Abstract: A photogrammetric plotter having a photocarriage which is supported on a trolley carried by a first set of way bars which in turn are supported from the trolley element on a second set of way bars mounted on a base for viewing by binocular optical system of a pair of stereo photos on a pair of photo carriers on the photocarriage. A constraint free drive apparatus is provided for driving the photocarriage along each of the set of way bars each of constraint free drive apparatus including a drive rod supported, in a preferred embodiment on the trolley supporting the photocarriage, and at only one end of the drive rod is displaceable in a direction transverse of the longitudinal axis of the drive rod but not displaceable axially of the drive rod. A drive assembly constituted by V drive wheel and a servo motor driving the drive wheel engage the drive rod, and a clutch means constituted by a spring-solenoid controlled cam member permit the rapid engagement and disengagement of the drive rod with the drive wheel.