The satellite data is collected by different agencies, and provided to the public, in the form of a dem file.. There are a number of different files available for the same area, from the same agency and represent different data densities, which could also be described as topographic detail. So, generally, the larger the file the greater the detail. As with all files with graphical details, while presenting a prettier picture, they also create more overhead for your GPS cpu. If your looking for the most topographical of the US, try this file(you only need one file per geographical area).