Generating missing heights
Height value is one of the components of calculating wood volume. When sample trees are recorded, all of the diameter values of sample trees are taken but height is only measured for a smaller proportion of sample trees. The reasons for the incomplete coverage of height measurements include lack of resources as well as experience which suggests that no considerable improvement of accuracy results from comprehensive height measurements. It is also known that the defining wood stock using plotted height curves should be given preference during the processing of forest resource assessment data. Doing so provides a more favourable environment for comparing time series and also eliminates some of the distorting effect of potentially erroneous or salient height values.
Height curves are plotted for each tree species, which requires in-terrain measurement of a sufficient number of properly selected specimens to yield a sufficient number of elements. During the full forest inventory cycle, height measurements were taken of an average of 8 sample trees in sampling plots that included sample trees. That can be regarded as more than sufficient in terms of sampling, particularly in case we take into account that sampling plots with fewer than 8 sample trees are also taken into account for calculating this average.
An integrated function of the FieldMap Inventory Analyst called fitting functions with exponential curves was used to plot height curves and to calculate the missing height values. The program first tries to plot the height curve of the selected tree species for each sampling plot, and if the number of elements required for plotting the curve are not fully available, then it goes on to trying to plot a height curve for all of the specimens measured at a sampling plot, and if that fails, the national height value pertaining to the tree species is used.
Relying on the height curve plotted as described above, height values are calculated for each tree with reference to diameter data. This value is sued as the sample tree height for the purpose of determining wood volume even if height measurements of a certain specimen exist. That is to say function derived data overwrite measured data. Exceptions include specimens with broken crown or broken stem (each of which have measured height values), and the latter will be used for volume calculation purposes. Using height curve data instead would result in overestimating the wood volume of these sample trees.
Plotted as describe above, height curves are also suitable for supporting volume calculations for regions where diameter information is available but height values are missing.