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## What is Triangulation Surveying?

Triangulation surveying is the tracing and measurement of a series or network of triangles to determine distances and relative positions of points spread over an area, by measuring the length of one side of each triangle and deducing its angles and length of other two sides by observation from this baseline. Triangulation surveying was first introduced by a Dutch man named Sneli. Triangulation is preferred for hills and undulating areas, since it is easy to establish stations at reasonable distances apart, with intervisibility. In plane and crowded areas it is not suitable as the intervisibility of stations is affected. The difficulty is overcome by building towers which is quite expensive. The main disadvantage of triangulation is the accumulation of error in the lengths and direction of lines, since both of them, for successive lines, depend upon the computations for those of the preceding line, which necessitates the check bases. In triangulation, entire area to be surveyed is covered with a framework of triangles. For the triangle, the length of the first line, which is measured precisely is known as Base line. The other two Computed sides are used as new baselines for two other triangles interconnected with the first triangle. By extending this process, a chain or network of triangles can be spread over the entire area.**Operations in Triangulation Survey**

**The field work of a triangulation is carried out in the following well defined operations:**

- Reconnaissance
- Station preparation
- Baseline measurement
- Measurement of angles.

## Applications of Triangulation Surveying

- Establishing accurately located control points for plane and geodetic surveys of large areas.
- Establishing accurately located control points in connection with aerial surveying
- Accurate location of engineering projects such as Centre lines, terminal points and shafts for long tunnels, and Centre lines and abutments for long span bridges.

## Triangulation Systems

A system consisting of triangulation stations connected by a chain of triangles. The complete fig is called triangulation system or triangulation figure. The most common type of figures used in a triangulation system are- Triangles
- Quadrilaterals
- Polygons

**Geometric conditions to be fulfilled by above figures in triangulation system are:**

- The sum of interior angles should be (2n-4)x90
^{o}, where n = no.of sides of the figure - If all the angles are measured at a station, their sum should be 360
^{o}. - The length of sides calculated through more than one routes should agree.

### Triangles

1. A chain of triangles is very rapid and economical when a narrow strip of terrain is to be surveyed. 2. Angles less than 30^{o}or more than 120

^{o}are not permitted 3. For well-conditioned triangles, angles should not be less than 30

^{o}or more than 120

^{o}.

**Advantages of triangles:**

- This is simple and rapid
- Economical method

**Disadvantages:**

- Since it is used to survey a long narrow strip, a no.of base lines must be introduced frequently to reduce the accumulation of errors. Therefore, a single chain of triangles is never permitted in high order triangulation.
- Least accurate method.

**Quadrilaterals**

- These afford an excellent system since the various combinations of sides and angles can be used to compute the lengths of required sides, and checks can be made frequently
- The best quadrilateral is square. A quadrilateral with both diagonals having no station at their intersection is usually employed.
- This is best suited for hilly areas.

**Advantages**

- Most accurate system as the number of checks are more.

**Polygons**

1. When areas that are very wide in proportion to their lengths are to be surveyed then pentagonal or hexagonal figures may be economical.
2. These may or may not have a central station.
**Advantages**

- This is also more accurate as the desired number of checks are more.