- High precision video tracking of microscopic particles is limited by systematic and random errors. Systematic errors are partly due to the discretization process both in position and in intensity. We study the behavior of such errors in a simple tracking algorithm designed for the case of symmetric particles. This symmetry algorithm uses interpolation to estimate the value of the intensity at arbitrary points in the image plane. We show that the discretization error is composed of two parts:(1) the error due to the discretization of the intensity, b D and (2) that due to interpolation, b I. While b D behaves asymptotically like N-1 where N is the number of intensity gray levels, b I is small when using cubic spline interpolation.