a coordinate indicating and plotting system wherein compass heading signals are periodically integrated at frequencies related to the distance traveled by a water , air or land vehicle to obtain coordinate change signals for visual indication of vehicle coordinates . the coordinate change signals can be applied to a plotting system to record the path followed by the vehicle or applied to a counter to indicate the unit changes for the vehicle in each direction . desirably , the compass heading signals are digitized by converting the heading signals into frequencies and the signals are integrated by summing the pulses for each output direction during each sampling period . a novel crescent - shaped opening whose width approximately follows a sine function between zero and 180 ° is positioned angularly on the compass card about the center of rotation thereof . linear light sources on one side of the card and photometric detectors positioned in alignment with the light sources on the other side of the card measure the light passing through the crescent - shaped opening to produce signals representative of the heading components of the vehicle in each direction .