a system for monitoring the position of a floating offshore drilling rig in relation to the subsea wellhead by stretching a taut steel line from the rig to the ocean floor. An inclinometer measures the slope of the line at the rig, and, because the line is assumed to be straight from the rig to the ocean floor, any angle in the lines indicates that the rig has moved. The system's weakness is that the taut line can be distorted by currents and thus give inaccurate readings. Compare acoustic past reference and position-reference system.
