Classification of triangulation system

The triangulation system is classified into first order (primary), second order (secondary) and third order (tertiary) triangulation, according to the accuracy of the work. The classification depends on the length of the base line and sides, the closure of the triangles and the probable error of the computed lengths.

1. First order (primary) triangulation

• It forms the main framework of a country and is used for geodetic work, for determining the shape and size of the earth, and for control of the whole area.
• Highest accuracy: long sides (30 to 150 km), base line 5 to 15 km, average triangular error less than 1".
• Stations are on hilltops or towers; signals and precise theodolites are used.

2. Second order (secondary) triangulation

• Based on the primary stations as control; it divides the primary network into smaller parts.
• Sides 8 to 30 km, base line 1.5 to 5 km, average triangular error 3". Used for regional surveys and for fixing points for topographic mapping.

3. Third order (tertiary) triangulation

• Based on the secondary net; it gives closely spaced stations for engineering surveys (towns, roads, dams, bridges, tunnels).
• Sides 1.5 to 10 km, base line 0.5 to 3 km, average triangular error 6". Less precise instruments are acceptable.

($K$ is the distance in km. The figures are the common textbook values; Survey Department practice may differ slightly.)

Classification by figure

The triangulation system can also be classified by the type of figure used: (i) a chain of single triangles, (ii) a chain of braced quadrilaterals, and (iii) a chain of polygons with a central station. The best-conditioned figures have all angles between 30 and 120 degrees.