Compass surveying

This ppt presentation covers compass surveying, which explains principal of compass surveying, Types of compass, Difference between compass, Bearing, Definitions related to compass surveying etc.

1. Rakesh Kumar Verma
Assistant Professor,
Civil Engineering Department, P.I.E.T.
Parul University
Compass Surveying

2. Definitions
True Meridian
True Bearing /Azimuth
Magnetic Meridian
Magnetic Bearing /Bearing
Arbitrary Meridian
Arbitrary Bearing.
Grid Meridian
Grid Bearing

3. TN
A
O
AZ
MN
A
MB
O
Magnetic North
Magnetic South
Magnetic
Meridian

4. North
South
Meridians Prime
Meridians
Equator

6. λ=40
Latitudes
Longitudes
Understanding of Lat. & Long.
Equator

7. Equatorial Plane
Location of Man
λ
α
α- Longitude
λ- Latitude
Understanding of Lat. & Long.
Observer's Plane

8. (i) Whole Circle Bearing
(WCB)
(ii) Quadrantal Bearing
(QB)
WCB QB
Designation of Magnetic Bearing
O
N N
W
S
S
E E W
SE
NW
SW
NE
O O
A
B
A
B
Clockwise rotation has to be followed

9. Reduced Bearing (RB) Fore And Back Bearing
Similar to Quadrant
Bearing
Obtained when,

Depended on direction
of main survey line
Bearing
WCB QB
Convert
N
Direction of Survey
BM
A B
BB FB
North

10. WCB-----QB
Reduced Bearing (RB)
O
A
φ
α
WCB reading=α=220ᵒ
QB reading=φ=220ᵒ-180ᵒ
=S 40ᵒW
N
S
E W

11. N
W
S
E
α
α=φ
WCBNE
180ᵒ-α=φ
WCBSE
α-180ᵒ=φ
WCBSW
360ᵒ-α=φ
WCBNW
Conversion of WCB to RB

12. N
W
S
E
φ
α=φ
RB NE
180ᵒ-φ1= α
RB SE
φ2+180ᵒ= α
RB SW
360ᵒ- φ3=α
RBNW
Conversion of RB to WCB
φ2
φ1
φ3

13. Fore Bearing & Back Bearing
Bearing taken in forward direction of survey line-
FB



Bearing taken in backward direction of survey
line- BB

FB
BB
A
B
B
A

14. 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 of the true meridian,
the position is termed ‘Declination West’ ().

When the north end of the magnetic needle is
pointed towards the east side of the true meridian,
the position is termed ‘Declination East’

15. Magnetic Declination
True North
E
W
W
S
S
E

16. Isogonic and Agonic Lines
Lines Passing Through Points Of Equal
Declination are known as ‘isogonic’ lines.
The Survey of India Department has prepared a map
of India in which the isogonic and agonic lines are
shown properly as a guideline to conduct the
compass survey in different parts of the country.

17. Isogonic and Agonic Lines

18. Dip of the Magnetic
Needle
Inclination
from the
horizontal
plane

Zero
inclination at
the equator

19. Dip of the Magnetic Needle

20. Local Attraction
Error introduced in direction of magnetic needle due to
magnetic materials such as iron ore, steel structures,
electric cables conveying current; etc.
Then it does not show the actual north. This
phenomenon is known as ‘local attraction’.
If




Error compensation
Divide equally in FB & BB

Error of local attraction FB-BB≠ 180 degrees
FB-BB= 180 degrees Free from local attraction

21. Example
For example, consider the case when

Observed FB of AB = 60030’

Observed BB of AB = 24000’

Calculated BB of AB = 600300 + 18000’ = 240030’

Corrected BB of AB = 1/2 (24000’ + 240030’) = 240015’

Hence, Corrected FB of AB = 240015’ – 18000’ = 60015’

22. Principle of Compass Surveying
Traversing, which Involves A Series of Connected Lines.
The magnetic bearings of the lines are measured by prismatic
compass and
The distances of the lines are measured by chain.
Such survey does not require the formation of a network of
triangles.

Interior details are located by taking offsets from the main survey
lines. Sometimes subsidiary lines may be taken for locating these
details.

Compass surveying is not recommended for areas where local
attraction is suspected due to the presence of magnetic substances
like steel structures, iron ore deposits, electric cables conveying
current, and so on.

23. TRAVERSING
Well Defined Network of Connected Lines
The sides of the traverse are known as ‘traverse legs’.

Measurements



A traverse may be of two types –
Closed traverse
1.closed loop (i.e. when the finishing point coincides with the starting
point
Open traverse
1.connected lines extends along a general direction and does not return to
the starting point,
2.Open traverse is suitable for the survey of roads, rivers, coast lines, etc
Lengths of Lines
Directions
Chain
Compass or Theodolite

24. Methods of Traversing
Chain traversing (by chain angle)
Compass traversing (by free needle)
Theodolite traversing (by fast needle) and
Plane table traversing (by plane table)

25. Check on Angular Measurements
(a) The sum of the measured Interior Angles should be
equal to (2N – 4) x 90ᵒ where N is the number of sides of
the traverse.

(b) The sum of the measured Exterior Angles should be
equal to (2N + 4) x 90ᵒ

(c) The algebraic sum of the Deflection Angles should be
equal to 360ᵒ

Right-hand deflection is considered positive and left-hand
deflection negative.

26. Check on Linear Measurement
(a) The lines should be measurement once each on two
different days (along opposite directions). Both
measurements should tally.

(b) Linear measurements should also be taken by the
stadia method. The measurements by chaining and by
the stadia method should tally.

27. Check on Open Traverse
No Direct checks available, but following
methods can be adopted
1. Taking cut-off lines
2. Taking an auxiliary point

28. A
F
E
C
D
B
N
N
N
N
N
For Check
Taking Cut-off Lines
Cut-off lines are taken between some intermediate stations of the open traverse.
Let AD and DG be the cut-off lines. The lengths and magnetic bearings of the cut-off
lines are measured accurately. After plotting the traverse, the distances and bearings
are noted from the map. These distances and bearings should tally with the actual
records from the field

29. Taking an Auxiliary Point
A
D
C
B
E
P
N
N
N
N
N
A permanent point ‘P’ is selected on one side of it. If the survey is carried out
accurately and so is the plotting, all the measured bearings of P when plotted should
meet at the point P. The permanent point P is known as the ‘auxiliary point’

30. The prismatic compass The Surveyor’s compass
Types of Compass

31. Components of Prismatic Compass
Construction
Zero degree marked at
South and 90 degree at
North

32. Vanes
Eye Vane
Guide Vane
Magnetic Needle
Bubble Tube
Screw
Circular
Metal Box
Dia.-8 to
10cm
Fixed
Graduated
Plane

33. Sr.
No.
Base Of
Comparison

Prismatic Compass

Surveyor Compass
1 First look Prism at one end and slit on
other
No prism only Slit at both
end
2 Use of Tripod
Stand
May or may not use along,
Steady hold in hand also give
good results
Use of Tripod stand is
necessary
3 Observation/
Readings
Taken with help of prism
provided by eye slit
Directly read from top of
compass
4 Magnetic
Needle
Does not act as index Act as index
5 Graduation WCB system QB system
6 Graduation
marking

Appear inverted from top,
Zero at south & 180 degrees
at north
Mark directly Zero at North
and 90 degree at East
7 Graduated
circle
Attached with needle, Does
not rotate with line of sight
Permanently attached with
box, rotates with line of sight
Comparison between
Prismatic Compass and Surveyor Compass

34. Temporary Adjustment of Compass
Centering
Levelling
Focusing
1.Adjustment of prism
2.Bisecting/Observation of bearing

35. North
φ1
φ3
φ2
Determination of Reduced Bearing
A
O
C
B
Angle AOB=φ2-φ1

Angle BOC= φ3-φ2 Angle AOC= φ3-φ1

36. North
φ3
φ2
φ1
φ4
C
A
B
Field Work
Included Angles BAC= φ2
Included Angles ACB=φ3
Included Angles ABC=φ4
Determination of Included Angles
Dist. AB