While performing the site analysis, we also derive the site contours. But often, they get forgotten somewhere in the design process, or we simply don’t know how to design with them.
And I can’t forget the first time I tried to read a contour map. All I saw was a bunch of lines and deciphering it seemed like an impossible task. Designing for contours is the most efficient way to make the best use of the existing topography of the site, and that begins by understanding how to read a contour map.
The first step to designing with contours begins with how to read and understand a contour map.
When working with contours it is important to know and understand the following terms:
The difference in elevation between two contour lines is known as contour interval. The contour interval should stay constant throughout the contour plan. So the contour interval should increase or decrease at a constant ratio.
The taller or elevated parts of the site are referred to as ridges. They can be mountains, hills, dunes, etc.
The lower or depressed parts of the site are referred to as valleys. They can be rivers, streams, etc.
The lower and less undulating area between two high elevations is known as the saddle.
A surface is termed as undulating when it is a combination of ridges and valleys. The elevation constantly varies throughout the different parts of the site.
A slope occurs when the vertical height increases over a horizontal span. A slope is calculated by dividing the rise height by the run distance.
Slope = rise/run
To represent the slope as a percentage, the above ratio is multiplied by 100.
Imaginary lines connecting points of equal elevation are known as slope lines.
The steepest slope is often perpendicular to the slope line.
Deriving the ratio of the slope by calculating the slope in each contour helps us understand the shape of the contour.
The gradient is the measurement of how steep a slope is. This is calculated by dividing the vertical height by the horizontal distance. The gradient(slope) of a straight line helps us indicate the steepness of a slope.
A slope gradient is often divided into percentages and each percentage range is colour coded to show the gradient of the slope.
0-2% – White
2-5% – Yellow
5-10% – Orange
10-15% – Brown
In the above case, 2% slope refers to 2m height of slope for 100m in horizontal span.
In a contour map you will be able to notice 3 types of contour lines:
These are thicker solid lines that are labelled with a number – the elevation of the contour line.
These are solid lines that aren’t labelled and are thinner than the index lines. All the lines preceding an index line will often be intermediate lines. And one index line often occurs between five intermediate lines, but this can vary based on the scale of the map.
These are dotted lines that indicate a much flatter terrain in comparison to the intermediate line elevations.
In a contour map, the following can be found:
Now that we understand what a contour map comprises, let’s get into how we read one of these maps.
Step 1: Let us take a contour map whose contour interval is given as 10m in the map key. If you want to find the elevation of the fourth intermediate line, first read the index line.
Step 2: For each line preceding the index line, the elevation adds up by 10m. Hence, for the fourth intermediate line, the elevation will be as follows:
Elevation of 4th intermediate = Index Line value + elevation x no. of intermediate lines
= 500m + 10mx4
Reading a contour map might seem super intimidating at first. But by understanding the contour terms and the basic ideology behind the way the map is structured, you will be able to read any contour map the minute you see it.
At the basic level, contour mapping helps analyse the topography of the site and helps you identify the elevation of the site. Some of the advantages of contour mapping are:
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