Abstract: A system for measuring the course length of a well bore uses a probe containing accelerometers to generate a first output signal indicative of the time rate of change of the probe's velocity as it travels through the bore. The probe also contains two casing collar locators separated by a measured distance. A microcomputer in the probe integrates the first output signal twice with respect to time to generate a course length value indicating the total distance travelled by the probe. The microcomputer also calculates a computed distance value by integrating the first output signal twice with respect to time over the time interval between the generation of first and second joint-indicative signals as a result of the successive passage of one pipe joint by the two casing collar locators. The computed distance value is compared by the microcomputer with a stored distance value representing the measured distance between the casing collar locators.