Compass Survey

Definition: A branch of surveying in which directions of survey lines are determined with a  compass and lengths of lines arc measured with a tape or chain. Generally used to run a traverse.

Types of Meridians:

Meridian : Fixed line of reference about which directions or angles arc measured.

• True Meridian: line joining,true  north and true south. Established by Astronomical observations .
• Magnetic meridian: Direction shown by a  freely floating and balanced magnetic needle free from all other attractive forces (Line passing through magnetic north and south). Established by magnetic compass.
• Grid meridian: For survey of a state, the true meridian of central place is taken as a reference meridian for. whole state and is called Grid meridian.
• Arbitrary meridian : Meridian taken in any convenient direction towards a permanent and prominent mark or signal.

Used to determine relative  direction of various lines in a small traverse or small area. 

Types of Bearings: Bearing of a line is the angle between a meridian and a survey line

• True Bearing : True bearing of a line is horizontal angle between true meridian and the line. Also known as Azimuth. Does not change with time, it is a constant.
• Magnetic beating: Magnetic beating of a line is the horizontal angle which the line makes with magnetic north. Changes with time. Used for small areas. Measured with a magnetic compass.
• Grid bearing: Grid bearing of a line is the horizontal angle with Grid meridian.
• Arbitrary bearing : Arbitrary bearing of line is the horizontal angle with “Arbitrary meridian”.

Systems of Bearings:

Whole circle Bearing system (WCB):

• Bearing of line OA = θ1 ;OB = θ2 ; OC = θ3 ;OD = θ4
• Bearing of a line is measured always in clockwise from North end of reference.
• Bearing varies from 0° to 360°
• Prismatic compass is graduated in this system.
• Also called "Azimuthal system"

 Quadral Bearing(QB) system:

• Bearing   OA = NθE, OB = SαE, OC = SγW, OD = Nν W
• Bearing of a line is measured eastward or westward from North or South, which ever is nearer.
• Varies from 0° to 90°.
• Observed by surveyors compass
• Also called “Reduced bearings”.

Conversion of bearings from one system to other: very easy with the aid of a diagratm

•  Conversion of  W.C.B. Into R.B

Line	WCB	Rule for R.B or Q.B	Quadrant
OA	0° & 90°	Nϴ E	I
OB	90° & 180°	S ( 180°-ϴ ) E	II
OC	180° & 270°	S (ϴ - 180°) W	III
OD	270° & 360°	N( 360°-ϴ)	IV

•  Conversion of R.B into W.C.B

Line	WCB Between ()	Rule for R.B or Q.B	Quadrant
OA	N ϴ E	ϴ	I
OB	SϴE	(180°-ϴ)	II
OC	SϴW	(180°+ϴ)	III
OD	NϴW	(360°-ϴ)	IV

Note: When a line points towards North, East, South or West, it Is written as “Due North”, “Due

east”,"Due South” or “Due West” respectively.

Thus, N0° = Due North; N 90°E = S90°E=Due east

S0° = Due South; N90° W = S90°W =Due West

Fore bearing and backing hearing:

• The bearing of a line ¡n the direction of progress of survey indicated by an arrow is called fore. bearing (F.B).
• The bearing in an opposite direction to F.B or in the direction opposite to the survey is Back bearing (B.B) 
  

Determination of B.B for F.B:

1. F.B and B.B differ by 180°

• If F.B is given as W.C.B:    B.B = F.B + 180° if F.B < 180° and B.B = F.B- 180° if F.B > 180°
•  If F.B of a line is given as quadrantal bearing: B.B is numerically equal to F.B change ‘N’ for ‘S’ and vice versa. Change ‘E’ for ‘W’ and vice versa.

Important Note: Always better to draw rough sketch either for conversion of one system to

another or for working out B.B from F.B or vice versa.

Included ‘angle between bearings:

Can be calculated ‘very easily using a diagram

Whole circle bearings are given:

• Bearings of two lines measured from a common point. Included angle θ = θ2 - θ1 two lines from
  
• Bearing of two lines not measured from a common point.

             = B.B of previous line - F.B of next line

             =(180+ θ1)-θ2

Quadrantal bearings of two lines are given:

• Measured from a common point:

Calculation of bearings from Angles:

• Bearing of any one line (generally, the first line) is also required along with included angles.
• In a closed traverse, clock wise angles will be the interior angles if the traverse is run in the anti-clockwise direction.
• Bearing of any line = Bearing of preceding line + Included angle.
• If the sum is more than 180° deduct 180°. if the sum is less than 180° add 180°

Magnetic compass: 

It gives directly the magnetic bearings

Dip: is the inclination of the magnetic needle with the horizontal.

• Northern end is ,deflected down in the ‘Northern hemisphere” while the southern end is deflected down in the “Southern; hemisphere” 
• The dip is zero at equator and needle will remain horizontal.
• At a place near 70° North latitude and 96° West longitude, dip will he 90° This area is called north magnetic pole.
• Similarly near south magnetic pole, dip is 90°

Magnetic declination:---  (True bearing — magnetic bearing)

• Isogonic line is the line drawn through points  of same declination.
• Agonic line  is the line made up of points having a zero declination.
• If the magnetic meridian is to the right side (or eastern  side) of the true meridian, declination is Eastern or positive.
• If is  to the left side (or western side), it is said to  be western or negative.
• Magnetic declination at a place is not constant but varies from time to time.

Duirnal variation :-

• Variation ¡n a day
• More during day time and less during night time.
• More at magnetic  poles and less at equator.
• Considerably more in Summer  than in winter.
• Changes from year  to year.

Annual Varition: variation over a period of an year

Secular variation: variation over a period approximately 250 years.

Irregular variations : are due to magnetic storms, earthquakes and solar influences etc. 

Determination of true bearing

True bearing = magnetic bearing ± declination

+ sign for eastward declination

- Sign for westward declination. (WCB)

If Reduced bearing is given , it is advisable to draw diagram and calculate bearing.

Local Attraction

• If BB- FB ≠ 180
• The local attraction is due to influence of magnetic materials like heavy steel or nickel objects , electric poles, transmission lines etc.
• The local attraction due to key chains, steel buttons, steel pens should be ignored

Adjustments of Prismatic Compass :

Temporary adjustments : are those made at every set up of the instrument. They are

1. Centering: Process of keeping the instrument exactly over the station. Done by adjusting legs of tripod and using plumb bob. 
2. Levelling : Generally, tripod is provided with ball and socket arrangement with the help of which leveling is done.
3. Focusing the prism: Prism is moved up or down ¡n its  slice till the graduations in the ring are seen clear.

Permanent adjustments: Done when, the fundamental relations between parts  are disturbed.