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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.
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.
Contour Map explains the topography of the ground surface. It assists designers to understand highpoints, low points, hilly terrain, valleys and flat spot locations.
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, road cross-fall of primary road joins the cross-fall of 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 surfaces.
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 interval.
Major contour lines are recommended to show with prominent colour along with texts showing the contour interval value.
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 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, so these are known as Intermediate 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.
Following are the key factor affecting 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 as contour interval.
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 steep hill or vertical cliff or wall.
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