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Unit3- Chain and compass traverse survey
Introduction and purpose
- in Chain surveying area to be surveyed is divided into number of triangles (suitable for small area)
- but when area is undulated and crowded with many details 'triangulation' is not possible in this test method of traversing is used
- in traversing framework consist of number of connected lines the lengths are measured by chain or tape and direction identified by angle measuring instrument
- the angle measuring instrument is compass hence the process is known as compass traversing
- consideration of traverse in an anticlockwise direction is always convenient
Closed traverse
- if the serving forming framework ends on the starting point it is known as close traverse
Open traverse
- is the survey-line forming fromework ends elsewhere is it is said to be an open traverse
- i.e. starts from A and end at 'F'
Definitions
1) True meridian
- line or plane both passing through geographical north pole geographical South pole and any point on the surface of the Earth is known as true meridian
- angle between true meridian and a line is known as true bearing
- it is also known as 'azimuth'
2) Magnetic meridian
- when a magnetic needle is suspended freely and balance properly unaffected by magnetic substances it indicates a direction the direction is known as magnetic meridian
- the angle between the magnetic meridian and aligning known as magnetic bearing
3) Arbitary meridian
- sometimes for the survey of a small area a convenient direction is assumed as a meridian known as the arbitary meridian
- sometimes the starting line of a survey is taken as the arbitary meridian
- the angle between the arbitrary meridian and a line is known as the arbitary of bearing of the line
4) Grid meredian
- sometimes for preparing a map from state agencies as zoom several lines parallel to the true meridian for a particular zone these lines termed as grid lines
5) Designation of magnetic bearing
I) Whole Circle Bearing (WCB)
II) Quadrantal Bearing (QB)
I) Whole Circle Bearing (WCB)
- the the magnetic bearing of a line measured clockwise from the north pole towards the line is known as the whole circle bearing of that line
- search a bearing may have value between 0° and 360°
- whole circle bearing of a line is obtained by prismatic compass
II) Quadrantal Bearing (QB)
- the magnetic bearing of a line measured clockwise or counterclockwise from the north pole and South pole (which nearer the line) towards the east or West is known as quadrantal bearing of the line
- this system consists of four quadrants- NE, SE, SW, NW
- the value of of a quadrantal bearing lies between 0° and 90°
- but quadrants should always mentioned
- obtained by surveyers compass
For example, QB of AB = N Q1 E
QB of AC = S Q2 E
QB of AD = S Q3 W
QB of AE = N Q4 W
6) Reduced Bearing (RB)
- when the wcb of line converted to quadrantal bearing it is termed as reduced bearing
- similar to the quadrantal bearing
- Value lies between 0° and 90°
Basic Surveying All Units Notes :
Fore bearing and back bearing 1) Fore Bearing
The vearing of a line measured in the direction of the progress of survey line is called 'fore bearing' of the line
2) Back Bearing
The bearing of a line measured in the direction opposite to the survey is called the 'back bearing' of the line
Magnetic declination
- the horizontal angle between the magnetic meridian and true meridian is known as magnetic declination
- when the north end of the magnetic needle is pointed towards the west side the true meridian the position termed as declination west
- when the north end of the magnetic needle is pointed towards east side of the true meridian the position is termed as declination East
Isogonic and agonic lines :
- line passing through points of equal declination are known as 'isogonic lines'.
- lines passing through points of zero declination is said to be agonic lines
Local attraction and it's adjustmet
- when the prismatic compass is located over any station by centering then the freely suspended magnetic needle will point towards the north direction
- but if the compass is centred nearby iron or steel structure or electric cable line such as LT line or transmission tower, lamp post carrying High voltage, Steel button of shirt or a steel wristwatch bunch of keys still tape etc.
- then it is observed that the magnetic needle of prismatic compass will not find exactly towards north but will deflect from its normal position
- in short the magnetic needle is under the influence of some external forces causing the deflection of needle from its normal position and showing some error in bearing of lines
- therefore it is concluded that the deviation of the magnetic needle from its normal position (north direction) under the influence of external force is called as local attraction
- local attraction shows some error in bearing of lines, therefore while selecting the stations for compass survey objects made up of Steel shall possibly be avoided
Deflection of local attraction
- detected by observing the difference between fore bearing and back bearing of each line
- is the difference is not exactly 180° then the stations on the both sides of the lines are affected by local attraction
Note:
1) If FB-BB =180° ; free from local attractions/same local attraction
2) if, FB-BB ≠180° ; affected by local attraction
Adjustmet of local attraction or eliminating the effect of local attraction or correction
- in this method the difference in fore bearing (F.B.) and back bearing (B.B.) of all the survey lines on the traverse are observed
- if for the line difference between fore bearing and back bearing is exactly equal to 180° then this line is selected and the process of applying correction is started from bearing of this line itself
- is the observed bearing is less than the corrected bearing the error is negative and collection applied to bearing is positive
- is the observed bearing is more than the corrected bearing the error is positive and correction applied to the bearing is negative
