A depth measurement system for determining an absolute depth of a drill string uses a position acquisition device to determine a length value for a joint or stand being added to the drill string. The position acquisition device receives a signal from a target object associated with the added joint. The processed signal can be an optical signal, a radio signal, an acoustic signal, or other suitable signal. Using techniques such as time lapse, Doppler effect or phase shift, the depth measurement system determines a position parameter such as distance or position based on the received signal. Thereafter, the processor determines the absolute depth of the drill string by summing a length of each joint making up the drill string and correcting for the position of the newly added joint.