Low cost artificial planar target measurement techniques for terrestrial laser scanning

This paper presents both an internal and external accuracy assessment of four different methods for measuring the centroid of a signalized planar target captured by a terrestrial laser scanner. The planar targets used in this project are composed of a black background and a white circle printed on 8 by 11 inches plain sheet of paper using a consumer level LaserJet printer. The first two methods tested define the centroid of a target to be the mean and median of the cluster of points belonging to the white circle in the point cloud. The latter two methods are more advanced, and they take advantage of the planar nature of the target as well as the intensity difference between the circle and the background to strengthen the centroid derivation through a combination of least-squares plane fitting and circle fitting. The main benefit of the four presented methodologies is that no specialized and/or laser scanner dependent targets need to be utilized. And it will be demonstrated in this paper that using the two advanced methods can yield position measurement precision and accuracy far superior to the simple mean or median computations. In fact, sub-millimetre precision and accuracy is achievable from using low cost paper targets provided that an appropriate target measurement algorithm like the latter two methodologies proposed in this paper is adopted