# Classification of Triangulation System - Triangulation Surveying

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

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#### 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