Estimating and Plotting TLS Midrange Precisions in Field Conditions: Application to Dam Monitoring

Abstract

Terrestrial laser scanners (TLS) are already widespread among surveyors, with a huge field of applications. The range of scanning distances goes from tenths to a few thousand meters, depending on scanning technology, but precisions of scan measures are not well stated by manufacturers, in particular for medium and large distances. It is necessary to ascertain precisions to optimize the design of “in field” scanning jobs. This is the case of TLS dam monitoring that requires detailed information about precisions for a range of scanning distances and incidence angles, to deal with topographical constraints and ensure the measurement of small dam movements. This article presents a new approach to obtain a complete map-type plot of the precisions of TLS scanning with time-of-flight method at midrange distances. Tests were developed in field-like conditions, similar to dam monitoring and other civil engineering works. Taking advantage of graphic semiological techniques, a map in “distance–angle of incidence” coordinates was designed and evaluated for field-like conditions. A map-type plot was designed combining isolines and sized and colored points, proportional to precision values. Precisions under different field conditions were compared with specifications. For this purpose, processed point clouds were evaluated under two approaches: classical “plane-of-best-fit” and proposed “simulated deformation” that showed improved performance. These results lead to a discussion and recommendations about optimal TLS operation in civil engineering works.