Reading the ground against the map
A map is a promise; the ground is what actually happens. Learning to read one against the other is most of navigation.
Contour lines that bunch mean steep; a stream shown as a thin blue thread can be a real obstacle after rain. Yesterday the path marked on the map had been rerouted around a slip, and only the shape of the slope told me where I really was. Trust the ground first, the map second, the phone last.