Baseline:
Whose length is measured these stations form the vertices of a series mutually co nnected, triangles the complete figure being called ‘Triangulation system’. In this system of triangles one line say ‘AB’ and all the angles are measured with greatest care and lengths of all the remaining line in the system are then computed. For checking both the fieldwork and computation another line say GH is very accurately measured at the end of the system. The line whose length is actually measured is known as baseline or base and that measured for checking purpose is known as the check base.
Triangulation Figures:
The geometric figures used in triangulation system are (i) Triangles (ii) Quadrilaterals (ii) Quadrilaterals, Pentagon, hexagons with centre angle. This arrangement although simple and economical but less accurate since the number of conditions involve in its adjustment is small.
 Station adjustment ==> sum of angle is 180
 Figure adjustment ==> sum of angles is 400 grad or 360
 Quadrilateral; adjust ==> (all the angles are horizontal)
Quadrilaterals pentagons or hexagonal with central stations. For very accurate work a chain of quadrilaterals may be used. There is no station at the intersection of diagonals. This system is most accurate since the number of conditions in its adjustments is much greater. To minimize the effect of small errors in measurement of angles the triangles hold be well shaped or well proportioned i.e. they should not have angle less than 30 or greater than 120.The best shape triangle is equilaterals triangle and best shape quadrilateral is square.
Classification of Triangulation System
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 (310 miles)
Length of sides of triangle = 30 to 100 km or more (20100 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 [ 13 miles]
= 8 to 70 km [ 540 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 pts 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
Check on base = 1 in 500
Applications of Triangulation
 The establishment of accurately located control points for surveys of large.
 The accurate location of indirection work such as:
 Centre lines, terminal pts shafts for long tunnels
 Centers lines and abutment for bridges of longs spans.
 Complex highway interchanges.
 The establishment of accurately located control pts in connection with aerial surveying.
 Measurement of deformation of structure such as dams.
Trilitration:
Because of the development of highly accurate electronic measuring devices, a triangulation system can be completely observed, computed and adjusted by measuring the lengths o the sides in the network. This procedure is known as trilitration. No horizontal angle need to be measured because the lengths of the sides are sufficient to permit both the horizontal angles and the positions of the stations to be computed.
The surveying solution technique of measuring only the side of triangle is called triplication.
