Abstract: An apparatus for determining the position of an object in a vertical column includes a sighting window mounted on a slide movable along a vertical rail. An endless chain extends around a pair of cogged wheels rotatably mounted on rotating shafts at opposite ends of the rail and is connected to the slide so that vertical movement of the sighting window rotates the wheels. One of the wheels is connected to an optical encoder which converts rotation of the wheel to units corresponding to the vertical linear movement of the sighting window. The optical encoder converts the units to an electrical signal which can be delivered to a computer for providing a desired measurement reading base on the position of the object in the column detected by the apparatus. The rail may be mounted on horizontal tracks for movement along a plurality of horizontally spaced columns.