Granite surface plates are used as a reference plane for making dimensional measurements, such as flatness and height. They are widely used by industrial quality-control personnel. The surface plate must be calibrated to determine how flat it is.
Created in 1955 by J.C. Moody, this method of calibration is widely accepted because it costs less, can be performed by semi-skilled technicians and provides sufficient accuracy for most applications.
An autocollimator with its mirrors and a straight edge is the most common equipment used to calibrate a surface plate. Alternatives include other angle-measuring equipment, such as a laser interferometer with electronic levels.
Prior to calibration, the surface plate must be thoroughly cleaned. A series of measurement guidelines are drawn on the plate. Lines are drawn corner to corner along the length and width. Lines are also drawn diagonally corner to corner, from mid-length to mid-length and from mid-width to mid-width. These lines are used as guides for placing the mirrors during measurements.