External error modelling with combined model in terrestrial laser scanning

Terrestrial laser scanning (TLS) is a promising tool in geodetic applications. Data quality of outdoor TLS measurements is considerably limited by external errors caused by atmospheric variations and object related factors. In this study, external models related to atmospheric refraction, beam wander and incidence angle were introduced and integrated into a seven-parameter similarity transformation (7PST) model to compose a combined model. The combined model was rigorously adjusted by an iterative Gauss-Helmert (GH) model primarily to estimate external calibration parameters and transformation parameters simultaneously. In the application on a dam surface, a t-test showed that a significant external calibration parameter was the range scale, which mainly corresponded with the incidence angle model. The precision of estimated representative points was improved by approximately 50% with the proposed method. The presented models show a great potential for detecting external errors and registering multiple scans in outdoor applications.