What is Triangulation in Surveying? Explained With Video

Triangulation is a method followed in surveying to achieve horizontal control. In triangulation system, a number of interconnected triangles is traced and their angles measured to determine the relative positions of the points spread over an area. In this system, the length of only one line is determined, called as the base line and the angles are measured using an instrument and length of all the sides are determined using trigonometric formula. 

Principal of Triangulation

The principle of triangulation in surveying is that, "if one side and three angles are known for a triangle, then the remaining sides can be calculated using sine rule."

In the above figure, if the anyone side for example c and the angles A, B and C are known, then as per sine rule,

a/sin A = b/ sin B = c/ sin C;

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Triangulation System in Surveying

As mentioned, as series of interconnected triangles form the survey area. If we know the length of one side and the three angles, the length of other sides of each triangle can be computed. 

In this system, the apex of the triangle are called as the triangulation stations, and the whole figure is called as triangulation system or triangulation figure.

The common types of figures that are used in triangulation system are:

  1. Triangles
  2. Quadrilaterals
  3. Polygons
All the figures that are used in triangulation system for surveying must follow certain conditions. which includes:
  1. The sum of the interior angles should be (2n-4) x 90, where the number of sides of the figure is given by 'n'.
  2. The total sum of the angles joining at a station must be equal to 360 degrees.
  3. The length of the sides that are calculated through more than one route should match and agree.

Triangles for Triangulation

Among all the figures, triangles are most commonly used because they are very rapid and economical when a narrow strip is to be surveyed. The triangles used for triangulation must not have interior angles, less than 30 degrees and greater than 120 degrees. This is method is more economical, simple and rapid. But as it is used in a long narrow strip, number of base line is introduced frequently that would result in the accumulation of errors. Hence it is less accurate.

                  Triangles for Triangulation

Quadrilateral for Triangulation

This is an excellent figure system which can involve various combination of sides and angles. It can be used to compute lengths of required sides and checks can be made frequently. The best quadrilateral is square. A quadrilateral with both diagonals without no station at their intersection is followed. Quadrilateral figures are used in triangulation for hilly areas. It involves more number of checks, hence the method is accurate.


Polygons for Triangulation

For areas that are very wide, their length are surveyed either by a pentagonal or hexagonal area. This option is more economical compared to triangles when wider area is there. In case of polygon figures, they may or may not have a central station. It involves more number of checks, hence the method is accurate.

Polygons with Central Points

Where are Triangulation Survey Used?

Triangulation is mainly used for survey of hills and undulating areas. In these areas, triangulation method helps to establish stations at reasonable distances apart. But, this method is not suitable for plane and crowded areas, where intervisibility of stations are affected. In situations, where it is a must, building towers are constructed, but it is costly.

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 

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