Introduction
A contour is defined as an imaginary line of constant elevation on the ground surface can also be defined as the line of intersection of a level surface with the ground surfac Process of tracing contour lines on the surface of the earth is called contouring Fr example, the line of intersection of the water surface of a still lake or pond with the surrounding ground represents a contour line. A line joining points of equal elevations a called a contour line. It facilitates depiction of the relief of terrain in a two dimensionl plan or map. A contour map consists of contour lines of a given geographical region. T keep the contour map simple and easy to read, not every contour line is marked and heavier with its elevation reading. These marked or labeled lines are termed as Indes Contour Lines or major contour lines. Other unmarked, faint contour lines between two requi consecutive index contour lines are known as general or minor contour lines.
A. Definitions of the Terms
Contour line:
A line joining points of equal elevations is called a contour line.
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 fig 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. Horizontal equivalent depicts the nature of ground. If I in n be the gradient of ground and "CI' m be the contour interval then, horizontal equivalent is given by: Horizontal equivalent = (Cl×n)m.
Index contour:
A contour line is shown on a map in a distinctive manner for an ease of identification, being printed more heavily than other contour lines and generally labeled with a value (such as figure of elevation) along its course. It appears at regular intervals, such as every fifth of contour line (depending on the contour interval).
Value of Index contour = Multiple of [Contour interval × 5] and divisible by contourn iterval × 5 in whole number.
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 (CI) of the map Fig (a) shows contour interval of Im whereas fig (b) shows 10m. The choice of suitable contour interval in a map depends upon the following principe Acc considerations. These are:
Contour Interval:
1. Nature of terrain:
The contour interval depends upon the nature of the terrain. For flat ground, a small contour interval is chosen whereas for undulating and broke ground, greater contour interval is adopted. This is to avoid the unification of contour line for a given scale while plotting the contour map In flat ground, there s no or little level difference available for long distance which yields less no of contour line. If large value of Cl is chosen, contour line may not exist so small Ava value of CI should be chosen but in steep terrain, for a small distance, there exists: and large level difference which yields numbers of contour lines. If small value of Clis If th chosen for a given scale of map, contour lines will come close to each other yielding bad map. Thus, large value of CI is preferred suitable with given scale.
2. 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 which prevents the unification of contour lines in plotted map.
3. Accuracy:
Accuracy need of surveying work also decides 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 such as roads and railways where the desired accuracy is less, higher contour interval should be used. In such case, extent of survey is generally large.
4. Available Time and Funds (costs):
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.
5. Purpose of map:
The contour interval on a map also depends upon the purpose for whichthe map is to be prepared. If the map is prepared for setting out a highway on hill slopes,large contour interval may prefer. For construction of small project such as construction ofhousing, university a small contour interval is prefered for accurate work.
Fig: contour lines
B. Uses of Contour Map
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:
1. To decide the most economical and suitable sites for engineering construction works such as canals, drainages, reservoirs and highway.
2. To study the general features/characteristics of the tract of the country without visiting the site. The country may be classified as flat, undulations or mountainous with the help of knowledge of the characteristics of contours.
3. To visualize the nature of ground along a cross section of interest, a line say XY is being considered through the contour map. The intersection points between the linea nd contours are projected at different elevations of the contours are projected ando joined by smooth curve. The smooth curve depicts the nature of the ground surfacea long XY (Determination of intervisibility).
4. 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.
5. Storage capacity of a Reservoir: The storage capacity of a reservoir is determined from contour map. The contour line indicating the full reservoir level (FRL) is drawn on the contour map. The area enclosed between successive contours measured by planimeter. The volume of water between FRL and the river bed is finally estimated by using either Trapezoidal formula or Prismoidal formula.
6. To trace out the contour gradient of the highway and intervisibility.
7. To plot the longitudinal and cross sections to ascertain the nature of the ground.