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Pitkänen, Timo P. Improving TLS-based stem volume measurements by field data Poster
Timo P. Pitkänen1
(1) Finnish Natural Resources Institute, Finland

Accurate measurements on tree stem dimensions enables their volumetric calculations and, if repeated, detection of changes. Tree stems can be constructed as cylinders, which have been fit to the TLS point cloud. The quality of cylinder fitting, however, depends markedly on the data quality and scanning conditions. In suboptimal conditions, cylinder model is often deficient both in terms of vertical extents as well as the modelled diameters. This will further lead to erroneous stem volume measurements and deficiencies for further applications.

In our study, cylinder-based stem modelling is combined with field measurements to calculate more accurate stem volumes. First, cylinders are constructed automatically from the point cloud data. Next, vertical dimensions of the stem are adjusted based on field-measured height and a marker attached at a specific height on the tree, and the stem is cut into thin slices. Slice diameters are then re-modelled by circle fitting procedure, using tolerance limits based on reference diameters from earlier measurements. Finally, taper curve is constructed as a spline model, assisted by circle fitting results and field-measured diameters.

When this procedure was tested with 74 trees with accurately known stem volumes, volumetric RMS errors decreased from 6.15% (no field data used) to 3.08% (using all the available field measurements). For the same trees, commonly used volume function with field-measured DBH and H resulted in a 9.27% RMS error. The presented method requires tree-wise field measurements, but is capable of improving substantially taper curve and stem volume results.