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).
[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”)