1. A method of measuring a relative offset between a first array of marking elements and a second array of marking elements in a printer, the method comprising the steps of: printing a target by printing a first group of pixels using a plurality of marking elements from the first array and printing a second group of pixels using a plurality of marking elements from the second array; scanning the target to measure an optical characteristic of the target as a function of position along the target; and identifying a position at which an extreme in the optical characteristic of the target occurs.