The IGN hiking maps for all of France are available on your Apple iPad, iPhone and iPod Touch for the modest subscription fee of about 19 euros per year.
How It Works
Opening the "ign" application while you have internet access loads all of the map areas at and around your location (your iPad/iPhone does need the built-in gps). The maps are stored in the devise's cache, so they are available to use while you're hiking — far from internet access.
You can set up the app to load various map scales: the 1/25,000 scale (identical to the paper maps), a closer view at 1/12,500, and even some super-detailed "Cadastral" scales. Once loaded, zooming in and out takes you from one scale to another.
In addition to the maps, you can load satellite photo-views at scales 1/25,000 down to 1/3125. A slider lets you select the map-view only, the satellite-view only, or the satellite overlaying the map to any degree of opacity you choose.
What It Is
IGN is France's Institu Geographique National, responsible for the detailed maps of all of France. The relatively new company of Géoportail manages the detailed French maps via the Internet and for mobile devices, such as some GPS devices and, now (2010), the Apple iPhone, iPod Touch and iPad.
Being more cute than practical, Géoportail has chosen the unfortunate name of iPhiGéNie for the Apple iPhone/iPad app. The choice of the name has some significance to the Géoportail, but being of no interest to us, difficult to pronounce or spell, we call it our "IGN app".