Triangulation system may be classified according to
- Degree of accuracy required
- Magnitude of work
Primary or 1st order Triangulation:
In primary triangulation very large areas (such as the whole country) are covered and the highest possible precision is secured. Well proportioned triangles, most refined instrument and methods of observations and computation are used.
Average triangle closure = 1 second Max triangle closure = 3 second Length of baseline = 5 to 20 km or more (3-10 miles) Length of sides of triangle = 30 to 100 km or more (20-100 miles) Degree of Accuracy = 1 in 500,000 Check on the base = 1 in 25000.
Secondary or 2nd Order Triangulation
Within the primary triangles other points are fixed at closure intervals so as to form a secondary series of triangles. Which are comparatively small are used, the instars and methods are not of the same at most refined.
Average triangle closure = 3sec
Max = 8 sec
= 2 to 5km [ 1-3 miles]
= 8 to 70 km [ 5-40 miles]
Degree of Accuracy = 1 in 50,000
Check on the base = 1 in 10,000
Tertiary or 3rd order Triangulation
Within the secondary &delta points are established at short intervals to furnish horizontal control fro detail survey.
Average closure = 6 sec = 12 sec = 1 to 30 km [½ to ½] = 1.5 to 10 km [less then a mile to 60 miles] Degree of accuracy = 71 in 500 C Check on base = 1 in 500