In one embodiment, a method of determining a drift value is described that includes performing one or more X-axis translations of an excitation beam over a probe array; measuring light responsive to the excitation beam from at least two positional reference elements associated with the probe array for the X-axis translations; calculating a distance value for the X-axis translation using a positional relationship of a known location associated with each of the positional reference elements and positions of the positional reference elements from the measured light; and determining a drift value using a difference between the calculated distance value and an expected distance value.