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Mmjoker, I didn't see you or anyone else mention if it seems accurate in other locations you are at? I would think if you tried it at other obvious landmarks and it's ok there, then it's just a map error. If it's off by half a mile everywhere, then I'd send it to the factory for calibration or at least call and ask the factory for advice. That's just my 2 cents. Good luck with it!
I also have Lowrance Create maps and they are off at several locations. I have watched it make trails along side of the map roads and sometimes they can be more than a 100 yards off. When I travel this route at another time, the trail is marking another one on top of the first one and is very good at duplicating its co-ordinates but the map program is off and still shows me driving along side of the road instead of on it. Most of the map roads and places are right on but some of the map is way off.
I found my book, Check the Position Format and Position Correction Factor (PCF)
Rowdy
Try checking it against the intersection of two major roads very close to the lake. These are usually verified every few years.