Oblique view individual tree crown delineation

Abstract

Individual tree crown (ITC) segmentation supports numerous applications in forest management and ecology. In the latter context, special attention is dedicated to the study of angular reflection effects, caused by the interaction of incident sunlight with a canopy. High precision airborne analysis of these effects requires multi-view sensor systems and multi-view ITC segmentation. In particular oblique view image segmentation is difficult and has been addressed by numerous template based methods. This contribution identifies persistent shortcomings in the state of the art and tackles the problem by a multi-step workflow utilizing the digital surface model (DSM), derived from multi-view stereo data. A slightly revised version of the previously published levelset-watershed segmentation of the DSM is presented as the first step. In the second step, the contour of the visible part of a candidate tree in images with known orientation is obtained by means of ray casting and concave hull calculation. The method was tested on a deciduous, mixed and coniferous plot, whose aerial images were acquired using the 3K camera system in 2018 at Kranzberg Forest, Bavaria, Germany. Accuracies were assessed at hand of human operator generated groundtruth tree tops for the DSM as well as images with zenit angles of approximately 0,45 and 52 degrees. The resulting F1-scores, averaged over the plots, are 0.909/0.902/0.886/0.876 for the DSM/near-nadir-/oblique-/maximum-oblique-images, respectively.

Keywords

• Individual tree crown segmentation
• Individual tree delineation
• Oblique viewing
• Levelset segmentation
• Visibility determination
• Ray-casting