Terrestrial laser scanner and retro-reflective targets: an experiment for

Artificial targets are generally used in the terrestrial laser scanner (TLS) practice for data georeferencing, since they are well recognized and modelled from the point cloud, and their positions can be contemporarily measured by topographical techniques. The accuracy of target identification directly influences the georeferencing quality. In particular, retroreflective materials can cause anomalies in range measurement due to the too high amplitude of returned pulse. If the received pulse intensity exceeds the limits of the sensor dynamic range, the receiver saturates producing a truncated pulse preventing the correct time-of-flight computation. A series of experiments were performed in order to test the performances of a specific instrument (Optech ILRIS 3D) for the acquisition of artificial targets made of retro-reflective material resulting in very high reflectance. Dealing with ranges lower than about 300 m, two cases were clearly observed, that is wrong distance measurement of points over the high reflecting surfaces, and the presence of haloes around these surfaces. Neglecting these phenomena, has serious implications and can lead to a wrong georeferencing. The experiments were executed and data analyzed providing a qualitative and semi-quantitative phenomenon description. Finally, the design of a target that can be easily recognized and correctly modelled was ideated and proposed