Evaluation of laser scanner data in forest areas

Abstract

The laser data acquired by the airborne laser scanner, called TopoSys-1, in the forest area has been evaluated in this study. The test site was the boreal forest of 0.5 km by 2.0 km in size in southern Finland. The laser scanning system provided 127 measurements per scan line across the flight direction at the pulse repetition rate of 83 kHz. Both the First pulse and Last pulse modes were employed. The position of the carrying platform was determined using the integrated DGPS/INS system. The original laser data set used in the analyses was geocoded but unfiltered. The files only consisted of X, Y, Z coordinates of each sample. First, the whole laser data were classified in four classes, namely: ground, vegetation, buildings, and erroneous points using the TerraScan software on top Microstation SE. After that the ground points were further processed when the still present vegetation removed. Finally, the Digital Elevation Models (DEMs) were generated. The terrain variation was from 91.3 m to 128.4 m. The DEM made from the laser data was compared with the reference DEM available at that moment. The systematic error was 1.03 meters. The standard deviation between the two DEMs was 2.77 meters. Using ground control points, the systematic error can be overcome. However, a more accurate reference DEM is needed for the future comparison. In addition, a map of the true tree heights in the area was generated at the cell size of 1 m by 1 m. The clear-cut areas can be more easily seen than using the aerial photographs.