Why measuring upper diameters?
If we are interested in describing the shape of a stem in a more detailed manner, it is necessary to measure diameters at various heights of the tree. Then, a so called taper curve can be derived that describes / models that shape. With known upper diameters and with a taper curve we may then come to more exact volume calculations and also to optimized assortments. Upper diameters are most easily observed on felled trees. However, there are also situations in which upper diameters need to be measured at standing trees. Up to a certain height direct measurements can be done (for example with the Finn caliper), for higher points at the stem, optical measurement devices are being used that base on simple geometric measurement principles.
Stem volume can be modeled with dbh as predictor variable alone. However, volume models are more precise if an upper diameter is also entered into the model because then we add some information about the stem shape. Measuring upper diameter is time consuming and costly; therefore, it is an optimization decision whether the additional precision gained by measuring upper diameters is actually justifying the additional cost of measuring an upper diameter.
Upper diameters are either measured at a fixed height (such as 7 m with the Finn caliper) or at relative heights, that is at \(x %\) of total tree height. While the diameter measurement at a relative height describes the stem shape in a more informative manner it does also require to first measure the tree height in order to be able to determine the tree-specific value for the relative height.
