Angular measurement

Angular measurement
types of Survey
compass

1. Elements Of Civil Engineering Ch :-04 angular Measurement

2. Introduction There are two methods for angular measurement: 1) Triangulation Survey 2) Traverse Survey

3. Triangulation Survey  In the past it was difficult to accurately measure very long distances, but it was possible to accurately measure the angles between points many kilometres apart, limited only by being able to see the distant beacome . This could be anywhere from a few kilometres , to 50 kilometres or more. Triangulation is a surveying method that measures the angles in a triangle formed by three survey control points. Using trigonometry and the measured length of just one side, the other distances in the triangle are calculated.

4. Traverse Survey DEFINITION:- Traversing is that type of survey in which a number of connected survey lines form the framework and the directions and lengths of the survey lines are measured with the help of an angle measuring instrument and a tape or chain respectively. TYPES OF SURVEYING There are two types of traverse surveying. They are: Closed traverse: When the lines form a circuit which ends at the starting point, it is known as closed traverse. Open traverse : When the lines form a circuit ends elsewhere except starting point, it is said to be an open traverse.

5. Close Traverse Open Traverse

6. Checks on traverse: Closed traverse Check on closed traverse: Sum of the measured interior angles (2n-4) x 90° Sum of the measured exterior angles (2n+4) x 90 ° The algebric sum of the deflection angles should be equal to 360°. Right hand deflection is considered + ve , left hand deflection – ve Check on linear measurement The lines should be measured once each on two different days (along opposite directions). Both measurement should tally. Linear measurement should also be taken by the stadia method. The measurement by chaining and stadia method should tally.

7. Checks on traverse: Open traverse Taking cut-off lines: measured the bearings and lengths of cut off lines after plotting and tally with actual values. Taking an auxiliary point: Take P permanent point as auxiliary point measured bearings and lengths of P from each traverse point. If survey is accurate, while plotting all the measured bearing of P should meet at P.

8. 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.

9. Types of compass There are two types of magnetic compass they are as follows:- The Prismatic Compass The Surveyor’s Compass The Transit Compass

10. The Prismatic Compass

11. The Prismatic Compass

12. Elements of prismatic compass Cylindrical metal box : Cylindrical metal box is having diameter of 8to 12 cm. It protects the compass and forms entire casing or body of the compass. It protect compass from dust, rain etc. Pivot : pivot is provided at the center of the compass and supports freely suspended magnetic needle over it. lifting pin and lifting lever : a lifting pin is provided just below the sight vane. When the sight vane is folded, it presses the lifting pin. The lifting pin with the help of lifting lever then lifts the magnetic needle out of pivot point to prevent damage to the pivot head. Magnetic needle : Magnetic needle is the heart of the instrument. This needle measures angle of a line from magnetic meridian as the needle always remains pointed towards north south pole at two ends of the needle when freely suspended on any support. Graduated circle or ring : This is an aluminum graduated ring marked with 0ᴼ to 360ᴼ to measures all possible bearings of lines, and attached with the magnetic needle. The ring is graduated to half a degree. Prism : prism is used to read graduations on ring and to take exact reading by compass. It is placed exactly opposite to object vane. The prism hole is protected by prism cap to protect it from dust and moisture.

13. Object vane : object vane is diametrically opposite to the prism and eye vane. The object vane is carrying a horse hair or black thin wire to sight object in line with eye sight. Eye vane : Eye vane is a fine slit provided with the eye hole at bottom to bisect the object from slit. Glass cover : its covers the instrument box from the top such that needle and graduated ring is seen from the top. Sun glasses : These are used when some luminous objects are to be bisected. Reflecting mirror : It is used to get image of an object located above or below the instrument level while bisection. It is placed on the object vane. Spring brake or brake pin : to damp the oscillation of the needle before taking a reading and to bring it to rest quickly, the light spring brake attached to the box is brought in contact with the edge of the ring by gently pressing inward the brake pin

14. 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 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. 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.

15. Observing Bearing of Line 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 wrt north pole. The enlarged portion gives actual pattern of graduations marked on ring. NORTH OBJECT B A SOUTH LINE OF SIGHT 90 180 270

16. 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.

17. Working of Surveyor`s Compass 1) CENTERING 2) LEVELING 3) OBSERVING THE BEARING OF A LINE First two observation 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.

18. Meridian Bearing of a line is always measured clockwise wrt some reference line or direction. This fixed line is known as meridian. There three types of meridian: 1) Magnetic meridian : The direction shown by a freely suspended needle which is magnetized and balanced properly without influenced by any other factors is known as magnetic meridian. 2) True meridian : True meridian is the line which passes through the true north and south. The direction of true meridian at any point can be determined by either observing the bearing of the sun at 12 noon or by sun’s shadow. 3) Arbitrary meri dian: In case of small works or in places where true meridian or magnetic meridian cannot be determined, then ,any direction of a prominent object is taken as a reference direction called as arbitrary meridian.

19. BEARINGS The bearing of a line is the horizontal angle which it makes with a reference line(meridian). Depending upon the meridian , there are four type of bearings they are as follows: 1) True Bearing : The true bearing of a line is the horizontal angle between the true meridian and the survey line. The true bearing is measured from the true north in the clockwise direction. 2) Magnetic Bearing : the magnetic bearing of a line is the horizontal angle which the line makes with the magnetic north. 3) Grid Bearing : The grid bearing of a line is the horizontal angle which the line makes with the grid meridian. 4) Arbitrary Bearing : The arbitrary baring of a line is the horizontal angle which the line makes with the arbitrary meridian.

20. BEARINGS TRUE MERIDIAN MAGNETIC MERIDIAN TRUE BEARING MAGNETIC BEARING A B MN TN

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

22. Whole circle bearing system(W.C.B.) The bearing of a line measured with respect to magnetic meridian in clockwise direction is called magnetic bearing and its value varies between 0 ᴼ to 360 ᴼ. The quadrant start from north an progress in a clockwise direction as the first quadrant is 0 ᴼ to 90 ᴼ in clockwise direction , 2 nd 90 ᴼ to 180 ᴼ , 3 rd 180 ᴼ to 270 ᴼ , and up to 360 ᴼ is 4 th one.

23. Whole circle bearing system(W.C.B.)

24. Quadrantal bearing system(Q.B.) In this system, the bearing of survey lines are measured wrt to north line or south line which ever is the nearest to the given survey line and either in clockwise direction or in anti clockwise direction.

25. Quadrantal bearing system(Q.B.)

26. Reduced bearing (R.B) When the whole circle bearing is converted into Quadrantal bearing , it is termed as “REDUCED BEARING”. Thus , the reduced bearing is similar to the Quadrantal bearing. Its values lies between 0 ᴼ to 90ᴼ, but the quadrant should be mentioned for proper designation.

27. conversion of WCB to RB. Line W.C.B Between Rule For R.B Quadrant AB 0◦ and 90 ◦ R.B=W.C.B NE AC 90◦ and 180◦ R.B=180◦-W.C.B SE AD 180◦ and 270◦ R.B=W.C.B-180◦ SW AF 270◦ and 360◦ R.B=360◦-W.C.B NW conversion of RB to WCB . Line R.B Rule For R.B Quadrant AB Nq1E R.B=W.C.B 0◦ and 90 ◦ AC Sq2E W.C.B=180◦-R.B 90◦ and 180◦ AD Sq3E W.C.B=180◦+R.B 180◦ and 270◦ AF Nq4W W.C.B=360◦-R.B 270◦ and 360◦

28. Fore bearing and Back bearing The bearing of a line measured in the forward direction of the survey lines is called the ‘fore bearing’(F.B.) of that line. The bearing of a line measured in direction backward to the direction of the progress of survey is called the ‘back bearing’(B.B.) of the line.

29. FB of line AB BB of line AB (FORE BEARING) A (BACK BEARING) NORTH NORTH Θ 1 Θ 2 B FB of AB = Θ 1(from A to B) BB of AB= Θ 2(from B to A) Remembering following points: In the WCB system ,the differences b/n the FB and BB should be exactly 180 ᴼ. Remember the following relation : BB=FB+/-180ᴼ + is applied when FB is <180ᴼ - is applied when BB is >180ᴼ 2) In the reduced bearing system the FB and BB are numerically equal but the quadrants are just opposite.

30. Computation of Angles Observing the bearing of the lines of a closed traverse, it is possible to calculate the included angles, which can be used for plotting the traverse. At the station where two survey lines meet, two angles are formed-an exterior angles and an interior angles. The interior angles or included angle is generally the smaller angles(<180 ᴼ ). A B C D

31. Magnetic declination: The horizontal angle between the magnetic meridian and true meridian is known as magnetic declination. Dip of the magnetic needle: If the needle is perfectly balanced before magnetisation , it does not remain in the balanced position after it is magnetised . 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. Local Attraction 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. 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.

32. Problems: Convert the following WCBs to QBs (a) WCB of AB = 45°30’ ( Ans 45°30’) (b) WCB of BC = 125°45’ ( Ans 180- 125°45’ = 54° 15’) Fore bearing of the following lines are given. Find back bearing AB=S 30°30’ E BC=N 40°30’ W The magnetic bearing of a line AB is 135°30’ what will be the true bearing, if the declination is 5°15’ W.

33. Problems

34. Problems Included angle at A= 280-180-40=60 =FB of DA-180-FB of AB Included angle at B= 40+180-70= 150 =FB of AB+180-FB of BC Included angle at C= 70+180-210 =FB of BC+180-FB of CD Formula: FB of previous line+/-180-FB of next line

35. CHAIN SURVEYING SUMMARY & ABSTRACT:   Chain (Tape) surveying is the simplest form of detail surveying. In this method the lengths of lines marked on the field are measured, while the details are measured by offsets and ties from these lines. This field work will continue for 3 field hours. Every group is expected to submit a detailed report besides the final plan of the area surveyed.

36. OBJECTIVES :   This field work aimed to train the student on the following process: Selection of a frame work (chain or base lines) and control points. Direct method of linear measurements (horizontal distance measurement). Setting out right angles (offsets). Determining the direction of any line in the field with respect to magnetic north "bearing". Booking Method. Plotting Method.

37. APPARATUS & INSTRUMENT   2 Tapes 3 Ranging Rods Arrows/Marker paints Prism Square Prismatic Compass Booking Board

38. PROCEDURE: Make a reconnaissance of the area and select a suitable framework (chain lines) and stations based on the criterion given in Note 1. Mesure all the chain (based) lines once in each direction using the direct measurement method. See Note 2. Measure the offsets/or ties from every necessary point on the detail to the corresponding base line. Use the procedure explained in Note 3. You may need some measurements on the details (on the building sides…). Measure the bearing of one of the chain lines by using the prismatic compass. See Note 4. Record all the information and measurements properly in the booking papers as explained in Note 5. Make the necessary calculations and corrections. Draw the details in a suitable scale to produce a detailed map of the area. See Note 6.

39. Note 1: CHAIN LINES & STATIONS SELECTIONS   To locate a suitable stations and chain lines, a reconnaissance of the area should be undertaken by walking around the area required to be surveyed. Any obstacles should be noticed. The selected stations should produce a well formed linked triangles or braced quadrilaterals. The principles to be considered are: Few long lines should be used. Avoid any obstacles to ranging or chaining. Angles should be > 30 o & <120 o . Make check lines when possible to detect errors when plotting. The lines should be closed to the details (Avoid long offsets (>10m) and ties. After selection of the framework you should draw sketch of the area and mark the stations by wooden pegs or marker paints, and give a number for each station.

40. NOTE 2: DIRECT DISTANCE MEASUREMENT For measuring any distance between A and B, especially when it is longer than the tape length, the following ranging is needed: "ranging by eye". The ranging rode should be placed at the beginning point A and the end point B. The first man should place the tape zero point at A. The second man holding the third raging rod and the tape should move to the direction of B to a point of the end of the tape length. Then he should hold the ranging rod vertically. The first man at point A lines up the ranging rod with that on point B. "by sighting as low as possible". Then, the second man should straighten the tape and mark this point. Both men move ahead repeat the whole steps beginning from the first mark. As measurements processes, the second man should record the length.

41. Note 3: SETTING THE OFFSET   Any point on the detail can be related to the chain line by offset or ties. This is accomplished by measuring all X's and Y's as shown in the figure. To set out offsets, a prism square can be used. To locate the point at which a 1 from any point on the feature would meet the chain line (say AB) you can follow the following steps: One man should hold the ranging pole at the given point, while the other pole is placed at any point on the chain line AB. The observer holds the instrument and walk along the line AB until he see both poles coincide in each other. Then the distance to this point along the chain line can be measured beside the length of the perpendicular. Another method can be used by holding the zero point of the tape at the given point and swinging tape over the chain line and mark the point on the chain line at minimum reading.    

42. NOTE 4: MEASURING THE BEARING OF A LINE   To measure the angle that any line makes with the magnetic direction, you can use a prismatic compass. The procedure is as follow: Place the ranging pole vertically at point B. Place the prismatic compass over its tripod at point A and level it using the bubble and screws. Rotate the compass until it is directed to the pole and read the angle. Repeat the whole procedure for point B.

43. NOTE 5: BOOKING The field book should be neat and consistent: Each chain line is represented by double line drawn through the corner of the page. Entries start at the bottom of the page. Detail that is on the right-hand side of the line is booked on the right-hand side of the page and voice versa. The lengths from the beginning of the line are written inside the double lines while the offset lengths outside.

44. NOTE 6: PLOTTING   A. Plot the framework of chain lines by: Draw the longest line according to its bearing. Build up the other chain lines by using beam compasses. Draw the check lines, and if there are any errors, check the drawn lines in the incorrect triangles. (You may need to measure them in the field).   B. Draw the detail for each chain line based on offsets and ties information. Then connect these points to get the details.