Abstract: A digitizer for digitizing a trace on a two-dimensional surface, in which the surface is defined by a longitudinal Y direction and a transverse X direction. The digitizer includes a frame mounted on at least three wheels such that the frame is movable only along the Y direction, a cursor mounted on the frame such that the cursor moves in relation to the frame in the X direction; a rotary optical encoder for determining the direction and amount of movement of at least one of the wheels; a pair of linear optical encoders for determining the direction and amount of movement of the cursor; and electronic circuits, including a microcomputer unit, for receiving information from the optical encoders which is indicative of the X and Y position of the cursor and for transmitting data, serially, to a host computer. The information may then be displayed and processed on a host computer in accordance with known devices.