Which Lowrance do you have? And what lakes are looking at?
Premium Cards data is limited do to the small size of the card, it has what Navionics calls Nautical Chart data only, now if you get a Hotmaps Platinum or Nav + you get whats called SonarCHart layer which much more detailed.
Follow this link to the web app and here you can compare the two views, whats on the web app is on the cards. When you are on the web app click on the sonar for SonarChart view
http://webapp.navionics.com/?lang=en#@14&key=_l%7CnGn%7ByrP
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