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A quick google suggests that OutDoors France is the main one, but reviews on the AppStore aren't very good.
Any recommendations for best Map App for walking in the Alps in summer?
We used iPhiGéNie, maps of France - pay for 1 month - or 1 week free trial https://itunes.apple.com/gb/app/iphigénie-maps-of-france/id350346756?mt=8
Alternatively download the area you want onto Viewranger
to view maps on PC https://www.geoportail.gouv.fr/carte
+1 for ViewRanger. I've used it in France, Spain, Switzerland, Germany and Mallorca.
I use Viewranger. Its very good. You can buy IGN map tiles which are pretty close to UK OS maps or you can download open maps for free
I’ve used Motion X and downloaded the maps over WiFi before switching data off. Worked ok
Logged for future ref.
Second iPhiGéNie and I've found it really good. I've used it in France a few times and the maps are good plus you can cache maps using wifi instead of data. Also easy to add in gpx tracks.
I've used sitytrail for touring in France the last couple of summers. I think you'd still get a month free trial too.
Use this on the desktop/laptop as well - https://www.geoportail.gouv.fr/carte and there is a free app in the same name however it doesn't cash like Iphegenie does and has less features ... but it is free.