Offline maps for Norway

Positioning works without a signal. The map is the part you have to plan for.

The most common misunderstanding first: your phone does not need mobile coverage to know where it is. Satellite positioning is receive-only. What it needs coverage for is downloading map tiles, and that is entirely solvable in advance.

The sources

Kartverket is the Norwegian mapping authority. Its topographic data underlies most of what you will look at, and much of it is openly licensed.
Norgeskart is Kartverket's public web viewer. Useful for planning on a large screen and for reading contour detail properly.
UT.no is run in association with the national trekking association and carries marked route and cabin information that pure topographic layers do not.

Getting it offline

Download tiles for your area before you leave, over wifi, at the zoom levels you will actually use. Download a larger area than the route, because the day you need the map is the day you are not on the route. Then verify it: put the phone in flight mode at home and check the map still draws. Assuming a download worked and discovering otherwise in cloud is a bad sequence.

Battery

Continuous track recording with the screen waking constantly is what kills a phone, not the GNSS receiver itself. Record the track, keep the screen off, and check position deliberately rather than watching the dot move.

Carry a paper map

Not out of nostalgia. A phone is a single point of failure sensitive to cold, water and impact, all three of which are likely. A map and compass fail differently, which is the entire point of carrying them.

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