Yet another way to navigate across a mountainous terrain is by using a compass or altimeter. One can correlate his position with respect to specific landmarks in the surrounding terrain to identify your approximate location on a topographic map. This usually happens using the intersection of two reference lines:

Ridgeline and altitude (altimeter)
Ridgeline and bearing (compass)
Altitude and bearing (altimeter, compass)
Two bearings (compass)

Let's take a closer look at each scenario in detail.

First option - Ridgeline and altitude. Say we are walking along a ridgeline (line 1). We have an altimeter watch or barometer that tell us we are at 2050m altitude (line 2). Our current location is identified by the intersection of the ridgeline and 2050m contour line as shown below.