The goodness of a match between points, given their 3-D position estimates, can be evaluated by checking how close corresponding points can be brought to coincide by a rigid motion computed from the given data. The performance of this well-known technique, however, is affected by the data accuracy. In stereo imagery, the depth values (z coordinates) estimated are subject to a higher range of uncertainty than the other two coordinates. The authors present a novel pruning method that discards the z coordinates and uses only x and y coordinates of points to compute the motion. It is noted that this method is more effective in detecting false pairings than the traditional method, which uses full 3-D coordinates. Since only x and y coordinates are used, the method is virtually equivalent to computing motion from orthographic projections. The authors dervive a least-squares solution to this motion problem and show that the determination of rotation and translation can be decoupled. The locus of rotation is proved to be a great circle on a unit quaternion sphere. Results of testing this method on real and random data are shown.