contouring.pptx

Surveying for civil engineering

1. Contour An imaginary line on the ground surface joining the points of equal elevation is known as contour. In other words, contour is a line in which the ground surface is intersected by a level surface obtained by joining points of equal elevation . This line on the map represents a contour and is called contour line . 2 2

2. Contour Map A map showing contour lines is known as Contour map. A contour map gives an idea of the altitudes of the surface features as well as their relative p ositions in p lan serves the purpose of both, a plan and a section .

3. Contouring The process of tracing contour lines on the surface of the earth is called Contouring.

4. PURPOSE OF CONTOURING Contour survey is carried out at the starting of any engineering project such as a road, a railway, a canal, a dam, a building etc. For preparing contour maps in order to select the most economical or suitable site. To locate the alignment of a canal so that it should follow a ridge line. To mark the alignment of roads and railways so that the quantity of earthwork both in cutting and filling should be minimum.

5. PURPOSE OF CONTOURING (contd.) For getting information about the ground whether it is flat, undulating or mountainous. v) To find the capacity of a reservoir and volume of earthwork especially in a mountainous region. vi) To trace out the given grade of a particular route. vii) To locate the physical features of the ground such as a pond depression, hill, steep or small slopes. 6

6. CONTOUR INTERVAL The constant vertical distance between two consecutive contours is called the contour interval. HORIZONTAL EQUIVALENT The horizontal distance between any two adjacent contours is called as horizontal equivalent. The contour interval is constant between the consecutive contours while the horizontal equivalent is variable and depends upon the slope of the ground. 7

7. FACTORS ON WHICH CONTOUR - INTERVAL DEPENDS The contour interval depends upon the following factors:- i ) The Natu r e o f the G r o u nd country, In f l at and the c o n t our uniformly sloping i n terval i s small , bu t i n b r oken a n d mountainous region the contour interval should be large otherwise the contours will come too close to each other. 8

8. FACTORS ON WHICH CONTOUR - INTERVAL DEPENDS ii) The Purpose and extent of the survey. Contour interval is small if the area to be surveyed is small and the maps are required to be used for the design work or for determining the quantities of earth work etc. while wider interval shall have to be kept for large areas and comparatively less important works. 9

9. FACTORS ON WHICH CONTOUR - INTERVAL DEPENDS The Scale of the Map . The contour interval should be in the inverse ratio to the scale of the map i.e. the smaller the scale, the greater is the contour interval. Time and Expense of Field and Office work . The smaller the interval, the greater is the amount of field-work and plotting work. 10

10. COMMON VALUES OF THE CONTOUR - INTERVAL The following are the common values of the contour interval adopted for various purposes:- i) For large scale maps of flat country, for building sites, for detailed design work and for calculation of quantities of earth work; 0.2 to 0.5 m. 11

11. COMMON VALUES OF THE CONTOUR - INTERVAL ii) For reservoirs and town planning schemes; 0.5 to 2m. For location surveys. 2 to 3m. For small scale maps of broken country and general topographic work; 3m,5m,10m,or 25m. 12

12. CHARACTERISTICS OF CONTOURS All points in a contour line have the same elevation. Contour lines close together represent Steep- slope. If contour lines are far apart it represent gentle slope. If they are equally spaced uniform slope is indicated. 5. A plane surface when they are straight, parallel and equally spaced

13. CHARACTERISTICS OF CONTOURS 6. A s e r ies of closed contour li n es o n t h e map represent a hi l l , I f the higher values are inside. Contour lines are close near the top of a hill or high ground and wide apart near the foot. This indicates a very steep slope towards peak and flatter slope towards foot. A HILL 65 60 75 70 80 60 65 70 75 80 HILL

14. CHARACTERISTICS OF CONTOURS 7) A series of closed contour lines on the map represent depression when If higher values are outside contour lines are closer near the bank of a pond or depression and wide apart towards centre.This indicates a steep slope near the bank and flatter slope at the centre . A DEPRESSION 70 60 65 70 75 80 DEPRESSION 60 65 70 75 80

15. CHARACTERISTICS OF CONTOURS 8. Contour l i ne c r oss ri d ge o r vall e y li n e at right angles. If the higher values are inside the bend or loop in the contour, it indicates a Ridge. RIDGE LINE 100 90 80 70 60 50

16. CHARACTERISTICS OF CONTOURS 9 Contour l i ne c r oss ri d ge o r vall e y li n e at right angles. If the higher values are outside the bend, it r ep r es e n ts a Valley V A L L EY L INE 100 90 80 70 60 50

17. 10) Contour lines always forms a closed circuit .But these lines may be within or outside the boundary of map 18 CHARACTERISTICS OF CONTOURS

18. 11. Contour lines cannot merge or c r oss one anot h er o n map except in the case of an overhanging cliff . CHARACTERISTICS OF CONTOURS OVERH A NGING CL IFF 19

19. 12) Co n tour l i nes nev e r r u n in t o one the c a se of v e rti c al cli f f. anot h er e x cept in a In this c a s e conto u rs ,s e veral coinc i de and the horizontal equivalent becomes zero. CHARACTERISTICS OF CONTOURS 50 20 10 30 40 VE RTIC CLIFF 50 10 20 30 40 50 Vertical cliff 20

20. CHARACTERISTICS OF CONTOURS 13) Dep r essions between summits is called a saddle . It is represented by four sets of contours a dip i n a ri d ge or the ju n ction o f two ridge s . And i n the r a n ge , i t t akes form of a pass . S A D D LE as 70 80 shown. It represents 90 70 80 90 1 10 90 100 case of a mountain Line 21 p a s s ing t h r ough the the s a d d les and s umm its giv e s water shed line.

21. METHODS OF CONTOURING There are mainly two methods of locating contours:- (1)Direct Method and (2) Indirect Method. Direct Method : In this method, the contours to be located are directly traced out in the field by locating and marking a n u m b er o f points o n each c o n t o u r . These points a r e t h en surveyed and plotted on plan and the c o nto u rs drawn th r o u gh the m . DIRECT METHOD OF CONTOURING 50 48 46 B.M 22

22. METHODS OF CONTOURING Direct Method : This method is most accurate but very slow and tedious as a lot of time is wasted in searching points of the same elevation for a contour. This is suitable for small area and where great accuracy is required 23

23. Procedure : To start with, a temporary B.M is established near the area to be surveyed with reference to a permanent B.M by fly leveling. The level is then set up in such a position so that the maximum number of points can be commanded from the instrument station. The height of instrument is determined by taking a back sight on the B.M. and adding it to the R.L. of bench mark. The staff reading required to fix points on the various contours is determined by subtracting the R.L. of each of the contours from the height of instrument. 24

24. Example : If the height of instrument is 82.48m ., then the staff readings required to locate 82, 81 and 80m contours are 0.48, 1.48 and 2.48m r espe c t i vel y . T h e approximate p osit i on d o wn of point the s l o p e and u n til staff i s held o n an then the moved up and desi r ed r e a di n g i s o b t aine d . The po i nt is marked with a peg. 25

25. Ex a m p le(c o n t d .) : Similarly various other points are marked on each contour. The line joining all these points give the required contour. It may be noted that one contour is located at a time. Having fixed the contours within the range of the instrument, the level is shifted and set up in a new position. 26

26. Procedure (Contd…..) The new height of instrument and the required staff readings are then calculated in a similar manner and the process repeated till all the contours are located. The positions of the contour points are located suitably either simultaneous with levelling or afterwards. A theodolite , a compass or a plane table traversing is usually adopted for locating these points. The points are then plotted on the plan and the contours drawn by joining the corresponding points by dotted curved lines. 27

27. This method is suitable for small areas, where a single point in the centre can command the whole a r ea. laid Rad i al out c e n t r e l i nes a r e f r om the by common theodolite or compass and their positions are fixed up by horizontal angles and bearings. Direct Method By Radial Lines Lines : 70 65 60 55 Fig. RADIAL LINES METHOD OF CONTOURING 28

28. Direct Method By Radial Lines Lines (contd.) : Temporary bench marks are first established at the centre and near the ends of the radial lines .The contour points are then located and marked on these lines and their positions are determined by measuring their distances along the radial lines. They are then plotted on the plan and the contours drawn by joining all the corresponding points with the help of a plane table instrument. 29

29. 2. Indirect Method: In this method the points located and surveyed are not necessarily on the contour lines but the spot levels are taken along the series of lines laid out over the area .The spot levels of the several representative points representing hills, depressions, ridge and valley lines and the changes in the slope all over the area to be contoured are also observed. Their positions are then plotted on the plan and the contours drawn by interpolation . This method of contouring is also known as contouring by spot levels . 30

30. SQU A R E ME T HOD CON T OURI N G 31

31. The corners of the squares are pegged out and the reduced levels of these points are determined with a level. 2. Indirect Method : (Square Method) 13 14 15 9 10 11 5 6 7 8 12 16 1 2 3 4 SQURES LAID ON GROUND 100.4 97. 6 96. 5 9 4. 4 97. 4 5 9 9. 2 5 9 8. 7 5 99. 4 98. 6 5 9 9. 6 89. 5 5 98.00 98.00 9 7. 8 98.0 99.0 99.0 98,00 100. 9 99. 7 9 9. 8 5 98. 4 5 CONTOURS INTERPOLATED CON T OURI N G 32

32. This method is commonly employed in all kinds of surveys as this is cheaper, quicker and less tedious as compared to direct method. There are mainly three method of contouring in indirect method: (i) By Squares . In this method, the whole area is divided into number of squares, the side of which may vary from 5m to 30m depending upon the nature of the ground and the contour interval. The square need not be of the same size throughout. CON T OURI N G 31

33. 2. Indirect Method : (Square Method) 100.4 97. 6 96. 5 9 4. 4 97. 4 5 9 9. 2 5 9 8. 7 5 99. 4 98. 6 5 9 9. 6 89. 5 5 100. 9 98. 4 5 98.00 98.00 9 7. 8 98.0 99.0 99.0 98,00 99.70 99.85 SQUARE METHOD important CON T OURI N G 33 The points s q u a r es w i t hin t he may be taken when required and located by measurements from the corners. The sq u a r es a r e pl o tted an d the r edu c ed levels of the corners are written on the plan.

34. 2 . Indirect Method : By Cross- Sections : This method is most suitable for the survey of long narrow strips such as a road, railway or canal etc. 70.2 69.1 70.4 70.5 1.2 70.8 66.3 70.6 70.8 1.6 71.2 70.6 RD 580 70.6 RD 560 70.8 RD 540 7 RD 520 7 71 71 70 70 69 67 68 71 71 70 69.1 69 68.8 69 69.1 70.8 72.4 71.7 Fig. X-Section Method CON T OURI N G 34

35. 2 . Indirect Method : By Cross- Sections : This method is most suitable for the surveys of long narrow strips such as a road, railway or canal etc. Cross sections are run transverse to the centre line of the work and representative points are marked along the lines of cross-section. The cross-section lines need not necessarily be at right angles to the centre line of the work. This may be inclined at any angle to the centre line if necessary. The spacing of the cross-sections depends upon the topography of the country and the nature of the survey. CON T OURI N G 35

36. 2. Indirect Method : By Cross- Sections : The common value is 20 to 30 m in hilly country and 100m in flat country . The levels of the points along the section lines are plotted on the plan and the contours are then interpolated as usual as shown in the fig. CON T OURI N G 36

37. INTERPOLATION OF CONTOURS The process of spacing the contours proportionally between the plotted ground – points is termed as interpolation of contours . This becomes necessary in the case of indirect contouring as only the spot levels are taken in this method. While interpolation of contours the ground between any two points is assumed to be uniformly sloping. CON T OURI N G 39

38. INTERPOLATION OF CONTOURS There are three main methods of interpolation: By Estimation: The position of the contour points between ground - points are estimated roughly and the contours are then drawn through these points. This is a rough method and is suitable for small scale maps. By arithmetical calculation : This is very tedious but accurate method and is used for small areas where accurate results are necessary. The contours are interpolated as under: CON T OURI N G 40

39. INTERPOLATION OF CONTOURS ii) By arithmetical calculation : Suppose A and B are two points at a distance of 30 m and the reduced level of A and B are 25.45m and 27.54m respectively .Taking the contour interval as 1m , 26 and 27 m contours may be interpolated in between A and B . The difference of level between A and B is 2.09m .the difference of level between A and 26m ,and A and 27m is 0.55m and 1.55 m respectively. The r efo r e the h o r i z o n t a l di s tance be t we e n A a n d 2 6 m contour = 0.55/2.09 x 30m and Between A and 27 m contour = 1.55/2.09 x 30m. These distances are then plotted to scale on the map. CON T OURI N G 41

40. INTERPOLATION OF CONTOURS Graphical method of inter p olat i on is simpler a s c o mp a r ed to arithmetical met h od and also the r esults ob t ained a r e Out of gra p hical accurate. several met h od s , t h e most com m on i s as g i ven below: (ii i ) By Gr a p h ical method : 50 55 60 65 B A 51.5m 62.5m 55m 60m 1 2 CON T OURI N G 42

41. Graphical method : As shown in the fig. suppose the contour interval is 5m , then on a piece of tracing cloth, a number of parallel lines spaced at 0.5 m (usually 1/10th of the contour interval) are drawn. Every tenth line being made thick. Suppose it is required to interpolate contours between two points A and B of elevation 51.5m and 62.5m respectively . CON T OURI N G 43

42. INTERPOLATION OF CONTOURS (iii) By Graphical method : If the bottom line represents an elevation of 50m. Then the successive thick lines will represent 55m, 60m and 65m, etc. Place the tracing cloth so that the point A is on the third line from the bottom, now move the tracing cloth until B is on the fifth line above the 60m thick line. The intersection of the thick lines 1 and 2 representing elevations of 55m and 60 m and the line AB give the position of the points on the 55m and 60m contours respectively and are pricked through on the plan with a pin . 44 CONTOURING

43. DRAWING THE CONTOUR LINES Contour lines are drawn as fine and smooth free hand curved lines . Sometimes they are represented by broken lines .They are inked in either in black or brown colour. A drawing pen gives a better line than a writing pen and French curves should be used as much as possible .Every fifth contour is made thicker than the rest. The elevation of contours must be written in a uniform manner, either on the higher side or in a gap left in the line .When the contour lines are very long , their elevations are written at two or three places along the contour .In the case of small scale maps, it is sufficient to figure every fifth contour. CON T OURI N G 45

44. USES OF CON T OUR MAP A contour map furnishes information regarding the features of the ground , whether it is flat, undulating or mountainous. From a contour map , sections may be easily drawn in any direction Intervisibility between two ground points plotted on map can be ascertained It enables an engineer to approximately select the most economical or suitable site for an engineering project such as a road, a railway, a canal or a pipe line etc. A route of a given grade can be traced on the map. Catchment area and capacity of a reservoir may be determined from the contour map. Contour map may be used to determine the quantities of earth work. CON T OURI N G 45