Wayne, from what I have heard (maybe wrong info) the sonar charts are just guesses at what the contour lines should be doing, they aren't scanned items. Is that right? If yes, then they don't really interest me. The problem I seem to have with my new Platinum is with my lake water level. It's down 27' and it's been down for years. The premium shows all of the old shoreline but the Platinum is blank where a bunch of the shoreline is supposed to be. If my lake ever (doubtful) fills back up it seems that I will have a lake missing a bunch of the shoreline.
If any mapping company did a survey or re-survey when the water was down, that is all the data they got to generate a contour map. Sonar does not work on land. The Platinum may have been the result of a re-survey and it can't match an older map with a different water level.
Most mapping companies purchase contour mapping from other companies and have some older data too, but only Navionics HD locations will be their own surveys.
The Sonar Charts program started with preliminary contour lines for most all bodies of water that are computer generated fake contour lines. It is not Navionics data and is only accurate where data is uploaded by those participating in the Sonar Charts program.
It is the same as buying a mapping program and processing your sonar logs, but it is a FREE service by Navionics.
Sonar logs are uploaded to the Navionics server and those sonar logs are used to adjust the preliminary contours lines where the GPS track was recorded. ONLY the exact track that is recorded is accurate. The whole body of water won't be accurate until all of it is logged.
The example I posted was for a new lake that no mapping company had even an outline for. ONLY after I uploaded the first few sonar logs did Navionics create the shoreline (zero line) to apply my data. I do some sonar logs every time I am at that lake to keep improving the overall map accuracy.
An example of doing a correction is this area that I had not logged and I knew from the overall map so far that is was missing. I have been fishing this lake since it was opened to fishing and before there was the Sonar Charts program. SO I did this track to cover that area to get that detail added. I posted these same images on another website today but forgot which one.
Note the GPS co-ordinates in the two contour map pictures:
What it was:
The Google Earth track I recorded:
The corrected map for that location:
This is a composite Google Earth track map I got from a couple of years ago to see my progress of sonar logging for that lake. Until the whole lake is solid red, the whole lake will not be accurate, but it will be more accurate that any commercial map product since I am taking the time to do the detailed coverage. Commercial mapping does not and will not cover every square foot of terrain--takes to long. They do surveys in intervals and then fill in between the measured data points with fake (interpolated) lines.
If you want more detail, the Navionics sticky topic on the Lowrance and Humminbird BBC forums has some.