Calibration and Direct Georeferencing Analysis of a Multi-Sensor System for Cultural Heritage Recording

Abstract

A large amount of cultural heritage monuments and sites exist distributed worldwide that require easy to use, cheap and fast sensor orientation tools for recording, georeferencing, surveying and mapping. Ground control points are usually required on site for close range photogrammetry to achieve accurate surveys, limiting both the involvement of non-experts and the chance to know the right place of the monuments unless a ground reference system is considered. This paper presents the system calibration of an image-based multi-sensor system that integrates two consumer-grade cameras, one global navigation satellite system (GNSS) and one low-cost inertial system, i.e., a micro-electro-mechanical system (MEMS) based inertial measurement unit (IMU). The multi-sensor system is calibrated indoor regarding both the camera orientation parameters and the boresight (rotations and offsets) parameters of the two digital cameras. The boresight parameters will be used to correct the direct approach estimates. The performance of the system calibration is tested outdoor on an upside down pyramidal sculpture to deliver both accurate 3D points and high resolution 3D models. Both scenarios are considered free of magnetic anomalies. The results achieved with the GNSS/MEMS-IMU direct approach are compared with the indirect approach based on bundle block adjustment. Further extrapolations to object space through digital surface models are also determined. The testing of the system shows that GNSS/MEMS-IMU data are good enough to provide approximate exterior orientation parameters of the cameras but not to yield accurate 3D models (<1 - 2 cm) for cultural heritage applications.