COMPASS SURVEYING

             Muhammad Nadeem Amin Khokar

                    H.O.D Civi Technology

     Allama Iqbal Poly Technic Institute Khudian Khas Kasur 

COMPASS SURVEYING :

  Prismatic  compass  –  surveyor’s  compass  –  bearing  –  system  of

conversions – Local attraction – magnetic declination  – Dip – Traversing –

Traversing. 

                                          COMPASS 

 Compass Surveying .

The branch of surveying in which direction of survey line are

determine by a compass and their length by a chain or tape is called compass

surveying.

This type of survey can be used to measure large areas with

reasonable speed and accuracy.

prismatic compass 

Prismatic compass is a instrument used to measure the bearing of a line. It consists of a magnetic needle pivoted at the center and is free to rotate. The area below the magnetic needle is graduated between 0 to 360 degrees. The instrument cover consists of a sighting vane and vertical hair to align the compass along the instrument station and the staff station. 

 Traverse 

A series of connected straight lines each joining two points   is

called traverse. Traverse may be either a closed traverse (or) open traverse.

Traversing  using  compass  is  done  by  observing  magnetic  bearings  of surrounding points from a point and shifting the point of observation along the traverse direction.

Fore and Back bearing

A line may be expressed by two bearings observed from the either end of the line. Both the bearings expressed in whole circle bearing deter each other by 180°. The bearing of the line in the direction of progress of survey is called fore or Forward bearing while the bearing opposite to the direction of the progress of survey is known as reverse of Back bearing.

closing error

When a close traverse is plotted from the field measurements the ends stations of a traverse generally doesn’t coincide exactly in the starting station.  

This is due to the error in field observations such an error of traverse is known as closing error or error of closer.

local attraction in determining bearing of a line

Bearing of a line is determined using a magnetic needle. In

certain locations presence of high voltage cables, railway tracks, etc alters the orientation of the magnetic needle towards the north, there by erroneous readings are observed. This can be minimized by taking back bearings to the same line or by taking observations from intermediate points in between the stations.

Dip

When a magnetic needle is suspended freely it always points

north. Due to certain factors magnetic needle may not point true north, it points at a direction away from north called magnetic north. The included angle between magnetic north and true north is called dip or declination.

true and magnetic meridian.

True meridian is the angle between the observed line and

actual north. This is the bearing observed in the absence of local attraction or magnetic declination. Magnetic meridian is the angle between the observed line

and magnetic north. Magnetic north may deviate from true north due to local attraction or magnetic declination.

True Meridian = Magnetic Meridian ~ Dip

Compass Surveying:-

 The branch of surveying in which direction of survey line are determine by a compass and their length by a chain or tape is called compass surveying. The method of chain surveying is used for small areas of survey. If the area to be surveyed is comparatively large with undulations compass survey

is adopted, before recommending the compass survey for any area, it must be ascertained the area is not magnetically declined. 

Traverse:- 

A series of connected straight lines each joining two points   is called traverse.  Traverse  may  be  either  a  closed  traverse  (or)  open  traverse. Traversing  using  compass  is  done  by  observing  magnetic  bearings  of surrounding points from a point and shifting the point of observation along the

traverse direction.

Designation of bearings:-

 Bearings of survey lines are design as

(i)    Whole circle bearing 

(ii)    Quadrental bearing 

Whole Circle Bearing (WCB)

In this method bearings of lines are observed from magnetic north and are observed directly from the prismatic compass.

Quadrental Bearing (QB )

These are bearings of lines from north or south towards east or west.

These values are usually reduced from observed whole circle bearing for the ease of calculation of included angles and plotting.

Convert the following whole circle bearing to quadrental bearing. 

(i) 12° 45’   (ii) 160° 10’  (iii) 285° 15’

(i)    12° 45

Quadrental bearing of 12° 45’ is  N12° 45’E

(ii)    160° 10’

Quadrental bearing of 160° 10’ is S19.50’E

(iii) 285° 15’

         Quadrental bearing of 285° 15’ is N 74° 45’ W

Convert the follow quadrental bearing to whole circle bearing 

   (i) N 30° 30’E  (ii) S 70° 42’ E     (iii)  N 85° 10’ W

 Whole circle bearing of N 30° 30’E is 30° 30’

(ii) S 70° 42’ E  

   Whole circle bearing of S 70° 42’ E  is = 180° - 70° 42’ = 109° 18’ 

(iii) N 85° 10’ W

Whole circle bearing of N 85° 10’ W is = 360° - 85° 10’ = 274° 50’ 

Fore and Back bearing:-

A line may be expressed by two bearings observed from the either end of  the  line. The  bearing  of  the line  expressed  along  the  line  of  traverse  is called fore bearing and bearing observed opposite to the line of traverse is called  back  bearing.  Both  the  bearings  expressed  in  whole  circle  bearing deter each other by 180°. The bearing of the line in the direction of progress of survey is called fore or forward bearing while the bearing opposite to the direction of the progress of survey is known as reverse of Back bearing By  observation  of  fore  bearing  and  back  bearing  of  a  line  two observations  are  made  to  a  line  minimizing  error  and  improving  the accuracy.This  also  helps  in  identifying  local  attraction,  if  the  difference between the fore and back bearing expressed in whole circle bearing is not exactly  180°,  then  one  or  both  of    the  observation  points  suffers  local attraction.

The fore bearing of travels site are as follows:  

AB = 85° 10’ 

CD = 265° 51’

BC = 155° 30’      

DE = 355° 30’ 

Find their B

ack Bearings?

Back bearing of 

AB = 180° + 85° 10’  = 265° 10’

 Back bearing of BC = 180° + 155° 30’  = 335°

Back bearing of CD = 360° = 265° 5’ – 180°  = 85° 5

Back bearing of DE = 355° 30’ – 180° = 175° 30’

precautions  to  be  taken  during  compass  surveying  for  maximum

accuracy of the observations.

Precautions to be taken in c

ompass survey.   

The instrumental and observation error can be minimized by taking the

following precisions.

(i) Setup & level the co

mpass carefully. 

(ii) Stop the vibration of the needle by gently pressing the break pin so that it

may come to rest soon. 

(iii) Always lift along the needle and not across to avoid parallax. 

(iv) When  the instrument

is  not  in  use  its magnetic  needle  should  be  kept

above the pivot. If, not the pivot is subjected to unnecessary wear which may

cause  sluggishness  of  the  magnetic  needle.  Before  taking  a  reading  the

compass box should be gently tapped to ensure the magnetic needle is freely

swinging and has not come to rest due to friction of the pivot 

(v) Station’s should be selected such that they are away from sources of local

attraction. 

(vi) Fore & Back bearing of each line should be taken to guard against the

local attraction, if the compass cannot be set at the end of line, the bearings

may be taken from any intermediate point along the line. 

(vii) Avoid taking a reading in the wrong direction.

(viii) If the glass cover has been dusted with hand kerchief. The glass cover

may be charged with electro static current and needle adheres to the glass

cover. This may be avoided by applying moist finger to the glass. 

(ix) Objective and the eye piece must be straight before making observations.

Compute magnetic declination for the following observations.

S.No.	Station	F.B	B.B
1.	AB	80° 45’	260°
2.	BC	130° 30’	311° 35’
3.	CD	240° 15’	60° 15
4.	DA	290° 30’	110°10’

AB => 260° - 80° 45’            = 179° 15’

BC => 130° 30’  ~  311° 35’ = 181° 5’

DA =>  290° 30’ ~ 110° 10’= 180° 20’

Magnetic error 1° 30’ w 

Compute the included angles.

S.No	Station	Fore bearing	Back bearing	Inc Angle

           

 . Losing Error.

Adjustment of closing errors:-

When a close traverse is plotted from the field measurements the ends stations of a traverse generally doesn’t coincide exactly in the starting station.  This is due to the error in field observations such an error of traverse is known  as  closing  error or  error  of  closer.  When  the  angular  and  linear measurement is of equal proportion graphical adjustment of the traverse may be made. Correction is applied to the length as well as bearing of the lines in proportion  to  their  links.  Graphical  method  is  also  known  as  proportionate method of adjustment.

When  the  closing  error  is  to  be  adjusted  with  angles  alone  equal distribution  of  error  is  done  to  all  the  observed  angles.  Total  error  is calculated,  from  which  total  correction  is  arrived.  The  correction  is  equally divided to all the stations eliminating closing error. plane table surveying the methods employed in plane table surveying. 

                   Muhammad Nadeem Amin Khokar

                       H.O.D Civi Technology

     Allama Iqbal Poly Technic Institute Khudian Khas Kasur