Digital mapping with GPS

[tonyjor]

I have the Navionics app on my phone and I also have a newer Lakemaster MN chip in my Humminbird ci. The last three lakes I've been to and marked spots on turned out completely different depths than the maps on both units showed. The last one, Big Lake in Big Lake, MN I marked a couple spots at about 15-18 fow along a break. When I checked these spots I was in 27 fow. Sugar Lake showed 15 on the map, turned out to be 20+ along with Grand Lake more of the same and the sand bar is 100 yards away from what the map showed. Has anyone else had issues like this and is there a calibration I can do or something to fix the problem? Thanks for any help!

[delcecchi]

Some lakes have data that came from depression era mapping involving guys on the ice with surveying equipment (transits, chains etc), chisels, and strings with weights on the end. Make a grid, drill hole, measure depth.

The DNR has that data on the lakefinder site. Finally the chip guys went out with modern technology and mapped the lakes more accurately.

Mille lacs was the first one I think. Look at the web site for the mfg and see if the lakes you are looking at have been remapped. Usually they are labeled as HD or something similar.

Also, contributing to that is variation in the "Datum" used, which is what maps a flat map to the round earth, and the co-ordinates.

I think some GPS you can select which datum to use.