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

    00

    DE 9900 280

    00

    EA 209030 29

    030

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