This invention generally relates to accurately measuring image geometries in-line within a printing system. One or more two-dimensional array sensors may be located adjacent to a sheet of media printed with a predefined array of indicia marks so that the sensor field of view is contained within the plane of sheet transport. As the sheet is transported by the sensor, the sensors continuously record the locations of the indicia marks within its field of view. The spatial frequency of the indicia marks is arranged so that at least two indicia marks are within the field of view of the sensor at any instant for each of the process and cross-process directions. The two-dimensional array sensor can thus measure the absolute spacing between adjacent local indicia marks within the precision of the sensor construction. The cumulative spacing along each direction may then calculated to determine the overall dimensions of the indicia marks array. A measurement procedure is also provided to accurately measure image geometries, even in the presence of significant sheet velocity variations.