A method for pre-determining an underwater objects GPS position using a forward scan sonar unit linked to a boat trolling motor, magnetic compass and GPS receiver. This system determines the underwater objects GPS position using the objects distance, compass heading and a GPS receiver/sonar on a boat. This system will provide real time longitude and latitude positions of underwater objects seen with sonar at a distance from a boat, and will allow for precise autopilot navigation or fixed position fishing. The system can also be used to correct for GPS errors when using previously stored waypoints positions of an object. The computer determines an objects underwater GPS position using a forward scan sonar transducer and compass mounted on a 360 degree movable mechanism such as a trolling motor unit, and formulates the objects position based on the distance and heading of the object in relation to the boats current GPS position.