I just installed a new Lowrance LCX-28c HD on my boat. This unit has all the pre-loaded maps. How accurate should I expect the maps to be. For example, when I'm in my driveway, my position on the map is off by several hundread feet. When I'm on my local lake, and fishing shorelines, the map may show my trail on dry land. The lake has many narrow inlets and the channel of these inlets is indicated with a blue line on the map. If I follow the blue line, I know I'm not in the channel. Most of the time I would guess I'm off by 50'. I guess I expected accuracy to be within a couple of feet. Were my expectations too high or do I have a problem with my equipment?