@KirtH I believe you spend a lot of time on Marion. Any insight to the accuracy of the Navionics Sonar chart on Marion in Lakeville?
The left Navionics map contour (left map) is basically the 1958 DNR chart with 1 ft. Contours calculated to the chart with minimal updates to some of the contours and yes, is quite inconsistent and inaccurate to the (right map) SonarChart.
Bathymetric metadata originated in 2009 where SorarCharts are derived from.
"SonarChart™ (right contour map) is produced starting from the Nautical Chart and other regional or local sources offering additional data, as well as Navionics surveys. We then validate and integrate sonar logs recorded by end users and contributed from both plotter and mobile devices, adjusting for tide correction and filtering out “bad” data. A new SonarChart is published with updated contours on a daily basis.
Navionics generates a new SonarChart every day. After a user submits sonar logs for processing, the updated SonarChart will normally be available after about one week, with some exceptions. For example, processing may take from three up to eight weeks when sonar logs are submitted for an uncharted area, because a coastline correction is typically required and Navionics will first source new coastline information for the area."
SonarChart Live from users are usually collected is small areas. I can't really visualize a person running a boat east to west than north to south every 25 feet over an entire lake are the size of Marion to record depth. This is the daily data in the above note from Navionics on how this user captured data is added to the map. I do not believe it is possible to rife the data with depth errors as the only measurement from a sonar is the distance form the transducer and lake bottom which is not adjustable on any sonar. Minimal GPS errors may occur but only a few feet depending on the quality of the Smart Phone GPS. Extreme dense weed areas in shallow areas that effect depth are included in the "bad" data filtering.
Ice is freezing up. Shore lines are now frozen up where they were open. Ice is honeycombed but firm. River is open past the 22 access by 100 yards. I might just take out my small wheeler and do a walleye fishing on shoe this weekend.