Abstract: A device for accurately measuring the position of a traveling block in a drilling rig or the like which is supported by means of a cable wrapped around a drum winch assembly. A pneumatic sensor coupled to the drum provides a predetermined number of output pulses per revolution of the drum. During the paying out of a first layer of cable from the drum, the sensor pulses are coupled to a first pneumatic counter, which is preset so as to provide an output pulse corresponding to a predetermined increment of motion during the paying out of the first layer. Additional counters are provided with different preset limits so that they will provide output pulses corresponding to the predetermined increment of motion during the paying out of second and additional layers of cable from the drum. As the cable is paid out or wound in, the output of the drum motion sensor is coupled to the appropriate pneumatic counter by means of a switching assembly.