CONTOURING

CONTOURING

Contour line: A Contour line is an imaginary outline of the terrain obtained by joining its points of equal elevation. In our example of the cone, each circle is a contour line joining points of same level.

Contour interval: A contour interval in surveying is the vertical distance or the difference in the elevation between the two contour lines in a topographical map. Usually there are different contour intervals for the different maps.

Horizontal equivalent: The horizontal distance between two points on two consecutive contour lines for a given slope is known as horizontal equivalent.

  

USES OF CONTOUR MAPS

   Contour maps are extremely useful for various civil engineering works as explained below:

[1]. Preliminary selection of project sites: Characteristics of contour lines give considerable information about nature of ground. Sitting in the office studying contour lines, a civil engineer decides various possible sites for his project.

[2]. Drawing of sections: From contour plan, it is possible to study profile of the ground along any line, which is normally required for earthwork calculation along a formation ground.

[3]. Determination of intervisibility: If intervisibility of any two points is to be checked, using contour, profile of the ground along the line joining those two points can be drawn. Then the line joining those two points is drawn. If the ground portion is above this portion, the two stations are not intervisible.

 [4]. Location of routes: The routes of railway, road, canal or sewer lines can be decided with the help of contour maps. After deciding the gradient of the route, it can be set on the map as explained.

[5]. Determining catchment Area; The area on which fallen rainwater drains into river at a particular point is called catchment area of the river at that point. This area can be determined from contour plans. The catchment area is also known as drainage area. First the line that separates the catchment basin from the rest of area is drawn. This is called watershed time. It normally follows ridge line. Then the area within watershed line is measured. This area is extremely useful in studying flood level and quantity of water flow in the river.

[6]. Calculation of reservoir capacity: The submerged area and the capacity of a proposed reservoir by building bund or dam can be found by using contour maps. After determining the height of the dam its full reservoir level is known. Then area between any two contour lines and the dam line is measured by using plan meter to find the reservoir capacity.

CHARACTERISTICS OF CONTOUR

The principal characteristics of contour lines which help in plotting or reading a contour map are as follows:

[1]. Contour lines must close, not necessarily in the limits of the plan.

[2]. The horizontal distance between any two contour lines indicates the amount of slope and varies inversely on the amount of slope.

[3]. Widely spaced contour indicates flat surface (Fig. Con.2).

[4]. Closely spaced contour indicates steep slope ground (Fig. Con.2).

 [5]. Equally spaced contour indicates uniform slope.

[6]. Irregular contours indicate uneven surface (Fig. Con.3).

[7]. Approximately concentric closed contours with decreasing values towards centre indicate a pond (Fig. Con.4).

[8]. Approximately concentric closed contours with increasing values towards centre indicate hills (Fig. Con.5).

[9]. Contour lines with U-shape with convexity towards lower ground indicate ridge (Fig. Con.6.a).

[10]. Contour lines with V-shaped with convexity towards higher ground indicate valley (Fig. Con.6.b).

[11]. Contour lines generally do not meet or intersect each other. If contour lines are meeting in some portion, it shows existence of a vertical cliff (Fig. Con. 7).

       

[12]. Contours of different elevations cannot cross each other. If contour lines cross each other, it shows existence of overhanging cliffs or a cave (Fig. Con. 8).

    

[13]. The steepest slope of terrain at any point on a contour is represented along the normal of the contour at that point.

[14]. Contours do not pass through permanent structures such as buildings (Fig. Con. 9).

 

[15]. A contour line must close itself but need not be necessarily within the limits of the map.

(Next post on “METHODS OF CONTOUR”)