SURVEYING -I (CONTOUR)

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UNIT-III CONTOUR (Surveying-I): 

UNIT-III CONTOUR (Surveying-I) Prepared by C.Makendran M.E.,(PhD.,)

Representation of Vertical Distance: 

Representation of Vertical Distance The utility of a plan or map increases highly if it also depicts the relative vertical position of the points, natural and/or man made objects and/or features. The vertical positions of points can be represented in different ways such as shading, haching , spot height or contour line on a map. Of these, contour lines or contours are most widely used for representation of the relief of terrain.

Contour : 

Contour A contour is defined as an imaginary line of constant elevation on the ground surface.It can also be defined as the line of intersection of a level surface with the ground surface. For example: The line of intersection of the water surface of a still lake or pond with the surrounding ground represents a contour line.

Definition: 

Definition A line joining points of equal elevations is called a contour line. It facilitates depiction of the relief of terrain in a two dimensional plan or map.

Contour Interval: 

Contour Interval The difference in elevation between successive contour lines on a given map is fixed. This vertical distance between any two contour lines in a map is called the contour interval (C.I.) of the map. Figure 17.1(a ) shows contour interval of 1m whereas Figure 17.1(b) shows 10m.

Slide 8: 

The choice of suitable contour interval in a map depends upon four principal considerations. These are: Nature of the Terrain Scale of the Map Accuracy Time of Cost

Nature of the Terrain Table 17.1 Contour Interval ( CI) for different types of Survey: 

Nature of the Terrain Table 17.1 Contour Interval ( CI) for different types of Survey Sl. No Purpose of survey Scale CI (m) 1 Building site 1/1000 or less 0.2 to 0.5 2 Town planning, reservoir etc. 1/50,00 to 1/100,00 0.5 to 2 3 Location Survey, earthwork, etc. 1/100,00 to 1/200,00 1 to 3

Scale of the Map : 

Scale of the Map The contour interval normally varies inversely to the scale of the map i.e., if the scale of map is large, the contour interval is considered to be small and vice versa ( Table 17.2 ).

Table 17.2 CI for different scales and types of Ground : 

Table 17.2 CI for different scales and types of Ground SI.NO Map Scale Type of Terrain CI(m) 1 Large (1:1000 or less) Flat 0.2 to 0.5 Rolling 0.5 to 1 Hilly 1 to 2 2 Intermediate (1:1000 to 1: 10,000) Flat 0.5 to 1.5 Rolling 1.5 to 2 Hilly 2 to 3 3 Small (1: 10,000 or more) Flat 1 to 3 Rolling 3 to 5 Hilly 5 to 10

Accuracy : 

Accuracy Accuracy need of surveying work also decide the contour interval. Surveying for detailed design work or for earthwork calculations demands high accuracy and thus a small contour interval is used. But in case of location surveys where the desired accuracy is less, higher contour interval should be used

Time of Cost : 

Time of Cost If the contour interval is small, greater time and funds will be required in the field survey, in reduction and in plotting the map. If the time and funds available are limited, the contour interval may be kept large.

Horizontal Equivalent: 

Horizontal Equivalent The horizontal distance between two points on two consecutive contour lines for a given slope is known as horizontal equivalent. For example, in Figure 17.1 (b) having contour interval 10m, the horizontal equivalent in a slope of 1 in 5 would be 50 m. Thus, horizontal equivalent depends upon the slope of the ground and required grade for construction of a road, canal and contour interval.

Characteristics of Contour : 

Characteristics of Contour The principal characteristics of contour lines which help in plotting or reading a contour map are as follows: The variation of vertical distance between any two contour lines is assumed to be uniform. The horizontal distance between any two contour lines indicates the amount of slope and varies inversely on the amount of slope. Thus, contours are spaced equally for uniform slope ( Figure 17.2 ); closely for steep slope contours ( Figure 17.3 ) and widely for moderate slope ( Figure 17.4 ).

Characteristics of Contour: 

Characteristics of Contour The steepest slope of terrain at any point on a contour is represented along the normal of the contour at that point ( Figure 17.5 ). They are perpendicular to ridge and valley lines where they cross such lines. Contours do not pass through permanent structures such as buildings ( Figure 17.6 )

Characteristics of Contour: 

Characteristics of Contour Contours of different elevations cannot cross each other (caves and overhanging cliffs are the exceptions). ( Figure 17.7 ) Contours of different elevations cannot unite to form one contour (vertical cliff is an exception). ( Figure 17.8 ) Contour lines cannot begin or end on the plan. A contour line must close itself but need not be necessarily within the limits of the map.

Slide 19: 

A closed contour line on a map represents either depression or hill ( Figure 17.9(a) ). A set of ring contours with higher values inside, depicts a hill whereas the lower value inside, depicts a depression (without an outlet) Figure 17.9(b) . Contours deflect uphill at valley lines and downhill at ridge lines. Contour lines in U-shape cross a ridge and in V-shape cross a valley at right angles. The concavity in contour lines is towards higher ground in the case of ridge and towards lower ground in the case of valley ( Figure 17.10 ). Contours do not have sharp turnings.

Contouring: 

Contouring The method of establishing / plotting contours in a plan or map is known as contouring. It requires planimetric position of the points and drawing of contours from elevations of the plotted points.

Characteristics of Contour: 

Characteristics of Contour Contouring involves providing of vertical control for location of points on the contours and horizontal control for planimetric plotting of points. Thus, contouring depends upon the instruments used (to determine the horizontal as well as vertical position of points). In general, the field methods of contouring may be divided into two classes: Direct methods Indirect methods

Direct methods : 

Direct methods In the direct method, the contour to be plotted is actually traced on the ground. Points which happen to fall on a desired contour are only surveyed, plotted and finally joined to obtain the particular contour. This method is slow and tedious and thus used for large scale maps, small contour interval and at high degree of precision. Direct method of contouring can be employed using Level and Staff as follows:

Vertical control :: 

Vertical control : In this method, a benchmark is required in the project area. The level is set up on any commanding position and back sight is taken on the bench mark. Let the back sight reading on the bench mark be 1.485 m. If the reduced level of the bench mark is 100 m, the height of instrument would be 100 + 1.485 = 101.485 m

Vertical control :: 

Vertical control : . To locate the contour of 100.5 m value, the staff man is directed to occupy the position on the ground where the staff reading is 101.485 -100.500 = 0.985 m. Mark all such positions on the ground where the staff reading would be 0.985 m by inserting pegs. Similarly locate the points where the staff reading would be 101.485 -101 = 0.485 m for 101m contour.

Vertical control :: 

Vertical control : The contour of 101.5 m cannot be set from this setting of the instrument because the height of instrument for this setting of the instrument is only 101.485 m. Therefore, locating contours of higher value, the instrument has to be shifted to some other suitable position..

Vertical control :: 

Vertical control : Establish a forward station on a firm ground and take fore sight on it. This point acts as a point of known elevation, for shifting the position of the instrument to another position, from where the work proceeds in the similar manner till the entire area is contoured

Horizontal control :: 

Horizontal control : The horizontal control is generally provided by method of plane table surveying or locating the positions of points by other details in which will be discussed in later module ( Figure 17.11 ) .

Horizontal control :: 

Horizontal control :

Indirect methods: 

Indirect methods In this method, points are located in the field, generally as corners of well-shaped geometrical figures such as squares, rectangles, and spot levels are determined. Elevations of desired contours are interpolated in between spot levels and contour lines are drawn by joining points of equal elevation.

Indirect methods: 

Indirect methods Indirect methods are less expensive, less time consuming and less tedious as compared to the direct method. These methods are commonly employed in small scale surveys of large areas or during mapping of irregular surface or steep slope. There are two different ways usually employed for indirect method of contouring: Grid method and Radial line method

Grid Method : 

Grid Method In this method, the area to be surveyed is divided into a grid or series of squares ( Figure 17.12 ). The grid size may vary from 5 m x 5 m to 25 m x 25 m depending upon the nature of the terrain, the contour interval required and the scale of the map desired. Also, the grids may not be of the same size throughout but may vary depending upon the requirement and field conditions. The grid corners are marked on the ground and spot levels of these comers are determined by leveling.

Grid Method : 

Grid Method The grid is plotted to the scale of the map and the spot levels of the grid corners are entered. The contours of desired values are then located by interpolation. Special care should be taken to give the spot levels to the salient features of the ground such as hilltops, deepest points of the depressions, and their measurements from respective corners of the grids, for correct depiction of the features. The method is used for large scale mapping and at average precision.

Grid Method : 

Grid Method

Radial line method : 

Radial line method In this method, a number of radial lines are set out at known angular interval at each station and points are marked at the ground at convenient distance apart on the rays that are set. Spot levels of these points are determined by leveling.

Radial line method: 

Radial line method The points are plotted to the scale of the map and spot levels are entered. The contours of desired values are then located by interpolation. This method is convenient in hilly terrain with level stations chosen at high points so as to command a large area from each. Horizontal control may be obtained by taping ( Figure 17.13 ).

Radial line method: 

Radial line method

A Comparison between Direct and Indirect Methods of Contouring : 

A Comparison between Direct and Indirect Methods of Contouring Direct Method Indirect Method 1 Very accurate but slow and tedious Not very accurate but quicker and less tedious. 2 Expensive Reasonable cost 3 Appropriate for small projects requiring high accuracy, e.g., layout of building, factory, structural foundations, etc. Suitable for large projects requiring moderate to low accuracy, e.g., layout of highway, railway, canal, etc. 4 More suitable for low undulating terrain. Suitable for hilly terrain. 5 Calculations need to be carried out in thefield Calculation in the field is not mandatory. 6 After contouring, calculation cannot be checked. Calculations can be checked as and when needed

Drawing of Contours: 

Drawing of Contours Points of desired elevation, at which contours are desired to be drawn, are interpolated in between observed points. Then, contours are drawn by joining points of equal elevation by smooth curves keeping in mind the principal characteristics of contour.

Drawing of Contours: 

Drawing of Contours They are then inked in, preferably in brown to distinguish them from other features. The contour value is written down in a gap in the line provided for the purpose. Every fifth contour is drawn bolder to make it distinguishable from the rest.

Ex.17-1 On the basis od spot elevations in meters given in Fig. 1 draw contours at 20 m interval.: 

Ex.17-1 On the basis od spot elevations in meters given in Fig. 1 draw contours at 20 m interval .

Ex.17-2 Fig. 2 shows the same area with stream courses in addition to the spot elevations. Draw the contours, in this case also, at 20 m interval: 

Ex.17-2 Fig. 2 shows the same area with stream courses in addition to the spot elevations. Draw the contours, in this case also, at 20 m interval

Contour Gradient : 

Contour Gradient An imaginary line on the surface of the earth having a constant inclination with the horizontal (slope) is called contour gradient. 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.

Slide 43: 

When the inclination of a contour gradient is given its direction from a point may be easily located either on the map or on the ground by the methods discussed below.

Locating contour Gradient on a Map : 

Locating contour Gradient on a Map With the aid of contour plan, it is easy to trace a contour gradient of desired inclination on a paper, and even transfer it later to the ground ( Figure 18.1 ).

To locate a rising gradient of 1 in 100 from a point say P situated on 200 m contour on the map having contour interval 5 m at a scale of 1: 5000, draw an arc of radius : 

To locate a rising gradient of 1 in 100 from a point say P situated on 200 m contour on the map having contour interval 5 m at a scale of 1: 5000, draw an arc of radius

Slide 47: 

with radius at P. The arc cuts the 205 m contour at Q. Locate R and S on 210m and 215 m contours taking arcs of radius of 10 mm with centres at R and S, respectively. Join P,Q,R and S. The line P to S represents the contour gradient on the ground having constant slope of 1 in 100.

Locating contour Gradient on the Ground : 

Locating contour Gradient on the Ground To locate a rising gradient of 1 in 100 from the station P, a level is set up at a commanding position and back sight is taken at P. Let the back sight reading be 1.255 m. The staff reading at any point X on the contour gradient can be calculated from its distance from P ( Figure 18.2 ). For the distance XP of 20 m, the required staff reading would be

To locate a rising gradient of 1 in 100 from the station P, a level is set up at a commanding position and back sight is taken at P. Let the back sight reading be 1.255 m. The staff reading at any point X on the contour gradient can be calculated from its distance from P (Figure 18.2). For the distance XP of 20 m, the required staff reading would be : 

To locate a rising gradient of 1 in 100 from the station P, a level is set up at a commanding position and back sight is taken at P. Let the back sight reading be 1.255 m. The staff reading at any point X on the contour gradient can be calculated from its distance from P ( Figure 18.2 ). For the distance XP of 20 m, the required staff reading would be

Locating contour Gradient on the Ground : 

Locating contour Gradient on the Ground To locate the point X on the ground, the staff man holds the 20 m-mark of the tape, keeping the zero-mark at P, and moves till the staff reading of 1.055 m is obtained. Likewise, the staff readings for other points at known distance from P, are calculated, and the points are located.

Locating contour Gradient on the Ground : 

Locating contour Gradient on the Ground If the point Q is on the contour of 105 m, its distance from P would be 500 m in this case. The instruments such as Indian clinometer , theodolite and Ghat tracer may also be used for tracing the contour gradient on the ground.

Uses of Contours : 

Uses of Contours Contours provide valuable information about the nature of terrain. This is very important for selection of sites, determination of catchment area of a drainage basin, to find intervisibility between stations etc. Some of the salient uses of contours are described below.

Nature of Ground : 

Nature of Ground To visualize the nature of ground along a cross section of interest, a line say XY is being considered through the contour map ( Figure 18.3 ). The intersection points between the line and contours are projected at different elevations of the contours are projected and joined by smooth curve. The smooth curve depicts the nature of the ground surface along XY

To Locate Route : 

To Locate Route Contour map provides useful information for locating a route at a given gradient such as highway, canal, sewer line etc.

To Locate Route : 

To Locate Route Let it be required to locate a route from P to Q at an upward gradient of 1 in 100. The contour map of the area is available at a contour interval of 5 meter at a scale of 1:10000. The horizontal equivalent will therefore be equal to 100 meter. Then with centre at P with a radius of 2 cm draw an arc to cut the next higher contour, say at q. With q as centre, mark the next higher contour by an arc of radius 2 cm say at r. Similarly, other points such as s,t,u …. etc are obtained and joining the points provides the location of route. ( Figure 18.2 )

Intervisibility between Stations : 

Intervisibility between Stations When the intervisibility between two points can not be ascertained by inspection of the area, it can be determined using contour map. The intervisibility is determined by drawing a line joining the stations / points say PQ and plot the elevations of the points and contours intersected by PQ as shown in Figure 18.3 . If the intervening ground is found to be above A'B' line, the intervisibility is obstructed. In the figure, the ground is obstructing the line of sight.

To Determine Catchment Area or Drainage Area: 

To Determine Catchment Area or Drainage Area The catchment area of a river is determined by using contour map. The watershed line which indicates the drainage basin of a river passes through the ridges and saddles of the terrain around the river. Thus, it is always perpendicular to the contour lines. The catchment area contained between the watershed line and the river outlet is then measured with a planimeter ( Figure 18.4 ).

To Determine Catchment Area or Drainage Area: 

To Determine Catchment Area or Drainage Area

Storage capacity of a Reservoir: 

Storage capacity of a Reservoir The storage capacity of a reservoir is determined from contour map. The contour line indicating the full reservoir level (F.R.L) is drawn on the contour map. The area enclosed between successive contours are measured by planimeter ( Figure 18.5 ). The volume of water between F.R.L and the river bed is finally estimated by using either Trapezoidal formula or Prismoidal formula.

Storage capacity of a Reservoir: 

Storage capacity of a Reservoir