An electromechanical apparatus is described that can be used to determine lengths from 0 to 50 ft with a precision of 4 parts per million. A flexible wire and drum are used to convert linear to rotary motion. The drum rotation is instrumented with an optical incremental shaft resolver giving length resolution of 0.002 in. at linear velocities from 0 to 10 ft sec^-1. Being automatic, the method enables inbuilt correction for systematic errors. The complete system can be produced as an easily portable unit.