Compass traversing.

1. Traversing BY. M.Z. J

2. Introduction to traversing • In chain surveying the area is to be divided into number of triangles. This method is suitable for fairly level ground covering small areas . But when area is large , undulating, and crowded with many details , triangulation is not possible, in such cases method of traversing is adopted. • In traversing framework consist of a number of series of connected straight lines .

3. The length of Traverse line is measured by chain or tape and directions identified by angle measuring instrument( Theodolite, compass, etc) • Traverse are classified as • Open traverse and • Closed traverse.

4. 1)Open Traverse: Traverse which neither returns to its starting point nor closes on any other known station is called an open traverse. In this case a series of connected lines extends in the same general direction.

5. 2) Close Traverse : traverse which either originates from a station and return to its to same station completing a circuit or runs between two known station is called closed traverse. • Closed circuit of traverse legs is called traverse circuit.

6. Closed Traverse B A C D F E

7. Classification of traverse based on instrument • Chain Traversing • Compass Traversing • Theodolite Traversing • Plane table Traversing • Tacheometric Traversing etc.

8. Compass surveying • The branch of surveying in which direction of survey lines are determined by compass and their lengths by chain or tape is called compass surveying. • In compass freely suspended magnetic needle directs to north- south direction and the bearing of line is obtained by line of sight. • Compass in an anticlockwise direction is always convenient in running the survey lines

9. Principle of compass surveying • The principle of compass surveying is traversing; which involves a series of connected lines. • The magnetic bearing of the lines are measured by prismatic compass. • Compass surveying is recommended when the area is large, undulating and crowded with many 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 , and so on.

10. Compass • A compass is a small instrument essentially consisting of a graduated circle, and a line of sight. • The compass can not measures angle between two lines directly but can measure angle of a line with reference to magnetic meridian at the instrument station point is called magnetic bearing of a line. • In compass surveying two types of compass are most commonly used. • Prismatic compass and • Surveyor’s compass.

11. Prismatic compass

12. Prismatic compass • Aprismatic compass is a navigation and surveying instrument which is extensively used for determining direction, bearings of survey lines and included angles between them. • The compass calculates bearings of lines with respect to magnetic north. The included angles can then be calculated using suitable formulas in case of clockwise and anti-clockwise traverse respectively. For each survey line in the traverse, surveyors take two bearings that is fore bearing and back bearing which should exactly differ by 180 if local attraction is negligible. • The name Prismatic compass is given to it because it essentially consists of a prism which is used for taking observations more accurately. Used only to measure Whole circle bearing (0-360)

16. Temporary adjustment of prismatic compass • The following procedure should be adopted after fixing the prismatic compass on the tripod for measuring the bearing of a line. • Centering : Centering is the operation in which compass is kept exactly over the station from where the bearing is to be determined. The centering is checked by plumb bob or dropping a small pebble from the underside of the compass. If the pebble falls on the top of the peg then the centering is correct, if not then the centering is corrected by adjusting the legs of the tripod.

17. Leveling : Leveling of the compass is done with the aim to freely swing the graduated circular ring of the prismatic compass. The ball and socket arrangement on the tripod will help to achieve a proper level of the compass. This can be checked by rolling round pencil on glass cover. • Focusing: the prism is moved up or down in its slide till the graduations on the aluminum ring are seen clear, sharp and perfect focus. The position of the prism will depend upon the vision of the observer.

18. Observing Bearing of Line NORTH OBJECT B • Consider a line AB of which the magnetic bearing is to be taken. • By fixing the ranging rod at station B we get the magnetic bearing of needle w.r.t north pole. • The enlarged portion gives actual pattern of graduations marked on ring. 180 270 A 90 0 LINE OF SIGHT SOUTH

19. The Surveyor`s Compass • It is similar to a prismatic compass except that it has a only plain eye slit instead of eye slit with prism and eye hole. • This compass is having pointed needle in place of broad form needle as in case of prismatic compass. • Prismatic compass helps in taking Quadrant bearing which vary from 0-90

21. Working of Surveyor`s Compass • 1) Centering • 2) LEVELING • Centering and levelling are same as prismatic compass but third observation differs from that. • 3) OBSERVING THE BEARING OF A LINE : in this compass ,the reading is taken from the top of glass and under the tip of north end of the magnetic needle directly. No prism is provided here.

22. Meridian • The fixed direction on surface of earth , with reference to which , bearings of survey lines are measured is called meridian. • True meridian: the line or plane passing through the geographical north pole , geographical south pole and any point on surface of earth is true or geographical meridian. True meridian remains constant.

23. True Meridian

24. True meridian is most commonly used, maps prepared by national survey department are based on true meridian. • The horizontal angle between true meridian and survey line measured in clockwise direction is termed as true bearing of the survey line. • Azimuth is smaller angle which survey line makes with true meridian. Azimuth does not specify direction of measuring angle between true meridian and survey line.

25. N E W • Like in given figure NAB is a true bearing measured clock wise from North while an acute angle NAB measured anticlockwise from north is Azimuth. B S A

26. Magnetic Meridian: • When magnetic needle is suspended freely, unaffected by local attraction it indicates a direction is called magnetic meridian. • Magnetic meridian does not coincides with true meridian. • True bearing is obtained by sum or difference of magnetic bearing and magnetic declination. • The horizontal angle between Magnetic meridian and survey line measured in clockwise direction is termed as Magnetic bearing of the survey line.

28. Magnetic Meridian:

29. Magnetic bearing does not remains constant that’s why true bearing is usually used for surveying a traverse. • Horizontal Angle between True meridian and Magnetic meridian is called magnetic declination. • The lines passing through points of equal declination are known as Isogonic line. • The lines passing through points of zero declination are known as Agonic line.

30. Arbitrary Meridian: • The Convenient direction which is assumed as a meridian for measuring of survey line is called Arbitrary Meridian. Arbitrary Meridians are assumed for survey of small plots of land.

31. Designation of bearing • The bearing are designated in the following two system:- • 1) Whole Circle Bearing System.(W.C.B) • 2) Quardrantal Bearing System.(Q.B)

32. Latitude ( Northing or southing). Latitude of survey line is defined as its co-ordinate measured parallel to assumed meridian. sometimes latitudes are also called meridian. • Longitude or departure ( easting or westing). Longitude of survey line is defined as its co-ordinate measured perpendicular to assumed meridian. sometimes Longitude are also called perpendiculars.

34. Quadrant bearings. • Bearing is a horizontal angle measured from reference line to any survey line. • Magnetic bearing measured clockwise or counter clockwise from north pole or south pole whichever is nearer to line towards east or west is called Quadrant bearing. • It consist of 4 quadrants NE, SE, SW, NW.

35. Each quadrant begins with 0 and end on 90. • Quadrant bearings are always defined by the reference meridian. • Value of Quadrant bearings lies between 0-90, but quadrant must always be mentioned. • Like if a line is in first Quadrant making an angle of 49, so it will be written as N 49 E, or if line question is in second Quadrant , angle will be written as S 49 E.

36. N N50°E A D NW NE O W E SW SE C B S

37. Whole Circle Bearing N E W • Magnetic bearing measured clock wise from north reference line to any survey line is called Whole Circle Bearing. • Its value varies from 0-360 and is measured by prismatic compass. • The quadrant start from north and progress in a clockwise direction as the first quadrant is 0ᴼ to 90ᴼ in clockwise direction , 2nd 90ᴼ to 180ᴼ , 3rd 180ᴼ to 270ᴼ, and up to 360ᴼ is 4th one. D A S O C B

39. Reduce Bearing • When WCB is converted in Q.B it is called Reduce bearing. • It is similar to Q.B. • Following method is followed to change WCB in Q.B.

40. Dip of the magnetic needle: • If the needle is perfectly balanced before magnetization, it does not remain in the balanced position after it is magnetized. This is due to the magnetic influence of the earth. • The needle is found to be inclined towards the pole. This inclination of the needle with the horizontal is known as dip of the magnetic needle.

43. Fore Bearing and Back Bearing • The bearing of a line measured in the direction of progressing line is called Fore Bearing(FB). • The bearing of a line measured in the direction opposite to progressing line is called Back Bearing(BB). • Both bearings expressed in WCB differ by 180. • If expressed in Q.B they must have same value, but different meridian.

44. BB= FB+180 or FB-180 also FB= BB+ 180, or BB-180 depends either value is less than 180 or more than 180. N N N N Case-i Case -ii 180° A B S S S S FB BB BB FB B A

45. N N • Like , if fore bearing is N 45 E, back bearing will be S 45 W , 45 is same for both. • If difference of fore bearing and Back bearing is not 180 or value FB and BB in Q.B system is not same, the line will said be affected by local attraction. S S B BB FB FB E W A

46. Angle between two lines more than 180 is called exterior angle and angle between two lines less than 180 is called interior angle or included angle. Exterior angle Interior angle B A

47. How to calculate interior and exterior angle M E EXTERIOR ANGLE A BB FB B FB /_A A BB C B AT STATION B ,FB of BB OF AB were measured . Difference of these two bearing will give you exterior angle B. INTERIOR ANGLE B=BB OF AB-FB OF BC EXTERIOR ANGLE B=360ᴼ-INTERIOR ANGLE INCLUDED ANGLE AT STATION A,FB of AB and BB of EB were measured. Difference of these two bearing give interior angle A. Angle A =FB OF AB – BB OF EA

48. Local Attraction • North end of freely suspended magnetic needle of compass always points to the magnetic north, if it is not influenced by any other external force except the earth magnet field . • It is common experience that magnetic needle gets deflected from its normal position , if placed nearer magnetic rocks, iron ores, cables carrying current or iron electric poles , such disturbing forces is known as local attraction.

49. If difference of FB and BB of a line is not 180 , it means it is affected by local attraction. • Local attraction must be detected by suitable method. • Method of correction for traverse: • First method: Sum of the interior angle should be equal to (2n-4) x 90. if not than distribute the total error equally to all interior angles of the traverse. Then starting from unaffected line the bearings of all the lines are corrected using corrected interior angles.

50. Second method: Unaffected line is first detected. Then, commencing from the unaffected line, the bearing of other affected lines are corrected by finding the amount of correction at each station. • Third method: Least affected line is first detected. Then, error is divided equally in BB and FB, then commencing from that the bearing of other affected lines are corrected by finding the amount of correction at each station.