Left: number of spiral arm detections per tracer for the different observer positions P1, P3, P3, and P4, respectively. Here, only the tangent points C–H are considered leading to a maximum of six possible detections. Right: deviation of tangent point angles with respect to the angle of CO of all spiral arms. Vertical lines represent the minimal and maximal deviation for each of the four observer positions P1-P4. The colors correspond to the tracers considered in this paper and the dots are the average value.