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 25900 BC 120020 301050 CD 170050 350050 DE 230010 49030 EA 310020 130015 Line F.B B.B included angle AB 80010 25900 A = 5005 BC 120020 301050 B =138040 CD 170050 350050 C = 1310 DE 230010 49030 D = 120040 EA 310020 130015 E = 99010 Sum of included angles = 539035 Line F.B B.B included angle AB 80040 260040 A = 50010 BC 121055 301055 B = 138045 CD 170050 350050 C = 13105 DE 23005 5005 D = 120045 EA 310050 130050 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 226010 BC 96055 27705 CD 29045 209010 DE 324048 144048 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 329045 BC 7800 256030 CD 42030 223045 DE 315045 134015 EA 220015 40015 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 - 5020 ) E = S (410 ) E (2) S 78040 W TB = MB Declination = S (78040 - 5020 ) 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 4008 E PQ S 10012 W 2004 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 260030 BC 32015 17300 CD 106015 20800 DE 9900 28000 EA 209030 29030