It's my understanding that one can use the Navionics Webapp to compare sonar data currently on the map card with the more-detailed SonarCharts data that can be downloaded from the Navionics website.

While using the Navionics Webapp recently to view the stretch of the Sabine River near downtown Orange where the Bassmaster Elites launched last year I noticed that the depths shown with the SonarCharts button switched on and off differ greatly. Surprisingly, the "high-resolution" data, while it has 1-foot contour lines, is totally erroneous! It shows mid-river depths of 9-14 feet. The low resolution view shows mid-stream depths of 28-40 feet, which is much closer to actual! Check it out for yourself!

So, have I misinterpreted what the Navionics Webapp is supposed to represent, or is Navionics fudging the data to encourage people to anti-up for access to SonarCharts?

Read this, I think you and a lot of folks might get some questions answered. I have NO DOG in this, no relationship with Lowrance or any other depthfinder, mapper, etc. Just thought it was interesting and I never knew it:

http://insightgenesis.wordpress.com/2014/07/31/forget-the-myth-of-one-foot-contours/#more-282

As I mentioned on a previous thread, I've been using sonar logs and the Insight Genesis program to survey the bottom on my lake. I've probably got 30% of it done (it does take some time).

It's amazing how inaccurate the built-in Insight Map (Lowrance) and the Navionics Web App are. I have no idea when the survey was made on our lake - maybe they're using old data. But the fact is that some areas do not reflect the reality at all.

I know that when Amistad dropped 60 + feet in a few months....My Navionics chip was WAY off in some areas, it would get you in trouble !!! I ran into a hump that the chart showed we were in 40 feet of water. A valuable lesson learned.. after that incident I found a lot of places that the chart was not EVEN close.
I have two different Navionics chips and one lake master for Amistad .. So far the lake master is a lot more reliable.

I do not have the time or desire to put a fine tooth comb to it but the navionics app that I have on my phone seems to be reliable on my home lake - in terms of when it shows bunched up contour lines, I go to that area and there is a good ledge/steep drop off which usually has some fish around it somewhere

Phish, Would you show me the area you are talking about? I pulled it up and did not see where you meant, not saying there isn't discrepancies or mistakes on the mapping, but if such areas are found, the best solution is to report them so that they can be checked into as times it can something as simpl as a software glitch, since you pointed that area out, I did see that the 0 is missing on 30 in several places, not sure what is going on there but I will find out. I did a sonar chart event a couple weeks back and there were many tracks turned in to improve the amount and the accuracy of the mapping for Orange and the Sabine.

The area I was viewing when I discovered this issue is in the neighborhood of the old Navy piers near downtown Orange and the City Boat Ramp, 30 deg 05' 50.00" N, 93 deg 43' 15.00" W (per Google Earth). Sorry, I couldn't get screen shots to upload, even when reduced quite small.