The 'Maps' app allows maps to be downloaded to Windows Phone for offline use so no data connection is necessary. This is useful. I need a map with more detail though that also shows hiking routes. OpenStreetMap has the level of detail I need. How can I download these more detailed maps onto Windows Phone for offline use? The best I've found so far is the 'GPX travel map' app which caches OSM.
Try app NaviComputer (https://www.microsoft.com/en-US/store/apps/NaviComputer/9WZDNCRFHXWR)
It can download and upload gpx-files to and from your pc. An additional pc client is needed. This is explained on their website.
I have been using it for a year and am very satisfied.