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UNIT-6Triangulation SurveyQ1.What is triangulation ?A1) Triangulation is a surveying method that measures angles in a triangle formed by three survey control points. Using trigonometry and measured length of just one side, the other distances in the triangle are calculated.Q2.What are the system of framework?A2) The common figures or system used in the triangulation system are:

Single Chain of Triangles

Double Chain of Triangles

Central Point Figures

Quadrilaterals

1. Single Chain of Triangles in Triangulation A single chain of triangles is used to survey narrow strip of terrain. This figure or system is rapid and economical. The figures made in this method is not so accurate and is not used for primary works. The number of conditions that are used to fulfil figure conditions is relatively small. In this method, two independent routes cannot be used to determine the solution of the triangles. Hence, to reduce the accumulation of errors, it is recommended to introduce baseline frequently. 2. Double Chain of Triangles Double chain figures are used for covering a larger area. 3. Centered Figures It is used to cover the area in a flat country. It provides satisfactory results. The centred figures can be quadrilaterals, hexagons, or pentagons with central stations. The figure provides desired checks during computations. The progress of centred figures work is slow as they require more instrument setting.

Fig-1 Centered Figures4. Quadrilaterals The quadrilateral with four corner stations and observed diagonal forms the best figures. It is best suited for hilly country. Since the computed length of the sides can be carried through the system by different combinations of sides and angles, the system is most accurate.Q3.What is baseline measurement and equipment of its?A3) It is the alignment as it forms the basis for the computations of the triangulation system the length of the baseline depends upon the grades of the triangulation.Equipment for Baseline Measurements

Tape

Straining Device

Spring balance or weight and pulley

Thermometers and a finely divided scale

Marking tripod or station rakes

Supporting tripods or stakes

Q4.Explain extension of baseline correction?A4) Extension of baseline correctionChain / Tape corrections1)Correction due to standardizationCorrection per chain length

Total correction

True length of line

Alternate solution of Q →(1) Core (1) L = 30m andL' = 30.40 m

1) C' = L'-L = 30.40-30 = (+) 0.40m

2) Number of chains = n =

3) Total correction

= 42.3×0.40=(+)169.20

4) Total length

l = 12690+169.20 = 16459.2 m

1) Correction due to slope

Fig-2 Correction due to slope This correction is always a negative correction.- Correct distance between any two points on ground is horizontal distance.Correction required = L –

approximate formulaIf

angle is given

2) Correction due to wrong alignment

Fig-2 Correction due to wrong alignmentSame as slope correctionIf endpoint of chain is not put on the straight line between points to be measured

Correction is always negative.3) Temperature correctionIf

is the temperature at the time of standardization.

is the temperature at the time of measurementL = length of chain / tape

= Coefficient of thermal expansion

Length of chain	Noted down value	Error	Correct
Increase	Less	Negative	Positive
Decrease	More	Positive	Negative

Table 1 Temperature correction4) Pull correction

= value of pull applied at the time of standardization

value of pull applied at measurementL = length of chain / tape

Cross sectionarea of tapeE = Young's modulusPull correction =

	Error	Correction
	Negative	Positive
	Positive	Negative

Table 2 pull correction5) Sag correction

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Fig-3 Sag correction

This collection is always negative.Formal tensionThe value of pull

For which positive pull correction is exactly equal to negative sag correction is called normal tension.

Q5.Explain measurement of angles?A5) Measurement of horizontal and vertical anglea) Horizontal angle:-- Horizontal angle is used to find bearing and direction in control survey.- It is also used for locating detail when mapping.b) Vertical angle:--It is used to find out the height of points and to calculate slope connections.Measurement of horizontal angle1) To measure the angle of ABC, the instrument is set to B.2) Upper clamp is loosened and the lower clamp is fixed.3) Telescope is turned and vernier A is a set O and Vernier B to 180°.4) Now lower clamp is loosened and tightened scope is pointed to A and bisect ranging rod at A.5) Now lower clamp is tightened and lower tangent screw is turned to perfectly bisecting ranging rod at A.6) Now upper clamp is loosened and the telescope is turned clockwise to bisect the C which is tightened at the upper clamp.7) Now, vernier reading A and B is noted.8) Vernier A result in angle directly and vernier B result reading by subtracting initial reading from final reading.Q6.What is satellite station?A6) Satellite station

It is also known as an eccentric station.

Satellite station is a false station because it is not possible to set the instrument over the structure false station point is assumed.

It is also a place where the movement of the satellite is followed and receive information from them.

Q7.What is principle of least squares?A7) Least squares’ is a powerful statistical technique that may be used for ‘adjusting’ or estimating coordinates in survey control networks. The term adjustment is one in popular usage but it does not have any proper statistical meaning. A better term is ‘least squares estimation’ since nothing, especially observations, are adjusted. Rather, coordinates are estimated from the evidence provided by observations. The great advantage of least squares overall methods of estimation, such as traverse adjustments, is that least-squares are mathematically and statistically justifiable and, as such, is a fully rigorous method. It can be applied to any over the determined network but has the further advantage that it can be used on one-, two- and three-dimensional networks. A by-product of least squares solution is a set of statistical statements about the quality of the solution. These statistical statements may take the form of standard errors of the computed coordinates, error ellipses or ellipsoids describing the uncertainty of a position in two or three dimensions, standard errors of observations derived from computed coordinates and other meaningful statistics described later. The major practical drawback with least squares is that unless the network has only a small number of unknown points, or has very few redundant observations, amount of arithmetic manipulation makes method impractical without the aid of a computer and appropriate software. The examples and exercises in this material use very small networks to minimize computational effort for the reader, while demonstrating the principles. Real survey networks are usually very much largerQ8.What is signal ?

A8) It is a small device which is used to define the location of triangulation station such that it is easily observed from other stations.

The signal must be vertical over the station.

It is placed centrally over station mark.

It is important to place over the station mark because the accuracy of the triangulation system depends on the signal that is entering the signal.

Q9.What are types of signal?A9) Signal:- Opaque signalLuminous signalOpaque signal:-Is a type of signal used where less accuracy and for short sight distance ( i.e. not more than 30 kilometres)It must be used in day time only.Cheaper than the luminous signal. Example of opaque signal (i)Target signal – suitable for distance up to 30 km(ii) Pole signal- distance up to 6 km(iii) Elevated signal – elevated tower is used as a signal and suitable for distance up to 30 to 40 km.(iv)Stone signpost(v)Pole and brush signal b)Luminous signal:- This type of signal used for geodetic survey because of quite distinct and have clear visibility even for long-distance (more than 30 km).It is used for the day as well as night also. Luminous signal-Sun signal-Light signal (i) Sun signal - also known as heliotrope. Heliotrope reflects sunrays towards the station. (ii) Night signals - observation is at night. Eg. Drummond's light and magnesium lamp with a parabolic reflector. principle of least squares- Least squares’ is a powerful statistical technique that may be used for ‘adjusting’ or estimating coordinates in survey control networks. The term adjustment is one in popular usage but it does not have any proper statistical meaning. A better term is ‘least squares estimation’ since nothing, especially observations, are adjusted. Rather, coordinates are estimated from the evidence provided by observations. The great advantage of least squares overall methods of estimation, such as traverse adjustments, is that least-squares are mathematically and statistically justifiable and, as such, is a fully rigorous method. It can be applied to any over the determined network but has the further advantage that it can be used on one-, two- and three-dimensional networks. A by-product of least squares solution is a set of statistical statements about the quality of the solution. These statistical statements may take the form of standard errors of the computed coordinates, error ellipses or ellipsoids describing the uncertainty of a position in two or three dimensions, standard errors of observations derived from computed coordinates and other meaningful statistics described later. The major practical drawback with least squares is that unless the network has only a small number of unknown points, or has very few redundant observations, amount of arithmetic manipulation makes method impractical without the aid of a computer and appropriate software. The examples and exercises in this material use very small networks to minimize computational effort for the reader, while demonstrating the principles. Real survey networks are usually very much largerQ10.Explain reduction to centre?A 10)It is defined as the process of computing the correct angle from an observed angle is called reduction to the centre.This method is often applied in triangulation when it is not possible to occupy a triangulation station.

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