contouring in surveying

 contouring in surveying

A contour line is a imaginary line which connects points of equal elevation. Such lines are drawn on the plan of an area after establishing reduced levels of several points in the area. A numerical value placed upon a contour line to denote its elevation relative to a given datum, usually mean sea level is called Contour Value. The contour lines in an area are drawn keeping difference in elevation of between two consecutive lines constant. Alternatively , a contour or a contour line may be defined as the line of intersection of a level surface with the surface of ground. This means every point on a contour line has the same altitude as that of the assumed intersecting surface. The process of tracing contour lines on the surface of the earth is called contouring and the maps upon which these lines are drawn are called contour maps.

The constant vertical distance between two consecutive contours is called the Contour Interval and the horizontal distance between any two adjacent contours is termed as the horizontal equivalent. The horizontal equivalent depends upon the slope of the ground The contour interval of a contour map is the difference in elevation between successive contour lines. For example, Figure shows contours in an area with contour interval of 1 m.

The contour interval depends upon the following factors:

(i) The nature of the ground:

In flat and uniformly sloping country, the contour interval is small, but in broken and mountainous

region, the contour interval should be large otherwise the contours will come too close to each

other.

(ii) The purpose and extent of the survey:

Contours interval is small if the area to be surveyed is small and the maps are required to be

used for the design work or for determining the quantities of earth work etc., while wider

interval shall have to be kept for large areas and comparatively less important works.

(iii) The scale of the map:

The contour interval should be in the inverse ratio to the scale of i.e. the smaller the scale, the

greater the contour interval.

(iv) Time and expense of field and office work:

The smaller the interval, the greater is the amount of field -work and plotting-work.

Characteristics of Contours

• Contour lines must close, not necessarily in the limits of the plan.

• Widely spaced contour indicates flat surface.

• Closely spaced contour indicates steep ground.

• Equally spaced contour indicates uniform slope.

• Irregular contours indicate uneven surface.

•Depression between summits is called a 

saddle. It is represented by four sets of

contours . 

It represents a dip in a ridge or the 

junction of two ridges.

And in the case of a mountain range, it

takes the form of a pass. Line passing

through the saddles and summits gives 

water shed line.

•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

In this case, several contours coincide and the 

horizontal equivalent becomes zero.

• If contour lines cross each other,

it shows existence of overhanging

cliffs or a cave.