Effect of disk flaring on spiral motion measurement. Different disk flaring will change the spiral location determination in the deprojection, which affects the spiral motion measurement. Left panel: Spiral motions derived based on different disk flaring. Right panel: Corresponding uncertainties obtained using Equation D.1. The red dot represents the velocity of the spiral motion and its corresponding uncertainty based on the best-fit disk model (h = 0.22 × r^1.04) from Benisty et al. (2017).