Doesn't the map use the phone's built-in compass, which should be reasonably accurate even while not moving? What prevents the app from knowing the phone's orientation accurately while stationary?
Always been curious about this.
For example, near me all shores covered by the metal-concrete structures, and also I seen magnet anomalies on just ordinary streets, looks like there where unstable surface (my district built on sand) and there created underground metal-concrete structure, so road will not break every day.
I also noticed, near large bridges magnet anomalies are largest, and usually are like roots of tree - extend about few hundreds meters beyond bridge itself.
Also exists some other large metal underground structures - pipes for water, for gas, etc.
Problem is that those underground metal where not demagnetized, in civilian building nobody cares, so only in very pure rural locations, you could be sure that nothing will disturb magnetic compass.
At other side, when move, GPS give very reliable direction vector, it is not affected by any magnet anomalies or others, so now nearly all map soft/hard using GPS direction when move and switches to unreliable magnetic compass when not move.
A compass will tell north, but it doesn't tell which way your head is pointed.