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24. Jan 2023
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• 1. Er Er Er Er. . . . Pramesh Pramesh Pramesh Pramesh Hada Hada Hada Hada B B B BE E E E C C C Ci i i iv v v vi i i il l l l, , , , M M M MS S S SC C C C U U U Ur r r rb b b ba a a an n n n p p p pl l l la a a an n n nn n n ni i i in n n ng g g g Assistant Professor Nepal Engineering College, Changunarayan,Bhaktapur By:- CHAPTER 7 Er. Pramesh Hada, Assistant Professor, nec 1
• 2. Chapter 7. Contouring (4 hour) • 7.1 Basic Definitions in Contouring –pu 2010 • 7.2 Contour Interval – pu 09,10,14 • 7.3 Characteristics of Contours – pu 08,09,10,11,13,14) • 7.4 Methods of Contouring – pu 09,10,11,13,14 Er. Pramesh Hada, Assistant Professor, nec • 7.5 Interpolation of Contours • 7.6 Uses of Contour Maps – pu 09,11,13 • 7.7 Instruction on field Works 2
• 3. •Contour An imaginary line on the ground surface joining the points of equal elevation is known as contour. •It facilitates depiction of the relief of terrain in a two dimensional plan or map. •In other words, contour is a line in which the ground 7.1 Basic Definitions in Contouring –pu 2010 •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. •Contouring is the science of representing the vertical dimension of the terrain on a two dimensional map. 2 CONTOURING CONTOURING 2 Er. Pramesh Hada, Assistant Professor, nec 3
• 4. Contours Er. Pramesh Hada, Assistant Professor, nec 4 Contour interval = 30 m
• 5. Er. Pramesh Hada, Assistant Professor, nec Different contour maps 5 Different contour maps
• 6. Er. Pramesh Hada, Assistant Professor, nec 6
• 7. Relief or shape of the land is shown by Contours • Contours are those light brown or orange lines that can see on maps. • A contour line is an imaginary line that joins points of equal height above sea level. • The lines are numbered giving • The lines are numbered giving the height above sea level in metres. • On 1:50000 map the contours will be drawn for every 10 metre rise. Although on this map they are every 3m so always remember to check. Er. Pramesh Hada, Assistant Professor, nec 7
• 8. • It is important to remember that the closer together the contour lines the steeper the slope. Steep slopes Er. Pramesh Hada, Assistant Professor, nec 8 Area that is flatter with only a gentle slope XXXX
• 9. • 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 positions in plan serves the purpose of both, a plan and a section. and a section. • Contouring : The process of tracing contour lines on the surface of the earth is called Contouring. Er. Pramesh Hada, Assistant Professor, nec 9
• 10. Contour Line : A Contour line is an imaginary outline of the terrain obtained by joining its points of equal elevation. Pu 09,10,14 Contour Interval (CI) – It is the vertical distance between any two consecutive contours. Suppose a map includes contour lines of 100m, 98m ,96 m and so on .The contour interval here is 2 m. This interval depends upon (i) the nature of the ground (i.e. whether flat or sleep). Er. Pramesh Hada, Assistant Professor, nec (i) the nature of the ground (i.e. whether flat or sleep). (ii) the scale of the map (iii) the purpose of the survey. Contour intervals for flat country are generally small, e g. 0.25 m, 0.5 m, 0.75m. etc. Contour interval for a steep slope in a hilly area is generally greater. e.g. 5m. 10 m, 15 m etc. It should be remembered that the contour interval for a particular map is Constant. 10
• 11. Er. Pramesh Hada, Assistant Professor, nec 11
• 12. • Horizontal Equivalent (HE) Horizontal equivalent is the horizontal distance between two consecutive contour lines measured to the scale of the map. – It is not constant. It varies according to the steepness of the ground. – For steep slopes, the contour lines run close together, and for flatter slopes they are widely spaced. Er. Pramesh Hada, Assistant Professor, nec 12
• 13. CHARACTERISTICS OF CONTOURS i) All points in a contour line have the same elevation. ii) Flat ground is indicated where the contours are widely separated and steep-slope where they run close together. iii) A uniform slope is indicated when the contour lines are iii) A uniform slope is indicated when the contour lines are uniformly spaced. iv) A plane surface when they are straight, parallel and equally spaced. CONTOURING CONTOURING 13 Er. Pramesh Hada, Assistant Professor, nec 13
• 14. examples Er. Pramesh Hada, Assistant Professor, nec 14
• 15. Er. Pramesh Hada, Assistant Professor, nec 15
• 16. CHARACTERISTICS OF CONTOURS (Pu 08,09,10,11,13,14) v) A series of closed contour lines on the map represent a hill , if the higher values are inside 65 70 75 80 HILL are inside A HILL 60 65 60 65 70 75 80 HILL CONTOURING CONTOURING 14 Er. Pramesh Hada, Assistant Professor, nec 16
• 17. CHARACTERISTICS OF CONTOURS vi) A series of closed contour lines on the map indicate a depression if the DEPRESSION 60 65 70 75 80 depression if the higher values are outside A DEPRESSION 70 60 65 70 75 80 60 15 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 17
• 18. CHARACTERISTICS OF CONTOURS vii) Contour line cross ridge or valley line at right angles. If the higher values are inside the bend or loop in the contour, it indicates a Ridge. 100 90 80 RIDGE LINE 70 60 50 16 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 18
• 19. CHARACTERISTICS OF CONTOURS vii) Contour line cross ridge or valley line at right angles. If the higher values are outside the bend, it represents a Valley 100 90 VALLEY LINE 90 80 70 60 50 17 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 19
• 20. viii). Contours cannot end anywhere but close on themselves either within or outside the limits of the map. CHARACTERISTICS OF CONTOURS of the map. 18 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 20
• 21. ix). Contour lines cannot merge or cross one another on map except in the case of CHARACTERISTICS OF CONTOURS 10 20 30 40 except in the case of an overhanging cliff. 40 30 20 10 OVERHANGING CLIFF 10 19 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 21
• 22. x) Contour lines never run into one another except in the case of a vertical cliff. In this case ,several CHARACTERISTICS OF CONTOURS 10 20 30 40 VERTICA CLIFF 50 this case ,several contours coincide and the horizontal equivalent becomes zero. 40 30 20 50 OVERHANGING CLIFF 10 10 50 20 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 22
• 23. CHARACTERISTICS OF CONTOURS XI) Depressions between summits is called a saddle. It is represented by four sets of contours as shown. It represents a dip in a ridge or the junction of two ridges. And in the case of a mountain range ,it takes the form of a SADDLE 70 80 90 70 80 90 range ,it takes the form of a pass . 90 110 100 90 21 CONTOURING CONTOURING Line passing through the saddles and summits gives water shed line. Er. Pramesh Hada, Assistant Professor, nec 23
• 24. Think where is the steepest slope on this island? 150 200 XXXX 50 100 Er. Pramesh Hada, Assistant Professor, nec 24
• 25. Steepest slope – contour lines are closest together Sparrow point 227 .76 150 200 Sometimes spot heights can be shown as a dot with a note of the height of that particular place. A trigonometrical point (or trig point) shows the highest point in an area (in meters) and is shown as a blue triangle .76 50 100 Er. Pramesh Hada, Assistant Professor, nec 25
• 26. the shape and pattern of the contours Er. Pramesh Hada, Assistant Professor, nec 26
• 27. Er. Pramesh Hada, Assistant Professor, nec 27
• 28. Er. Pramesh Hada, Assistant Professor, nec 28
• 29. Er. Pramesh Hada, Assistant Professor, nec 29
• 30. Er. Pramesh Hada, Assistant Professor, nec 30
• 31. PURPOSE (uses) OF CONTOURING/CONTOUR MAP 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. i) contour maps is prepared in order to select the most economical or suitable site. economical or suitable site. ii) It helps to locate the alignment of a canal so that it should follow a ridge line. iii) It helps to mark the alignment of roads and railways so that the quantity of earthwork both in cutting and filling should be minimum. 4 CONTOURING CONTOURING 5 Er. Pramesh Hada, Assistant Professor, nec 31
• 32. iv) It helps for getting information about the ground whether it is flat, undulating or mountainous. v) It helps to find the capacity of a reservoir and volume of earthwork especially in a mountainous region. vi) It helps to trace out the given grade of a particular route. PURPOSE(uses) OF CONTOURING/CONTOUR MAP route. vii) It helps to locate the physical features of the ground such as a pond depression, hill, steep or small slopes. (Undulating – rising falling) CONTOURING CONTOURING 6 Er. Pramesh Hada, Assistant Professor, nec 32
• 33. USES OF CONTOUR MAP (Pu,09,11,13) - 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 45 CONTOURING CONTOURING - 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. Er. Pramesh Hada, Assistant Professor, nec 33
• 34. FACTORS ON WHICH CONTOUR -INTERVAL DEPENDS The contour interval depends upon the following factors:- i) The Nature of the Ground - In flat and uniformly sloping country, the contour uniformly sloping country, the contour interval is small , but in broken and mountainous region the contour interval should be large otherwise the contours will come too close to each other. CONTOURING CONTOURING 8 Er. Pramesh Hada, Assistant Professor, nec 34
• 35. 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 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. CONTOURING CONTOURING 9 Er. Pramesh Hada, Assistant Professor, nec 35
• 36. FACTORS ON WHICH CONTOUR - INTERVAL DEPENDS iii) 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. the scale, the greater is the contour interval. iv) Time and Expense of Field and Office work- The smaller the interval, the greater is the amount of field-work and plotting work. CONTOURING CONTOURING 10 Er. Pramesh Hada, Assistant Professor, nec 36
• 37. 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 building sites, for detailed design work and for calculation of quantities of earth work; 0.2 to 0.5 m. CONTOURING CONTOURING 11 Er. Pramesh Hada, Assistant Professor, nec 37
• 38. COMMON VALUES OF THE CONTOUR - INTERVAL ii) For reservoirs and town planning schemes; 0.5 to 2m. iii) For location surveys. 2 to 3m. iii) For location surveys. 2 to 3m. iv) For small scale maps of broken country and general topographic work; 3m,5m,10m,or 25m. CONTOURING CONTOURING 12 Er. Pramesh Hada, Assistant Professor, nec 38
• 39. METHODS OF CONTOURING (PU 09,10,11,13,14) 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 50 48 B.M field by locating and marking a number of points on each contour. • These points are then surveyed and plotted on plan and the contours drawn through them. DIRECT METHOD OF CONTOURING 48 46 22 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 39
• 40. Er. Pramesh Hada, Assistant Professor, nec 40
• 41. Methods of Contouring (PU 09,10,11,13,14) The location of a point in topographic survey involves both horizontal as well as vertical control. The method of contouring depends upon the instruments used. The method may be divided into two classes. The Direct Method The indirect method In the direct method • The contour to be plotted is actually traced on the ground. Only those points are surveyed which happen to be plotted and • The contour to be plotted is actually traced on the ground. Only those points are surveyed which happen to be plotted and contours are drawn through them. • The method is slow and tedious and is applicable for small areas and where great accuracy is required.(as a lot of time is wasted in searching points of the same elevation for a contour). In the indirect method some suitable guide points are selected and surveyed(the guide points should not necessarily be on the contours but serve as basis for interpolation of contours) Er. Pramesh Hada, Assistant Professor, nec 41
• 42. Direct Method Each contour is located by determining the positions of a series of points through which the contour passes . Vertical Control: • The points on the contours are traced either with the help of a level and staff or hand level. • The level is set at a point to command as much area as is possible and is leveled. • The staff is kept on BM and the height of the instrument is determined. If BM is not nearby, fly leveling may be carried out to establish a temporary bench mark in that area. • Having known the height of the instrument, the staff reading is calculated so that the Having known the height of the instrument, the staff reading is calculated so that the bottom of the staff is at an elevation equal to the value of contour to be plotted. Eg. If height of instrument is 800.250 meters the staff reading to plot a contour of 800 meters will be 0.250 meters. Taking one contour at a time( for 800m contour) the staff man is directed to keep the staff on those points where staff reading of 0.250 m is obtained every time. Er. Pramesh Hada, Assistant Professor, nec 42
• 43. Direct method b) Horizontal Control: • After having located the points for various contours vertically they are to be surveyed with a suitable control system for horizontally located. • For this, the location of points are traced by the method of plane tabling, chain survey or theodolite. • Then the contour are drawn through these points. • For accurate contouring sufficient number of the points at close interval are required. • The plotted positions of the consecutive points of any contour are simply joined by straight lines to form contour lines Er. Pramesh Hada, Assistant Professor, nec 43
• 44. Er. Pramesh Hada, Assistant Professor, nec 44
• 45. 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. 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 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 45
• 46. 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 respectively. •The staff is held on an approximate position of point and then moved up and down the slope until the desired reading is obtained. The point is marked with a peg. 25 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 46 •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.
• 47. 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 Procedure (Contd…..) 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 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 47
• 48. 2. Indirect Method: •In this method some suitable guide points are selected and their elevations are found. These point may form well shaped geometrical figures. •The location of such points are plotted by plane tabling and contours are drawn by interpolation. These guide points do not fall except by coincidence, on the contours to be located. • While interpolating, it is considered that the slope between any two adjacent guide points is uniform. adjacent guide points is uniform. •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. • Other Guide points 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 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 48
• 49. • 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 Indirect Method: • 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 corners of the squares are pegged out and the reduced levels of these points are determined with a level. 31 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 49
• 50. SQUARE METHOD 31 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 50
• 51. Indirect Method: (Square Method) 13 14 15 16 100.4 97.60 96.05 94.40 98.00 98.00 10 11 6 7 1 2 3 4 5 8 9 12 SQURES LAID ON GROUND 97.45 99.25 98.75 99.40 98.65 99.60 97.80 89.55 100.90 99.70 99.85 98.45 98.00 98.0 99.0 99.0 98,00 CONTOURS INTERPOLATED 32 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 51
• 52. 2. Indirect Method: (Square Method) 100.4 97.60 96.05 94.40 98.00 98.00 •The important points within the squares may be taken when required and located by measurements from the corners. 97.45 99.25 98.75 99.40 98.65 99.60 97.80 89.55 100.90 99.70 99.85 98.45 98.00 98.0 99.0 99.0 98,00 SQUARE METHOD 33 CONTOURING CONTOURING from the corners. • The squares are plotted and the reduced levels of the corners are written on the plan. Er. Pramesh Hada, Assistant Professor, nec 52
• 53. 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 aignment etc. • In this method cross sections perpendicular to the centre line of the area are set out. • The spacing of the cross-section depends upon the contour interval, scale of plan and the characteristic of ground. interval, scale of plan and the characteristic of ground. • The common value is 10 to 20 m in hilly country and 20-30m in flat country. •The centre line and cross sections are plotted along with important features on the desired scale and their RLs are entered. •The contours are then interpolated with respect to these RLs. 35 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 53
• 54. Indirect Method: By Cross- Sections: 70.6 69.1 68.8 69.1 70.8 70.8 70.2 69.1 70.4 70.5 RD 580 RD 560 70 70 69 69 •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. 70.8 70.2 69.1 70.4 70.5 71.2 70.8 66.3 70.6 70.8 71.6 71.2 70.6 72.4 71.7 RD 560 RD 540 RD 520 71 71 70 69 68 67 71 71 Fig. X-Section Method 34 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 54
• 55. 2. Indirect Method: (iii) By Tacheometric method: • A tacheometer is a transit theodolite having a diaphragm fitted with two stadia wires, one above and other below the central wire. Fig. Stadia Wires at Diaphragm central wire. • The horizontal distance between the instrument and staff station may be determined by multiplying the difference of the staff readings of the upper and lower stadia wires with the stadia constant of the instrument, which is usually 100. • Thus the tacheometer is used for both the vertical as well as horizontal measurements. 37 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 55
• 56. 2. Indirect Method: iii) By Tacheometric method (contd…): •This method is most suitable in hilly areas as the number of stations which can be commanded by a tacheometer is far more than those by a level and thus the number of instrument settings are considerably reduced. •A number of radial lines are laid out at a known angular •A number of radial lines are laid out at a known angular interval and representative points are marked by pegs along these radial lines. •Their elevations and distances are then calculated and plotted on the plan and the contour lines are then interpolated. 38 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 56
• 57. By Tacheometric method Er. Pramesh Hada, Assistant Professor, nec 57
• 58. Difference between Direct and Indirect Methods of Contouring Er. Pramesh Hada, Assistant Professor, nec 58
• 59. 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. method. • While interpolation of contours the ground between any two points is assumed to be uniformly sloping. 39 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 59
• 60. INTERPOLATION OF CONTOURS There are three main methods of interpolation: i) By arithmetical calculation: • This is very tedious time consuming but accurate method . • The positions of contour points between the guide points are located by arithmetic calculation. • used for small areas where accurate results are necessary. • used for small areas where accurate results are necessary. 40 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 60 •Find distance from A for a contour of 99m elevation. (Note – Square = 10*10m) •Distance from A for contour of 99m = (99—98.75)/ (100.75 —98.75) *10 = 1.25m
• 61. INTERPOLATION OF CONTOURS The contours are interpolated as under: Example- 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 By arithmetical calculation: A B 30 m 25.45m 27.54m 26m 27m Diff (A-B)=2.09m 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 0.55mand 1.55 m respectively. Therefore the horizontal distance between A and 26 m contour =0.55/2.09 x 30m ((Diff of A 26m / Diff of A B)* Dist.AB)) and Between A and 27 m contour =1.55/2.09 x 30m. These distances are then plotted to scale on the map. 41 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 61
• 62. INTERPOLATION OF CONTOURS •Graphical method of interpolation is simpler as compared to arithmetical method and also the results (ii) By Graphical method: 60 65 B 62.5m 60m 2 and also the results obtained are accurate. • Out of several graphical methods, the most common is as given below: 50 55 A 51.5m 55m 1 42 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 62
• 63. As shown in the fig. suppose the contour interval is 5m, then on a piece of tracing cloth or tracing paper, a number of parallel lines spaced at 0.5 m (usually 1/10th of the contour Graphical method: INTERPOLATION OF CONTOURS (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. 43 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 63
• 64. INTERPOLATION OF CONTOURS 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 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 CONTOURING Er. Pramesh Hada, Assistant Professor, nec 64
• 65. iii) By Estimation: – The position of the contour points between ground - points are estimated roughly and the contours are then drawn through these points. INTERPOLATION OF CONTOURS – This is a rough method and is suitable for small scale maps. • Accuracy of work depends upon the skill and experience of surveyor. Er. Pramesh Hada, Assistant Professor, nec 65
• 66. 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 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. 45 CONTOURING CONTOURING Er. Pramesh Hada, Assistant Professor, nec 66
• 67. Contour Gradient • Contour gradient is a line lying throughout on the surface of the ground and preserving a constant inclination to the horizontal • The inclination of a contour gradient is generally given either as rising gradient or falling gradient, and is expressed as ratio of the vertical height to a specified horizontal distance. If the inclination of a contour gradient is 1 in 50, it means that for every 50 m horizontal distance, there is a rise (or fall) of 1 m. rise (or fall) of 1 m. • To locate the contour gradient in the field a clinometer, a theodolite or a level may be used. • The inclination of a contour gradient is generally given either as rising or falling gradient and is expressed as ratio of the vertical height to a specified horizontal distance. Er. Pramesh Hada, Assistant Professor, nec 67
• 68. Er. Pramesh Hada, Assistant Professor, nec 68