Towards the empirical determination of correlations in terrestrial laser scanner range observations and the comparison of the correlation structure of different scanners

The determination of a fully populated variance-covariance matrix (VCM) of a point cloud acquired with a terrestrial laser scanner (TLS) is not straightforward as physical correlations between TLS observations are not sufficiently known. Ignoring correlations leads to biased estimates, wrong modeling of surfaces, or the misinterpretation of geometric changes. For this reason, this study aims at progressing the empirical determination of correlations in TLS range observations by investigating short-scale correlations (mm to cm) of TLS rangefinders. Therefore, two methods are proposed to determine spatial correlations between TLS range observations. Both methods are evaluated for simulated laser scans, and then applied to empirical data of four different terrestrial laser scanners. The results demonstrate that short-scale correlations exist, and it is possible to quantify the relevant parameters to fill the VCM. It also shows that the correlation differs between the scanners and with the distance. Furthermore, we analyze the change of the correlation length with changing scanning configurations such as distance, incidence angle, and reflectivity with the result that this must be considered while applying the aforementioned methods to point clouds of large objects. All in all, this study trustfully quantifies physical short-scale correlations yielding from the rangefinder and suggests a guideline to apply this to point clouds of large objects.