Test of the influence of including the spiral arm contribution to the combined distance PDF. We generated fake sources on a uniform grid with pure circular rotation. For sources to the left and below of the red dashed lines, where no spiral arm information is used, the program returns the input locations with reasonable accuracy (except for slight deviations in regions near ﹩{\ell }\approx 0﹩ and near the “tangent points” where small velocity changes result in large changes in distance). However, to the right and above the red dashed lines, spiral arm information was used, resulting in distance estimates biased to spiral arms.