ADVANCED SURVEYINGTRIANGULATION

Prof. Ujjval J. Solanki, Darshan Institute of Engineering and Technology-RAJKOT

Discussed points� Difference between Geodetic and Plane surveying� What is Triangulation.� Objectives of Triangulation.� Principle of Triangulation.� Triangulation figures or System.� Classification of Triangulation.� Inter visibility of triangulation stations ( Examples)� Selection of triangulation stations.� Measurements of Horizontal angle

Points to be discussed� Strength of figures� Field work in Triangulation.� Station Marks.� Signals� Towers and scaffolds� Base line.� The base Net.� Extension of Base line.� Correction to base line.� Satellite station and Reduction centre

Well conditioned triangle� The triangles of such a shape, in which

any error in angular measurement has aminimum effect upon the computedlengths, is known as well-conditionedtriangle.E:\DARSHAN TEACHING\DEGREECIVIL\ADVANCESURVEYING\Triangulation_survey8.955tri-net.pdf

Strength of figures� The strength of figure is a factor to be considered in

establishing a triangulation system to maintain thecomputations within a desired degree of precision.� For a given change in the angles, the sine of smallangles change more rapidly than those of largeangles. This suggests that smaller angles less than 30°should not be used in the computation oftriangulation.� If, due to unavoidable circumstances, angles less than30° are used, then it must be ensured that this isnot opposite the side whose length is required tobe computed for carrying forward the triangulationseries.

Field work in Triangulation� ROUTINE OF TRIANGULATION SURVEY� The routine of triangulation survey, broadly consists of� (a) field work, and (b) computations.� The field work of triangulation is divided into thefollowing operations :� (i) Reconnaissance� (ii) Erection of signals and towers� (iii) Measurement of base line� (iv) Measurement of horizontal angles� (v) Measurement of vertical angles� (vi) Astronomical observations to determine the azimuthof the lines.

Station Marks.� The triangulation stations should be permanentlymarked on the ground so that the Theodolite andsignal may be centered accurately over them.

� Points considered while markingthe exact position

Station Marks.

Signals�A signal is a device erected todefine the exact position of atriangulation station so that it canbe observed from other stations

Signals – Requirement � (i) It should clearly visible, kept at least 75 cm above the station mark.� (ii) Capable of being accurately centered over the

station mark.� (iii) It should be suitable for accurate bisection from

other stations.Classification of signals

� (i) Non-luminous, opaque or daylight signals� (ii) Luminous signals.� (iii) Night Signals

� Pole signal Target Signal Pole and bush signal

� Stone cairn Beacon � Non Luminous signalNon Luminous signal

Luminous signal - Heliotrope

Night signal

BASE LINE� The accuracy of an entire triangulation system depends

on that attained in the measurement of the base line.�� Selection of site for base lineSelection of site for base line1. The site should be fairly level2. The site should be free from obstructions3. The ground should be firm and smooth.4. The two extremities of the base line should be inter visible.5. The site should be such that well-conditioned triangles can

be obtained.6. The site should be such that a minimum length of the base

line as specified, is available.

BASE NET

EXTENSION OF BASE LINE� Selected shorter length due to

(i) Not possible to get suitable site for longer base.(ii) Difficult and expensive to measure a long base� Selection of base net

(i) small angles opposite to known side must avoid(ii) Make well condition triangle(iii) Length of base line as long as possible � Extension of base line.

Two ways – avoid ill-conditioned triangle(1) Base extension by prolongation and(2) Base extension by double sighting

Base extension by prolongation

Base extension by double sighting

CORRECTION TO BASE LINE MEASUREMENT

� TAPE CORRECTION1. Correction to absolute length 2. Correction to temperature3. Correction for pull or tension4. Correction for sag.5. Correction for slope6. Correction for alignment7. Reduction to mean seal level

Learn in sem. III so refer book

CORRECTION TO BASE LINE MEASUREMENT

� TAPE CORRECTIONSr.No Correction Nature of correction1 Correction to absolute length + ve OR – ve -Cumulative2 Correction to temperature + ve OR – ve -Cumulative3 Correction for pull or tension + ve OR – ve -Cumulative4 Correction for sag. – ve -Cumulative5 Correction for slope Always negative6 Correction for alignment Always negative7 Reduction to mean seal level Negative

Measurement of Horizontal angles� Two methods� 1) Method of Repetition.� 2) Method of reiteration

Satellite station

Satellite stationTo secure well-conditioned triangles or to havegood visibility, objects such as chimneys, churchspires, flat poles, towers, lighthouse, etc., areselected as triangulation stations.Such stations can be sighted from other stationsbut it is not possible to occupy the stationdirectly below such excellent targets for makingthe observations by setting up the instrumentover the station point.�

Satellite station/Eccentric stationAlso, signals are frequently blown out ofposition, and angles read on them have to becorrected to the true position of thetriangulation station. Thus, there are two typesof problems:1. When the instrument is not set up over thetrue station, and2. When the target is out of position.Such subsidiary stations are called assatellite or eccentric or false stations

Different position of Satellite station and Reduction to center

Satellite station –Position of satellite station-West side*� A,B and C Triangulation station� C-True station� S its satellite station.� Eccentric distance –d� Angle ACB =Ø� Angle ASB = θ� Angle BSC = γ� O –Point of intersection AC and BS.� ∝- Angle SAC� β -Angle SBC� a , b, and c side of CB, CA and AB respectively oftriangle ABC.

* Consider upper side North ↑

Satellite station –North side

Locations of satellite station with reference to triangulation stations C

� North side � South side

�

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