Evaluating ultrasonic RTLS pose constraints for smartphone-based indoor photogrammetric reconstruction

Smartphone-based photogrammetry offers a low-cost approach to indoor reconstruction but typically requires extensive ground control point (GCP) networks. This study provides a controlled proof-of-concept that commercial ultrasonic real-time location system (RTLS) constraints can reduce ground control requirements while meeting the U.S. Institute of Building Documentation Level of Accuracy 20 (LOA20) specification of 2–5 cm for architectural documentation. A ZeroKey Quantum RTLS supplied camera poses for two experiments evaluated against terrestrial laser scanner references. In the sparse experiment, the recommended configuration achieved 2.6 mm inter-target distance root mean square error (RMSE) under position-only constraints—comparable to the 2.9 mm GCP baseline—and 11.5 mm under combined position and orientation constraints, both within LOA20. Other position-only configurations produced a linear scale error; combined constraints additionally introduced higher-order error components consistent with an uncalibrated boresight matrix. Where scale error was present, it was correctable through minimal ground control: a scale bar for reconstructions in the RTLS local frame, or a minimum of three GCPs for external alignment. The dense reconstruction illustrated this: GCP-based scale correction during registration reduced the check point RMSE from 32.1 mm to 20.5 mm, within the LOA20 specification, against a 10.1 mm GCP baseline.