a digital measuring instrument includes a housing containing a measuring line , a rotatable drum engaging the line whereby movement of the line causes rotation of the drum , a storage spool mounted in the housing for storing the line , a crank assembly for rotating the storage spool for winding the measuring line about the storage spool , and a rotatable disc that rotates simultaneously with the measuring drum . the instrument includes an optical sensor in communication with the rotatable disk for compiling data related to rotation of the disk , and electronic circuitry in communication with the optical sensor for calculating a length of the line drawn from the housing . the housing has a well for receiving the storage spool and a side cover with a slideable lock for covering the well .