unless you have a BIG boat I don't think you would really see a difference in where you place your antenna, via the unit. GPS has an error percentage from satellites to your antenna and youll never be in an exact location every time. I think I read somewhere that it can be off by 30 feet or so... I can be parked at our dock in our marina and sometimes it will show my boat about 15-20 feet away from the dock. Sometimes its right against it. but yes it reads from the location of the antenna.