Sources of angle-dependent errors in terrestrial laser scanner-based crop stand measurement

In past years, research was conducted to investigate the potential of laser rangefinders and scanners for vehicle-based measuring of crop stand physical parameters. High coefficients of determination (R-2 > 0.90) were found between the crop biomass and the laser rangefinder and scanner readings in the form of the mean height of reflection points of the laser beam. It was observed that the height of the reflection point increased depending on the detecting angle of the laser beam in a considerable manner. This phenomenon indicates that farther crop plants generate higher reflection points, resulting in an overestimation of crop height respectively of crop biomass. In the face of these unsolved problems, the object of the paper was to investigate the measuring properties of a chosen laser scanner depending on the inclination angle and the scanning angle and to analyze the error sources for vehicle,based laser scanner measurements in crop stands. Therefore, the scanner was investigated in two test series (May 30, 2008 in winter wheat, and June 10, 2009 in winter rye) along a transect (tramline) with a length of approximately 700 m. The performed comparisons demonstrated that one part of the observed overestimation of the reflection point height can be explained theoretically by the geometry of the laser beam. The main part of overestimation was explained by the recognizability of the gap fraction in crop stands. Because no sufficient theoretical compensation algorithm for overestimation resulting from gaps exists, it must be concluded that for each specific laser rangefinder type and crop species, the overestimation depending on detection angles has to be investigated individually in field tests. (c) 2013 Elsevier B.V. All rights reserved.