Surveying - Module II - compass surveying

Orphanage polytechnic Edavanna

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MODULE II : Compass surveying
1. Explain the concept of traverse? What is closed and open traverse?
Traversing is the type of survey in which a number of connected survey lines forms
the frame work. The directions and length of survey lines are measured using
suitable instruments.






2. Define compass surveying?
Compass surveying is the type of surveying in which the direction of the survey
lines are measured with a compass and the length of the survey lines are measured
with a tape or chain in the field.
3. Write the names of the instruments used in compass surveying?
(i) Instruments for the direct measurement of directions:
a) Surveyor’s compass
b) Prismatic compass.
(ii) Instruments for the measurement of angles:
a) Sextant
b) Theodolite
4. What is mean by bearing?
Bearing is the horizontal angle between the meridian and one of the extremities of
line. They are of three types
1. True bearing
2. Magnetic bearing
3. Arbitrary bearing
5. What is mean by fore bearing and back bearing?
Fore bearing: If the bearing of line AB is measured from A towards B
(direction of the progress)
Back bearing: If the bearing of line AB is measured
from B towards A




Closed traverse: Survey lines
forms a closed figure or circuit.
Orphanage Polytechnic College, Edavanna
REVISION PACKAGE
Surveying - I ( Revision 2015)
Open traverse: Survey lines forms
an open figure. eg: canal, roads, etc

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6. What is mean by local attraction? What are its sources?
It is a term used to denote any influence, such as magnetic substances, which
prevents the needle from pointing to the magnetic north in a given locality.
Sources of Local attraction
• Magnetic ore, Wire carrying electric current
• Steel structures. Iron pipes
• keys, High voltage transmission line
• Electric pole etc.
7. Briefly explain how local attraction is detected and eliminated?
Detection : By observing F.B & B.B , and checking whether the
difference 180
0
 There is local attraction
We confirm the local attraction only if the difference is not due to
observational or other personal or natural errors. If ( FB - BB ) = 180,
then the bearings measured in that station is free from local attraction. If
detected, that has to be eliminated
Correction: Two methods (1) First method (2) Second method
First method:
 Find the included angles
 Sum of the included angles = (2n-4) x 90 , n – number of lines
 If not, then distribute the total error equally to all interior angles of the
traverse
 Then starting from unaffected line the bearings of all other lines are corrected
using corrected included angles
Second method:
 Find the Unaffected line
 Then, Starting from the unaffected line, the bearing of other affected lines
are corrected by finding the amount of correction at each station.

8. What is mean by closing error in closed traverse? State the methods of
adjusting of closing error?
When a closed traverse is plotted, the finishing and starting points may not coincide.
The distance by which the traverse fails to close is called closing error
• If the closing error exceeds a permissible limit
( 15’√ , NNumber of stations) the field work should be repeated.
• If it is within permissible limits, it can adjusted graphically
by Bowditch’s rule.

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9. Give in a tabular form, the difference between prismatic compass and
surveyor’s compass.
Prismatic compass Surveyor’s compass
The reading is taken with help of prism No prism for reading
sighting and reading simultaneously not simultaneously
Tripod not sufficient Tripod is sufficient
Object vane: metal vane with vertical
hair
Eye vane: small metal vane with slit
Object vane: metal vane with vertical
hair
Eye vane: small metal vane with a fine
slit
Graduated ring doesn't rotate along with
line of sight
Graduated ring rotate along with line of
sight
Graduations in WCB Graduations in RB
‘Broad type magnetic needle’ doesn’t act
as index
‘Edge bar’ magnetic needle act as index
also
Graduations are marked inverted Graduations are marked erect

10. What do you mean by meridian? Explain each?
Meridian is some reference direction based on which direction of survey line is
measured. They are of three types.
1. True meridian ( Constant)
2. Magnetic meridian ( Changing)
3. Arbitrary meridian
True Meridian:
 Line passing through geographic north and south pole and observer’s position
 Position is fixed
 Established by astronomical observations
 Used for large extent and accurate survey (land boundary)
Magnetic meridian:
 Line passing through the direction shown by freely suspended magnetic needle
 Affected by many things i.e. magnetic substances
 Position varies with time (why? not found yet)
Arbitrary meridian:
 Line passing through the direction towards some permanent point of reference
 Used for survey of limited extent
 Disadvantage: Meridian can’t be re-established if points lost.

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11. The following are the bearings taken on a closed traverse. Compute the interior
angles and correct them for observational errors. Assuming the observed
bearing of the line CD to be correct, adjust the bearing of the remaining sides.






Ans:








Theoretically, Sum of the included angles = (2n-4) x 90= 540
Error = -25’  Correction = +25’
Five included angles

Apply +5’

correction for all included angles







12. Draw a neat sketch of a prismatic compass and identify the parts.






Line F.B B.B
AB 80
0
10’ 259
0
0’
BC 120
0
20’ 301
0
50’
CD 170
0
50’ 350
0
50’
DE 230
0
10’ 49
0
30’
EA 310
0
20’ 130
0
15’
Line F.B B.B included angle
AB 80
0
10’ 259
0
0’ ∠ A = 50
0
5’
BC 120
0
20’ 301
0
50’ ∠ B =138
0
40’
CD 170
0
50’ 350
0
50’ ∠ C = 131
0

DE 230
0
10’ 49
0
30’ ∠ D = 120
0
40’
EA 310
0
20’ 130
0
15’ ∠ E = 99
0
10’
Sum of included angles = 539
035’
Line F.B B.B included angle
AB 80
0
40’ 260
0
40’ ∠ A = 50
0
10’
BC 121055’ 301
0
55’ ∠ B = 138
0
45’
CD 170
0
50’ 350
0
50’ ∠ C = 131
0
5’
DE 230
0
5’ 50
0
5’ ∠ D = 120
0
45’
EA 310
0
50’ 130
0
50’ ∠ E = 99
0
15’
Sum of included angles = 540
0

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13. The following bearings were observed with a compass. Calculate the interior
angles:





Ans:







14. What do you mean by :
(i) True bearing of a line (ii) Isogonic line (iii) Agonic line
(iv) Magnetic bearing (v) Magnetic declination (vi) Magnetic dip

True bearing of a line: It is the horizontal angle between the true meridian and a
survey line. It is also called as azimuth of the line.
Isogonic line: It is the line drawn through the points of same declination
Agonic line: It is the line joining points of zero declination
Magnetic bearing: The angle between the magnetic meridian and a survey line is
known as magnetic bearing. It changes with time.
Magnetic declination: The horizontal angle b/w the magnetic meridian and true
meridian. They are of two types, eastern and western declination.
Magnetic Dip: Due to the magnetic influence of the earth, the needle does not
remain in the balanced position. This inclination of the needle with the horizontal is
known as the dip of the magnetic needle.

15. Following fore and back bearings were observed in running a traverse. At what
stations do you suspect local attraction and determine the corrected bearings.
Line F.B B.B
AB 45
0
45’ 226
0
10’
BC 96
0
55’ 277
0
5’
CD 29
0
45’ 209
0
10’
DE 324
0
48’ 144
0
48’




Line F.B
AB 64
0
30’
BC 130
0
0’
CD 47
0
50’
DE 210
0
30’
EA 310
0
30’

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Ans:











16. Observed bearings are given below. Determine the corrected included angles?
Line F.B B.B
AB 150
0
30’ 329
0
45’
BC 78
0
0’ 256
0
30’
CD 42
0
30’ 223
0
45’
DE 315
0
45’ 134
0
15’
EA 220
0
15’ 40
0
15’

Ans:

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17. The value of magnetic declination at a place is 5
0
20’ W. convert the magnetic
bearings into true bearings
(1) S 46
0
20’ E
(2) S 78
0
40’ W
Ans: (1) TB = MB Declination
TB = MB - Declination
= S (46
0
20’ - 5
0
20’ ) E
= S (41
0
) E
(2) S 78
0
40’ W
TB = MB – Declination
= S (78
0
40’ - 5
0
20’ ) W
= S (73
0
20’) W
18. Find the magnetic bearing of the lines from the following true bearing and
declination



Ans:
Line AB




Line PQ





Line True bearing Declination
AB 48
0
12’ 4
0
08’ E
PQ S 10
0
12’ W 2
0
04’ W

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19. The following bearings were observed in traversing with a compass at a place
where local attraction was suspected. At what station do you suspect local
attraction? Determine the corrected bearings of the lines.






ans:

Line F.B B.B
AB 80
0
30’ 260
0
30’
BC 32
0
15’ 173
0
0’
CD 106
0
15’ 208
0
0’
DE 99
0
0’ 280
0
0’
EA 209
0
30’ 29
0
30’