What is Contour Interval? 4 Important Points

Table of Contents

What is Contour?

What is the Purpose of Contour Maps?

Purpose of Contour Maps of Existing Ground:

Purpose of Contour Maps for Proposed Surface:

How to generate Contour Maps?

How to Read Contour Lines and Calculate Contour Interval?

What are the 3 types of Contour Lines?

Index Lines:

Intermediate Lines:

Supplementary Lines:

What are the factors affecting Contour Interval?

How do you find the Contour Interval?

Why can't Contour Lines cross?

What Contour Map does not cover?

As a Civil Engineer, you must have seen Contour Maps, but do you know what is Contour maps and Contour intervals?

Please read this blog for details.

What is Contour?

Contours are imaginary lines passing through equal elevation points on the ground surface, normally created by using the triangulation surface model. In simple words when we analyse a 3D surface and observe some equally elevated points and draw a line joining those equal elevation points, the line is called a contour line.

When the contour lines are drawn underwater, they are termed as submarine contours.

What is the Purpose of Contour Maps?

Contour Map explains the topography of the ground surface. It assists designers to understand highpoints, low points, hilly terrain, valleys and flat spot locations.

Purpose of Contour Maps of Existing Ground:

1. In order to design a new physical infrastructure, it is very important to know where are the low points or high points across the project area. This will help designers to design the levels of the proposed road or railway lines in a similar manner to minimise extensive cuttings or embankments.
2. In an urban or rural setting, low-lying locations are commonly used for drainage outfalls and connect to existing rivers or ponds or canals. So generating a contour map of existing ground helps designers to plan the proposed drainage network accordingly.
3. Low-lying areas (close to rivers or canals) are prone to flooding. Meteorological Departments use Contour Maps to inform the public about flood-prone locations. Design organisations use this information’s plan new infrastructure projects in such a way that further damage or further flooding can be avoided.

Purpose of Contour Maps for Proposed Surface:

1. Contour Maps are created to check the design proposals. In Rural Highways or Urban Road Projects, contour maps assist designers to check the flat spots, flow arrows, steep slopes, uneven surfaces over the junction.

2. If flat spots are found within the contour map, it means those locations could create ponding and need to redesign to eliminate flat spots.

3. Flow arrow helps drainage designers to identify low points and propose drainage gully pots and kerbs drainage systems accordingly. It also helps designers to review if the low points are coming across the pedestrian crossing points (at road junction) or not. If they are then it would be recommended to eliminate those low points.

4. Steep slopes or sub-standard cross-falls along the footways would not be comfortable for wheelchair and pushchairs users. It is not recommended to provide a footway cross-fall of more than 3.3%. So, contour maps are used to check the steep slopes of proposed surfaces.

5. Over the urban junction, the road cross-fall of the primary road joins the cross-fall of the secondary road. Therefore, it is very important to design the cross-fall at the interface location precisely and contour maps assist designers to eliminate uneven surfaces and make them like a smooth surface.

How to generate Contour Maps?

It is recommended to generate contour maps at 25mm intervals. By doing this it would be very easy to read and review the topography of the surface. To make it easy to read drawings, it is recommended to show minor contour lines with less prominent colour and without any text for contour intervals.

Major contour lines are recommended to show with prominent colour along with texts showing the contour interval value.

How to Read Contour Lines and Calculate Contour Interval?

1. Contour lines very close to each other represent steep slopes, whereas contour lines with comparatively larger distances represent gentle slopes. If there is a significant space between two contour lines, then it represents a flat spot location.
2. Increasing order of contour interval represents a hilly terrain surface and decreasing order of contour interval represents a valley or ridge type surface.
3. Contour lines never cross each other. If you find a location where a contour line crosses another contour line, it means there is an error while generating the contour map. However, contour lines can merge to one point. For example, if there is a vertical cliff at one point, the contour lines will be shown as merging at one point.
4. Concentric circles shape of contour line with increasing order of contour interval represents a cone-shaped hilly terrain.
5. Concentric circles shape of contour line with decreasing order of contour interval represents a cone-shaped deep valley.
6. Equally spaced contour represents a uniform slope and these types of contour maps are commonly seen in a complaint road design proposal. Irregular contour lines represent an uneven ground surface and are commonly seen when maps are created from a topographic survey of greenfield land.

What are the 3 types of Contour Lines?

Index Lines:

Index Lines are the prominent contour lines represented with a round number contour interval. For example, in a contour map of 0.25m interval, every fifth contour line will be a round number (like 1.00, 2.00, 3.00 etc) and they will be known as Index Contour Lines.

Intermediate Lines:

Intermediate Lines are lines between Index Lines and represent by standard contour intervals. In the above example, the lines between index lines of 1.00 and 2.00 for the contour interval of 0.25m are 1.25, 1.50, 1.70, etc, so these are known as Intermediate Lines.

Supplementary Lines:

Supplementary Lines normally appear when there is flat terrain for the wider area. In this case, supplemental lines are shown with dotted lines as notional information.

What are the factors affecting Contour Interval?

Following are the key factor affecting contour interval.

• Terrain
• The scale of the Contour Map
• Extent of Map

How do you find the Contour Interval?

First, determine the difference between two index contour lines (let’s say the value is X). Then, count the number of intermediate lines and one index line (let’s say it’s Y). Now divide the X by Y and that is termed a contour interval.

Why can't Contour Lines cross?

One contour line represents one elevation level so if there are two contour lines then both will represent two different levels. That’s why two contour lines cannot cross each other, however, they can merge at one point in case of a steep hill or vertical cliff or wall.

What Contour Map does not cover?

• Contour Map does not cover trees, shrubs, plants, etc although these features are recorded within the topographical survey.
• Contour Map does not cover the size and shape of any physical infrastructure such as manholes, gully pots, fencing, lighting poles, etc.

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