A coordinate position digitizing system incorporates apparatus for detecting the X and Y positions of a voltage detecting device relative to a position determining plate employing apparatus for generating data corresponding to the X and Y positions of said voltage detecting device on said plate at successive sampling times, means for calculating the difference corresponding to the difference in the indicated position at two successive sampling periods, and rejecting signals indicating positions which differ from previous positions by distances which are greater than a predetermined amount, corresponding to the maximum distance that the voltage detecting device can be moved between successive sampling periods. In this way, position-indicating signals which are corrupted by noise pulses or the like are rejected.