The France National Geographic Institute has released for public access on its website some of its map library, free to download and use . If we could get before on other sites actual and today's maps or sat views of the regions for which we have an interest , they released the "cartes de l 'état-major", a set of maps, 1:40000, covering the whole french territory purposely done for the Military . This huge work was done during the 2nd half of the 19th century and give the reader a very nice view of the land, towns, roads , railways that I find totally compatible with our OFF terrain. During WW1, these maps were no longer up to date for military use and other more recent maps were used with different scales by the different services of the protagonists. The Air Services maps were around 1:30000 IIRC generally.
The IGN did something great in thet they merged all the maps to make a whole France , superposable with sat views or road map.
The site is not very user-friendly (for a French, what about a non-French speaking) and lacks intuitivity and explanantions on how to have the best benefit of its huge features and I am of the lazy kind ,so I 'm stuck at watching the maps and zooming in and out, so I will try to use them on a mobile phone or a pad (don't have one...yet) while "flying " my OFF missions.
For instance, the link to the map of the Arras region, but from it you can scroll the whole territory
http://www.geoportail.gouv.fr/donnee/56/carte-de-l-etat-major-en-couleur?c=2.7594067997943945,50.29197441499255&z=0.00033503704418998587&l=GEOGRAPHICALGRIDSYSTEMS.ETATMAJOR40$GEOPORTAIL:OGC:WMTS(1)&permalink=yes
Hope that can give you some more realism and comfort too in the OFF experience. If I can be of any help , don't hesitate to ask.
Cheers.