Comparison of Aerial Photographs and Laser Scanning Data as Methods for Obtaining 3D Forest Stand Parameters.

The aim of this study is an evaluation of the use of (semi-) automatic software solutions for the derivation of crown surface models and further forest information like vegetation height and vertical stand structure. It gives an overview of the up-to-date reachable accuracies of crown surface models and vegetation heights from traditional aerial images in comparison with the results from laser scanner data. Concerning the surface models, two algorithms (feature-based matching approach - ISAE and an area-based matching approach - RSG) were tested. The results were evaluated with three different methods. Despite careful investigations, the evaluations didn’t lead to a definite preference of one of the algorithms. On visual interpretation, the ISAE seemed to be too smooth, while the RSG method produced higher errors within the single tree height measurements. For the area based comparison, both led to quite the same accuracies. Concerning the calculation of the vegetation height, the second main part of the study was the evaluation of the available terrain model. As the laser scanner terrain model could be used as ground truth, the difference is depicted through statistical figures. It turned out, that the official terrain model with errors of more than 10 m is not appropriate for measuring vegetation height. It can be summarised, that both photogrammetric algorithms lead to erroneous results in forest stands with a strong, but small-patched vertical structure, but to good surface models in quite homogeneous stands. In combination with an accurate terrain model – for example derived by laser scanning - the mean stand height for these parts can be calculated with an appropriate accuracy.