Studio di possibili effetti sistematici nelle nuvole di punti SfM da APR: confronti con TLS, distorsioni e metodi di mitigazione

Structure from Motion (SfM) photogrammetry is based on the use of algorithms allowing the
automatically identification of a large amount of homologous points (or pixels) between images
overlapping areas. The coordinates of the homologous points (provided in different reference systems
for each acquisition), allow the images alignment meaning the internal and external camera
calibration. The number and the distribution of homologous points drives the entire procedure of
photogrammetric restitution. In some cases, due to a fatal combination of acquisition strategy, digital
cameras and software, anomalies could occur causing systematic effects in the point clouds
representing the observed surfaces. This paper shows the results obtained from SfM surveys carried
out as part of a project for the study of deformations due to the phenomenon of liquefaction by
comparing multi-temporal models. This is the only case we have observed in years of SfM survey
experience in which there are systematic effects preventing the direct use of the point clouds obtained
using Photoscan (versions 1.1.2 and 1.7.0). Some approaches aimed at mitigating the distortions by
balancing the distribution of homologous points and inhibiting the adjustment of internal camera
calibration are used to improve results. A reference Terrestrial Laser Scanning (TLS) survey allowed
the evaluation unexpected problems. Finally, the images data set processed by means of Metashape
(1.5.0), a new improved version of Photoscan, provide good results free from systematism proving the
greater efficiency of new algorithms for homologous points selection optimization. Since Photoscan
is a very widespread product and also used within the INGV, it is believed that this experience can be
useful to many operators involved in monitoring and studying surface deformations.