1. Apparatus for establishing at a first location a plane having a predetermined orientation and position and for locating a reference point on said plane at a second location comprising: means for projecting to said second location an image having an illuminated and unilluminated portion separated by a boundary region which includes said reference point; means at said second location for detecting said image at two detection points to produce first and second signals, each having an amplitude corresponding to the intensity of the image at a corresponding detection point; and means for establishing said reference point from a predetermined comparison of said amplitudes, including means for indicating said reference point by the position of one of said two detection points when said predetermined comparison between the amplitudes of said two signals exists.