Purposes of triangulation; tertiary triangulation

Triangulation is a method of horizontal control survey in which control stations are fixed at the vertices of a chain or net of triangles. One side (the base line) is measured very accurately, all the angles of the triangles are measured, and the lengths of the other sides are computed with the sine rule. Trilateration is the method in which the lengths of all the sides of the triangles are measured directly with an EDM (electromagnetic distance measuring) instrument, and the angles are computed from the sides using the cosine rule.

Purposes of triangulation and trilateration

• To establish accurate horizontal control (position of stations) for topographic and engineering surveys of large areas, with plane tabling, tacheometry or traverses based on the control stations.
• To determine the size and shape of the earth (geodetic surveys) and to fix positions on the earth by latitude and longitude.
• To provide control for photogrammetric maps and aerial surveys.
• To set out the exact alignments and centre lines of large engineering works, such as long bridges, tunnels, dams and canals.
• To monitor movement or deformation (of dams, landslides, the crust) by repeating the survey.
• To fix the boundaries of states or countries and provide a reference framework for cadastral surveys.

General specification of tertiary (third order) triangulation

Tertiary triangulation is based on the secondary stations and is used for closely spaced engineering control.