The line of intersection of level surface with the bottom surface is understood as contour line or just the contour. It also can be defined as a line passing through points of equal reduced levels.For example, a contour of 100 m indicates that each one the points on this line have an RL of 100 m.
Similarly, during a contour of 99 m, all points have an RL of 99 m, and then on (Fig. 6.1).
A map display only the contour lines of a region is named a contour map.
The vertical intervening space among any two consecutive contours is understood as a contour interval.
Guess a map comprehends contour lines of 100 m, 98 m, 96 m, and so on.
The contour interval herein is 2 m. This interval depends upon,
i) the character of the earth surface (i.e. whether flat or steep),
(ii) the size of the map, and
(iii) the aim of the survey.
Contour intervals for flat land are usually small, e.g. 0.25 m, 0.50 m, 0.75 m, etc.
The contour interval for a steep slope during a hilly area is usually greater, e.g. 5 m, 10 m, 15 m, etc.
Again, for a small-scale map, the interval could also be of 1 m, 2 m, 3 m, etc., and for giant scale map, it should be of 0.25 m, 0.50 m, 0.75 m, etc.
It should be remembered that the contour interval for a specific map is constant.
The aclinic space among any two consecutive contours is understood as horizontal equivalent. it’s not constant.
It varies consistent with the steepness of the bottom .
For steep slopes, the contour lines run close, and for flatter slopes they’re widely spaced.
Object Of Preparing Contour Map
The general map of a rustic includes the locations of roads, railways, river, villages, towns, and so on. But the character of the earth surface can’t be realised from such a map.
However, for all engineering projects involving roads, railways, and so on, a knowledge of the character of ground surface is required for locating suitable alignments and estimating the quantity of earth work.
Hence, the contour map is significant for all engineering projects. this is often why contour map is ready .
Uses Of Contour Map
The following are the precise uses of the contour map:
1. the character of the earth surface of a rustic are often understood by studying a contour map.
Hence, the possible route of communication between different places are often demarcated.
2. an appropriate site or a cheap alignment are often selected for any engineering project.
3. The capacity of a reservoir or the realm of a catchment are often approximately computed.
4. The intervisibility or otherwise of various points may be established.
5. an appropriate route for a given gradient will be marked on the map.
6. an area of the earth surface are often drawn in any direction from the contour map.
7. Quantities of earth work will be approximately computed.
Characteristics Of Contours
In Fig. 6.2, the contour lines are close near the highest of a hill or high ground and wide apart near the foot.
This means a really steep slope towards the height and a flatter slope towards the foot.
In Fig. 6.3, the contour lines are close near a bank of the pond or dumps and wide separately towards the centre, near the bank and a flatter slope at the centrum.
Uniformly gaped contour lines indicate a uniform slope (Fig. 6.4).
Contour lines always form a closed circuit. But these lines may be within or outside the limits of the map (Fig. 6.5).
Contour lines can’t cross one another, except in the case of an overhanging cliff . But the overlapping portion must be shown by dotted line (Fig. 6.6).
While the greater values are inside the loop, it indicates a ridge line. Contour lines cross ridge lines at right angles (Fig. 6.7).
While the smaller values are inside the loop, it Indicates a line. Contour lines cross the valley line at right angles (Fig. 6.8).
A sequence of closed contours constantly indicates a depression or summit.
The lower values essence beneath the loop indicates a depression and therefore the higher values being inside the loop indicates a summit (Fig. 6.9).
Depressions between summits are called saddles [Fig. 6.10(b)].
Contour lines joining at a point indicate a vertical cliff [Fig. 6.10(a)].
Method Of Contouring
There have two methods of contouring—direct, and indirect.
There may be two cases, as outlined below.
Case I When the area is oblong and cannot be controlled from a single station
In this method, the varied points on any contour are located on the earth surface by taking levels.
Then these points are marked by pegs. After this, the points are plotted on the map, to any compatible scale, by plane table, This method is extremely slow and tedious. But it gives exact contour lines.
1. Guess a contour map is to be made for an oblong area. a short lived bench-mark is ready up near the positioning by taking fly-level readings from a permanent bench-mark.
2. the level is then established at an appropriate position L from where maximum area are often covered.
3. The plane table is ready up at an appropriate station P from where the above area are often plotted.
4. A backsight measurement is taken on the TBM. Guess the RL of the TBM is 249.500 m that the BS reading is 2.250 m.
Then the RL of HI is 251.750 m. If the contour of 250.000 m is necessary, the staff reading should be 1.750 m.
If the contour of 249.000 m is necessary, the staff reading will be 2.750 m, and so on.
5. The staffman holds the staff at different points of the realm by moving up and down, or left and right, until the staff reading is precisely 1.750.
Then the points are marked by pegs. Let, A, B, C, D …., these are the points.
6. an appropriate point p is chosen on the sheet to represent the station P. After, by means of the alidade touching p, rays are drawn to A, B, C and D.
Then measured the distance of PA, PB, PC and plotted to an appropriate scale. during this manner, the points a, b, c and d of the contour of RL 250.000 m are obtained.
These points are joined to get the contour of 250.000 m (Fig. 6.11).
7. Likewise, the points of the another contours are located.
8. While required, the levelling instrument and the plane table are transferred and set up in a new position in order to continue the operation along the oblong area.
Case Il When the area is Small and can be controlled from a single station
In this case, the method of radial lines is adopted to obtain contour map, This is also very slow and tedious, but gives the actual contour lines.
1. The plane table is about up at an appropriate station P from where the entire area may be commanded.
2. some extent p is suitably selected on the sheet to represent the station P. Radial lines are then drawn in several directions.
3. a short lived bench-mark is established near the location . the level is ready up at an appropriate position L and a BS reading is taken on the TBM.
Let, the HI during this setting be 153.250 m. So, to search out the contour of 152.000 m RL, a staff reading of 1.250 m is required at a specific point, in order that the RL of contour of that time involves 152.000 m.
RL=HI – Staff reading
=153.250 – 1.250 =152.000 m.
4. The staffman holds the staff along the rays drawn from the plane table station in such how that the staff reading on it point is precisely 1.250.
During this manner, points A, B, C, D and E are located on the bottom , where the staff readings are exactly 1.250.
5. The distances of PA, PB, PC, PD and PE are surveyed and plotted to any suitable scale.
Thus the points a, b, c, d and e are obtained which are joined so as to get a contour of 152.000.
6.:The other contours could also be located in similar fashion (Fig. 6.12).
In this method, the RLs of various points (spot levels) are taken at regular intervals along a series of lines found out on the earth surface.
The positions of points are to be plotted on a sheet to any compatible scale. The spot levels are noted at the relatives points.
Then the points of contour lines are known by after which they’re joined to induce the specified contour lines.
Although very quick, this provides only the approximate positions of thc contour lines. This method are often adopted in two ways: (i) cross-sections, and (ii) squares.
Using Cross Sections during this method, a bottom line , centre line or profile line is taken into account.
Cross-sections are taken perpendicular to the present line at regular intervals (say 50 100 m, etc.). After this, points are marked along the cross sections at a daily intervals (say, 5 m, 10 m, etc).
A TBM (temporary bench-mark) is about up near the location . Staff readings are taken along the earth surface line and therefore the cross sections.
The readings are noted into the level book; the bottom line and therefore the cross sections should even be mentioned. The RL of every of the points calculated.
Then the bottom line and cross sections are plotted to an appropriate scale.
Then, the RLs of the respective points are noted on the map, after which the specified contour line is drawn by interpolation.
This method is worthy to the route survey, when cross sections are taken transverse to the longitudinal section (Fig. 6.13).
Using Squares In this method (Fig. 6.14), the realm is split into variety of squares.
The dimensions of those squares depends upon the character and extent of the bottom .
Generally, they need a sides varying from five to twenty meters.
The corners of the squares are numbered serially, as l, 2, 3 … a short lived bench-mark is about up near the positioning, and therefore the level is about up at an acceptable position.
Then the staff readings on the corners of the squares are aching and to be noted within the level book maintaining the sequence of the serial numbers of these corner, The RLs of all the corners had been calculated.
The skeletons of the squares are then plotted to an appropriate scale. The relatives RLs are to be noted on the corners, after which the contour lines are drawn by interpolation.
The contour join the points of same elevation directly or by interpolation technique. and therefore the points of same RL are joined to create a contour map..