs that are discussed in what follows: points, lines and plots.
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