Where positioning fails

Cities, canyons, cold.

Sources of error

Buildings and cliffs reflect signals, producing multipath errors of tens of metres. Tree cover and deep valleys weaken reception, and interference is now common near conflict zones.

Phones blend satellite, wifi and mobile data, which usually helps.
Reported accuracy is an estimate, not a guarantee.

Altitude

Vertical accuracy is markedly worse than horizontal. Barometric altimeters are often better, once calibrated against a known height.