ASABE/ISO Standard 12188-2 provides test procedures for positioning and guidance systems in agricultural vehicles during straight and level travel. While the standard provides excellent descriptions of test procedures, it does not provide detail on methods to carry out the calculations necessary to calculate relative cross-track error (XTE), which is the primary error statistic used to judge accuracy. Given the travel speed and sampling constraints provided by the standard, the difference between a method based on nearest points or one based on path interpolation could hypothetically be as large as 25 cm. In this project, the standard was used to estimate the guidance accuracy of a relatively low-accuracy vehicle at 1.25 and 0.5 m s-1. At 1.25 m s-1, a basic nearest point calculation overestimated mean XTE by 0.8 cm, or 8.2%. The location sampling density was much higher with a 0.5 m s-1 travel speed, and mean XTE was only overestimated by 0.1 cm with the nearest point method. There are clearly situations where the calculation method will affect results, and the use of the more complicated methods explained in this article are suggested when using this standard
