Let me explain myself a bit better than my orig post:
I was not using a GPS with Navionics on it. I just printed out a small section of the lake where we would be fishing. Nevertheless, I had quite an accurate idea of where I was from looking around me in the small cove we were in.If I believe my sonar readings, there should be a 'dip' or depression in the area that I checked----indicated by concentric closed loops. But there is no such indication on the map. The map shows the depth increasing more or less uniformly with further dist from the shore.
So, while GPS accuracy is a pertinent topic for this forum, it's not really relevant to the issue on this thread.(I guess I should have elaborated a bit more in the first place ).
I don't see the seasonal variation of water level to be particularly important. I.e. I'm interested in the relative depth-----the 'holes', bumps, ridges---the 'structure'--- rather than the absolute depth.
I have to wonder where Navionics gets their very detailed 'data'. Has somebody really surveyed the lakes in such detail?? Or, did they take the cruder contours shown on gov't charts and interpolate them like crazy??