Methods to calibrate a three-sphere scale bar for laser scanner performance evaluation per the ASTM E3125-17.

• Per ASTM E3125-17, TLS performance may be evaluated using long scale bars. • We report two methods to calibrate a scale bar with sphere targets mounted on it. • The four-orientation and two-face method is to calibrate horizontal bar length. • The comparison method is proposed for calibrating bar length in any orientation. • The validation experiments for the two methods are given and discussed. A scale bar with sphere targets is one way of realizing the symmetric and asymmetric length tests to evaluate Terrestrial Laser Scanner (TLS) performance per the ASTM E3125-17 standard. The length of the scale bar is required to be known with an expanded (k = 2) uncertainty that must be at least four times smaller than the manufacturer's maximum permissible error (MPE) specification of the instrument being tested. In this paper, we propose two methods to calibrate the scale bar length using a laser tracker. The first method, which we refer to as the four-orientation and two-face (FOTF) method, is proposed for calibrating the scale bar when it is in a horizontal orientation. The other method, which we refer to as the comparison method, is proposed for calibrating the length of the scale bar in any orientation. We describe the methods, present underlying theories, discuss validation experiments, and summarize results. The two calibration methods are beneficial for the realization of the ASTM E3125-17 standard for TLS performance evaluation. [ABSTRACT FROM AUTHOR]