Abstract: A transducer is mounted on the boom to produce an electrical signal proportional to the angle the boom has moved in a horizontal plane. A resistance related to the boom length receives the output of the transducer and supplies a current proportional to L Cos .theta. where L is the boom length. A potentiometer supplies a current proportional to the offset distance from the boom hinge point to the rotating axis of the crane and an indicator means receives the current proportional to both the L Cos .theta. and the offset distance for indicating the operational radius of the crane.