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

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  1. Primary Triangulation
  2. Secondary Triangulation
  3. 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
Specifications are:

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.
Specifications are:

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.
Specifications are:

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

Also Read: 

  1. What is Compass Surveying?
  2. What is Principle of Surveying?
  3. What is Chain Surveying?
  4. Reconnaissance Survey and Index Sketch in Chain Surveying
  5. What is a Site Plan? What is the Purpose of Site Plan? When is Site Plan Required? What are the Types of Site Plans?
  6. Linear Measurements in Surveying