# surveying - module ii - compass surveying

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• OPTC Edavanna Page 1

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) Surveyors 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

• OPTC Edavanna Page 2

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 1800 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 , NNumber of stations) the field work should be repeated.

If it is within permissible limits, it can adjusted graphically

by Bowditchs rule.

• OPTC Edavanna Page 3

9. Give in a tabular form, the difference between prismatic compass and

surveyors compass.

Prismatic compass Surveyors 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 doesnt 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 observers 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 cant be re-established if points lost.

• OPTC Edavanna Page 4

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 80010 259

00

BC 120020 301

050

CD 170050 350

050

DE 230010 49

030

EA 310020 130

015

Line F.B B.B included angle

AB 80010 259

00 A = 5005

BC 120020 301

050 B =138040

CD 170050 350

050 C = 1310

DE 230010 49

030 D = 120040

EA 310020 130

015 E = 99010

Sum of included angles = 539035

Line F.B B.B included angle

AB 80040 260

040 A = 50010

BC 121055 301055 B = 138045

CD 170050 350

050 C = 13105

DE 23005 50

05 D = 120045

EA 310050 130

050 E = 99015

Sum of included angles = 5400

• OPTC Edavanna Page 5

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 45045 226

010

BC 96055 277

05

CD 29045 209

010

DE 324048 144

048

Line F.B

AB 64030

BC 13000

CD 47050

DE 210030

EA 310030

• OPTC Edavanna Page 6

Ans:

16. Observed bearings are given below. Determine the corrected included angles?

Line F.B B.B

AB 150030 329

045

BC 7800 256

030

CD 42030 223

045

DE 315045 134

015

EA 220015 40

015

Ans:

• OPTC Edavanna Page 7

17. The value of magnetic declination at a place is 5020 W. convert the magnetic

bearings into true bearings

(1) S 46020 E

(2) S 78040 W

Ans: (1) TB = MB Declination

TB = MB - Declination

= S (46020 - 5

020 ) E

= S (410 ) E

(2) S 78040 W

TB = MB Declination

= S (78040 - 5

020 ) W

= S (73020) 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 48012 4

008 E

PQ S 10012 W 2

004 W

• OPTC Edavanna Page 8

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 80030 260

030

BC 32015 173

00

CD 106015 208