Triangulation surveying employs triangulation system which is a combination of interconnected triangles. The combination of set of triangles is called as triangulation system or triangulation figure.
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The classification of triangulation figures is performed based on the accuracy with which the length and the azimuth of a line of the triangulation are determined. The triangulation system opted can take different accuracies which is merely dependent on the extend and the purpose of the survey.
Classification of Triangulation System
- Primary Triangulation
- Secondary Triangulation
- Tertiary Triangulation
1. Primary Triangulation or First Order Triangulation in Surveying
- Highest order triangulation
- Determine control points or earth's figure
- Embraces the whole survey area
- It can be connected to secondary triangulation
SL.No |
Parameter |
Specification |
1 |
Average
Triangle Closure |
< 1 sec |
2 |
Maximum
Triangle Closure |
Not more
than 3 sec |
3 |
Length of
the Base Line |
5 to 15 km |
4 |
Length of
the sides of Triangle |
30 to 150
km |
5 |
Actual
Error of Base |
1 in
300,000 |
6 |
Probable
Error of Base |
1 in
1,000,000 |
7 |
Discrepancy
between two measures of the section |
10 mm sqrt (km) |
8 |
Probable
error of computed distance |
1 in 60,000
to 1 in 250,000 |
9 |
Probable
error in astronomic azimuth |
0.5 seconds |
2. Secondary Triangulation or Second Order Triangulation in Surveying
- Consist of number of points fixed within the framework of first order triangulation.
- Stations are placed close to obtain small sized triangles.
SL.No |
Parameter |
Specification |
1 |
Average
Triangle Closure |
3 sec |
2 |
Maximum
Triangle Closure |
8 sec |
3 |
Length of
the Base Line |
1.55 to 5 km |
4 |
Length of
the sides of Triangle |
8 to 65 km |
5 |
Actual
Error of Base |
1 in150,000 |
6 |
Probable
Error of Base |
1 in
500,000 |
7 |
Discrepancy
between two measures of the section |
20 mm sqrt(kms) |
8 |
Probable
error of computed distance |
1 in 20,000
to 1 in 50,000 |
9 |
Probable
error in astronomic azimuth |
2 seconds |
3. Tertiary Triangulation or Third Order Triangulation
- Have number of points fixed within the framework of the secondary triangulation.
- The points are immediate control for the detail engineering and other surveys.
- The triangles formed are smaller in size.
- Instruments used can be moderate precision ones.
SL.No |
Parameter |
Specification |
1 |
Average
Triangle Closure |
6sec |
2 |
Maximum
Triangle Closure |
12 sec |
3 |
Length of
the Base Line |
0.5 to 3 km |
4 |
Length of
the sides of Triangle |
1.5 to 10 km |
5 |
Actual
Error of Base |
1 in
75,000 |
6 |
Probable
Error of Base |
1 in
250,000 |
7 |
Discrepancy
between two measures of the section |
25 mm sqrt of kms |
8 |
Probable
error of computed distance |
1 in 50,000
to 1 in 20,000 |
9 |
Probable
error in astronomic azimuth |
5 |
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