Error distributions for the (x, y) coordinates of isophote center positions derived from ellipse fits to 16 simulated galaxy images. For each galaxy, a series of ellipses was fitted to the surface-brightness distribution, incrementing the SMA by two pixels at each step, as described in Section 3.1. The errors returned by the IRAF ellipse task are compared with the "true errors," defined as the difference between the known position of the center of the model galaxy and the center position returned by the ellipse fit. The errors on each isophote center returned by ellipse are broadly consistent with the true errors. The error distributions combine results from all 16 simulated galaxy images. The model images were derived from GALFIT fits to the following data: NGC 4261/NICMOS2, IC 1459/WFPC2-PC, NGC 1399/WFC3-IR, and IC 4296/ACS-HRC.