Navigation Basics – Locating your position using a ‘Transit’
A 'Transit' is a pair (or more) of visible identifiable features which lay along the same line of sight – these might be a stream junction and a lone building, or maybe a nearby peak.
If you are on a marked path or ridge, you can extend a line drawn between these two points on a map to cross the ridge or path on which you are travelling.
This will quickly, and relatively reliably identify your location along your path on a map.
Another such ‘transit’ might be a section of a nearby stream, ridge or path. So long as the section is largely straight, and when extended the line crosses the path that you are on, it can be used as a reliable transit.