GNSS–Camera Systems for Heritage Documentation. Accuracy assessment of measurements of inaccessible points and preliminary tests in photogrammetric applications

Abstract. The measurement of Ground Control Points is a crucial but time-consuming step in photogrammetric surveys, especially in the Built Heritage domain. This study investigates the use of a new generation Global Navigation Satellite System receiver equipped with an integrated camera to measure inaccessible points (to be used as GCPs, e.g. on building façades), combining satellite positioning and photogrammetry in a single device. The approach was tested on a historical building using both the tested GNSS-camera system and traditional Total Station topographic measurements as reference. Results show that the proposed method can achieve centimetric accuracy, with horizontal and vertical errors of about 3-5 cm (in line with the adopted NRTK service). While a small systematic shift was observed, the overall geometric consistency of the measured points remained within acceptable limits for many heritage documentation applications. Additionally, the georeferenced images acquired by the system were successfully used in a Structure from Motion (SfM) direct georeferencing workflow, producing 3D models and orthophotos with good metric reliability when compared to other data, such as TLS scans. The study demonstrates that GNSS-camera systems can significantly reduce fieldwork time while providing reliable spatial data, representing a promising solution to be integrated into heritage documentation pipelines in an efficient and flexible way.