1. A multi-printhead printing system, comprising: first and second printheads adapted to print on a receiver medium; a transport system adapted to move the receiver medium relative to the first and second printheads in an in-track direction; a digital image capture device; a data processing system; a memory system communicatively connected to the data processing system and storing instructions configured to cause the data processing system to implement a method for aligning image data printed on the receiver medium using the first and second printheads: printing a test pattern including a plurality of features on the receiver medium using first and second printheads as the receiver medium is moved relative to the printheads in the in-track direction, wherein some of the features in the test pattern are printed with the first printhead and some of the features in the test pattern are printed with the second printhead, wherein the features in the test pattern are separated by predefined test pattern feature separations; capturing a digital image of the printed test pattern on the receiver medium using the digital image capture device; analyzing the captured digital image to determine a first camera pixel separation between two features printed with the first printhead; determining a camera scale factor responsive to the determined first camera pixel separation and the corresponding test pattern feature separation; analyzing the captured digital image to determine a second camera pixel separation between a feature printed with first printhead and a feature printed with the second printhead; using the determined camera scale factor to scale the second camera pixel separation; comparing the scaled second camera pixel separation to the corresponding predefined test pattern feature separation to determine an alignment error; and using the determined alignment error to adjust the alignment of image data printed with at least one of the first and second printheads.