Abbé Error - (Named after Dr. Ernst Abbé, 1800's)

An error in measuring a feature's spatial dimension (such as diameter or length) or linear displacement which results from a changing angular orientation between object and measurement reference component of the instrument . This effect is observed when the measured feature or reference point of motion does not lie along the same line of the measurement reference. The spatial separation between measured point and reference line is known as the Abbé offset.  See also Abbé principle.

The magnitude of the error equals the Abbé offset times the sine of the orientation angle change.