I like the Navionics WebApp for topo purposes. I think there is also a feature you can turn on and it (if my understanding is correct) uses data collected from units that have been out on the water to give more recent depth numbers. I think that's the function and how it works but I'm not entirely sure. Either way it's pretty awesome.

 

The Navionics Web App has three functions now.

One is the Nautical maps by Navionics

One is the Sonar Charts that is user generated

One is satellite images for Ski maps.

 

In all cases, the data is updated daily.

 

The Sonar Charts layer is the one I use to create HD contour maps of waters that have no maps or no accurate maps.

I use Googe Earth Acme Mapper and Bing Maps

 

Bing Maps are especially useful for increased clarity in some areas of Lake of the Woods, Ontario