Improvised Long Test Lengths via Stitching Scale Bar Method: Performance Evaluation of Terrestrial Laser Scanners per ASTM E3125-17.

Periodic performance evaluation is a critical issue for ensuring the reliability of data from terrestrial laser scanners (TLSs). With the recent introduction of the ASTM E3125-17 standard, there now exist standardized test procedures for this purpose. Point-to-point length measurement is one test method described in that documentary standard. This test is typically performed using a long scale bar (typically 2 m or longer) with spherical targets mounted on both ends. Long scale bars can become unwieldy and vary in length due to gravity loading, fixture forces, and environmental changes. In this paper, we propose a stitching scale bar (SSB) method in which a short scale bar (approximately 1 m or smaller) can provide a spatial length reference several times its length. The clear advantages of a short scale bar are that it can be calibrated in a laboratory and has potential long-term stability. An essential requirement when stitching a short scale bar is that the systematic errors in TLSs do not change significantly over short distances. We describe this requirement in this paper from both theoretical and experimental perspectives. Based on this SSB method, we evaluate the performance of a TLS according to the ASTM E3125-17 standard by stitching a 1.15 m scale bar to form a 2.3 m reference length. For comparison, a single 2.3 m scale bar is also employed for direct measurements without stitching. Experimental results show a maximum deviation of 0.072 mm in length errors between the two approaches, which is an order of magnitude smaller than typical accuracy specifications for TLSs.