SOME ALGORITHMS FOR VIRTUAL DEFORESTATION (VDF) OF LIDAR TOPOGRAPHIC SURVEY DATA

Lidar topographic surveys of forested terrain generate XYZ positions for laser returns from numerous points, some on the ground and some from vegetation. Extracting a ground surface model from such data requires ‘virtual deforestation’ (VDF), preferably by au tomatic means. A simple error budget for lidar topography of forested terrain suggests that the dominant source of error—and the greate st room for improvement—lies in VDF procedures. We discuss a despike VDF algorithm that classifies returns as ground or not-ground on the basis of the geometry of the surface in the neighborhood of each return. The despike algorithm is fully automatic, effective, and can recover breaklines. It fails to ident ify some negative blunders, rounds some sharp corners off the landscape, and as implemented is slow. There are clear paths to improve it s speed. If multiple-return data are available, a no-multiple-returns VDF algorithm robustly defines areas where all returns are ground returns. Many groups are using variations on block-minimum VDF algorithms, but these do not work well on slopes and typically require su bstantial human involvement to adjust block size as the fraction of ground returns changes. Fully automatic VDF algorithms are desirable not only to minimize survey costs but also to produce topography for which all nec essary interpretive biases and assumptions are explicit. The development of effective VDF algorithms has been hindered by the tendency of some commercial and academic practitioners to keep their work proprietary. Open dialogue is needed.