When the unit is turned on in the boat and the boat is in a river for example, the boat position is shown maybe 100m to starboard of it's true position so often I am saling through houses etc. Same in open water when passing marker buoys, I am always shown on the screen about 100m to the starboard of my true position. Also when I zoom in close to get more details on marker buoys for example, the screen just goes blank. Any help would be appreciated for these very annoying problems. Garry