Contouring in Surveying - Contour Lines & Contour Intervals

In addition to the relative positions in a plan, if a map shows the spatial configuration or relief of the topographic features, then it adds real value. 

There are various ways to represent relief through relief models, shading, form lines and contour lines. Among which, contour lines indicate elevations directly and quantitatively. 

Contouring in Surveying - Contour Lines & Contour Intervals

We will discuss what is contouring and the important features of contouring used in civil engineering surveying.

What is Contouring?

A contour line is a continuous line or curve that is obtained by drawing the points at the same elevation on the ground above a chosen datam. It can also be defined as the intersection of level surface with the surface of the earth.

In a map or plan, contours are represented as contour lines. The preparation of a contour plan for survey work and other office works are called as contouring. 

Contour represent the topographic configuration with better facility, accuracy and definiteness compared to other symbols. 

Contour Interval in Contour Maps

The vertical distance between two consecutive contours is termed as contour interval. It is desired to have three constant contour intervals throughout the map.

But for certain areas, which extreme steep topography, variable contour intervals can be adopted. But, it is adopted for simple cases as it would give a false impression of steepness. 

So contour intervals are dependent on:

1. Scale of the Map
2. Purpose of the Map
3. Nature of the Country
4. Time
5. Funds

1. Scale of the Map

Contour interval is inversely proportional to the scale of the map. The contour interval for different terrain for different scale are mentioned in the table below.

In general, contour interval =  (25/ number of cm per km ) m

2. Purpose of the Map

• For highways and railways, the contour interval is kept up to 2m
• For earthworks, building sites, dams etc. the interval is kept as small as 0.5m
• For city surveys, a contour interval of 0.5 m may be adopted
• For geological surveys, 6 to 15 m can be adopted
• For town planning surveys, 0.5 to 2m interval can be chosen, for scale 1cm = 15 to 100m
• For Location Surveys, interval of 2-3m for scale 1 cm = 50 to 200 m can be adopted

3. Nature of the Country

Contour intervals varies based on topography. It is large for steep grounds and small for flat grounds.

4. Time

When the project time is less, the contour intervals are kept large to finish the work fast.

5. Funds

When funds are short and limited, the contour intervals are kept large.

Horizontal Equivalent in Contouring

1. Horizontal equivalent is defined as the horizontal distance between the consecutive contours. 
2. Horizontal equivalent is not a constant value.
3. Horizontal equivalent varies from point to point with the steepness of the ground
4. Steeper the ground, lesser is the Horizontal equivalent

Figure-3: Horizontal Equivalent for a Regular Section

In the figure-3 above, the section of a truncated cone is given with different elevation from A to H. Their respective contour lines are also drawn as shown in plan. The horizontal distance between the consecutive contours A and B is called as Horizontal equivalent in the plan. 

The figure-4 above shows a contour for a irregular section. For the steeper portion BB' in the elevation, you can see smaller horizontal equivalents are taken.

Contour Gradient

Contour gradient is the line on the ground surface that maintains a constant inclination to the horizontal. It is determined using clinometer, theodolite or a gradienter. 

Contour gradient is determined as a field operation in locating roads and railways over a range of hills. 

Read More On: Characteristics of Contour Lines - How to Read Contour Lines in a Contour Map?

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Declination and Angle of Dip in Surveying

Bearings in Surveying - Meridian & Types of Bearings

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