A method of surveying; the area is divided into triangles and the length of one side and its angles with the other two are measured, then the lengths of the other sides can be calculated WordNet 3.0 & Open English WordNet

In surveying, triangulation is the process of determining the location of a point by measuring only angles to it from known points at either end of a fixed baseline by using trigonometry, rather than measuring distances to the point directly as in trilateration. Wikipedia

A process used in surveying to determine unknown distances, angles and positions from those that are known Wikipedia Disambiguation

A technique in which distances and directions are estimated from an accurately measured baseline and the principles of trigonometry; (countable) an instance of the use of this technique. Wiktionary

Surveying technique. Wiktionary (translation)