Summary: A misprint occurred on page 34 in our paper [W. Nagel and V. Weiss, Image Anal. Stereol. 27, No. 1, 29–37 (2008; Zbl 1168.60006)]. The equation \[ \frac 32 U^P_1=U_1=\frac{\sqrt\pi}{2}U^V_1, \] should correctly read as \[ \frac 23 U^P_1=U_1=\frac{\sqrt\pi}{2} U^V_1, \]