A method for measuring flatness of a factory floor upon which machinery is to be mounted using a rotating laser beam that emits a rotating laser beam in an essentially horizontally plane. A photosensitive position sensor delivers an electrical pulse, which is identified by its timing and phase angle, during illumination by the laser beam. The phase angle and timing of these pulses constitute a measure of the position of the sensor in the coordinate system. Measurements are taken with the sensor positioned at each location at which the machinery is to be fixed to a factory floor. The method determines any differences in height between the measurement points and the amount, if any, that the floor needs to be adjusted at each mounting point to insure that a machine that is affixed to the floor these points will be level.