FishNChip is the setting to display SONAR CHARTS.
Sonar Charts is contour mapping layer created by sonar logs uploaded by those participating in that program. It is not Navionics data and it is not sold, it is a free service for those participating in collecting that data and for those wanting to view it. If is free with an update for Navionics Hotmaps Platinum and Navionics+ card owners. It is also viewed on the Navionics website's WEB APP and a map layer on the Mobile App.
Before anyone did any of that, most all locations got preliminary contours lines to be adjusted by those sonar logs. ONLY a location that has been recorded and uploaded will be close to accurate and only as accurate as the person doing the data recording.
You cannot assume any part of a Sonar Charts map is accurate unless you did the sonar logs yourself or verify with your sonar that the contours are accurate at a particular location.
This is a composite of a lot of sonar log tracks that I did at a particular lake. ONLY what is exactly under the red lines is accurate. Only until that satellite view is solid red will the contour map be accurate everywhere.