Streaming maps on the FATMAP app requires an internet connection.
However, with Premium you are able to download as many areas as you like on to your mobile device (we recommend downloading over WiFi). When you have done this, you will not need mobile/cellular data in order to use FATMAP.
GPS functionality does not use data but real-time location sharing and external links, such as the weather and avalanche forecast, will require data.
To update the app with the latest user comments and conditions you will need data, but this can be done over WiFi if you do not have data enabled.