A method for optically scanning a document having preprinted marking positions is disclosed. According to the invention, the document has a specified format including rows of preprinted marking positions to be scanned. Document size in at least one direction is automatically measured at each row of preprinted marking materials before that row is scanned. The scanning of each row of preprinted marking positions is automatically adjusted to compensate for random variations in shrinkage and expansion of document size. Document skew relative to the scanning camera is automatically measured at each row of preprinted marking positions to be scanned before scanning each row, and an indication that scanning should not continue is provided if any skew measurement exceeds a predetermined skew limit.