Understanding the different characteristics of contour lines, their types and features helps one to easily read a contour line from a contour map.
We will discuss each important feature that will help read contour lines of a topography.
Types of Contour Lines
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| Types of Contour Lines |
We will encounter three types of lines in a contour map. They are:
- Index lines
- Intermediate lines
- Supplementary lines
Index lines are thickest lines among the three contour lines and are usually numbered along the line. These lines tell the elevation lines with respect to sea level as datum.Intermediate lines are thinnest lines among the contour lines that are seen between the index lines. After five intermediate lines, there appears an index lines.
Supplementary lines appear as dotted lines that indicates a flatter terrain.Characteristics of Contour Lines
1. Slopes in Contour Map
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| Contour Lines of steep and gentle slope |
- Closer contour lines represent steep slopes
- Wide contour lines represent flat slopes
- Uniformly spaced contour lines indicate uniform slope
- Series of straight, parallel, equally-spaced contour lines represent plane surface
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| Plane Surface Contour Lines |
2. High-Lying Forms or Hills in Contour Map
High-lying forms means elevated grounds. Mostly, they can be hill, hillock and plateau. The contours of high-lying grounds are bit circular in shape and have increasing contour values inwards as shown below. The figure below shows the general contour line patters of a hill, hillock and a plateau.Â
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| High-Lying Forms or Hills in Contour Map |
3. Low-Lying Forms and Depressions in Contour Map
The most common low-lying forms or depressions are ravines, and valleys etc. In both the cases they have decreasing contour values inwards.
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| Valley, Depression and Gorge Contour Lines |
- A ravine is a depression in the form of a trough on the earth's surface that is elongated in one direction with the bottom inclined towards one side as shown below. It is a depression that is washed out in the ground by the flowing water.Â
- Ridge is a convex form of terrain that is gradually declining in one direction and a counter part of ravine.
- Valley is a broad ravine with a gentle sloping bottom, usually V-Shaped. The contour lines have a V pattern.
- When the valley floor is very narrow and have steep sides on a level terrain, it is called as gorge. It is also termed as canyons in mountain region. Due to steepness, contours are crowded.
- When the ground is low compared to surrounding land with a gentle slope, it is called as depression with contour line few far apart.Â
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| Ride and Valley |
Always remember that the contour lines repeat or occur in pairs on opposite sides of ridges and valley.Â
4. Valley Line and Ridge Line
The slope of contour lines of ravine intersect along a line called as line of discharge. When the slope of valley intersect along a line, then the line is called valley line.Â
When the slope of contours of ridge intersect a line, it is called a ridge line or axis of ridge or watershed or watershed line, which is generally wavy in nature.
All the intersection is at right angles, where at a ridge line, the contour lines curve around with concave side along increasing ground. At a valley line, the contour lines curve around with convex side along the increasing ground.
So contour lines cross ridge or valley line at right angles.
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| Ridge Line and Valley Lines |
5. Saddle
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| A Saddle |
A saddle is a low point between two sections with higher elevation. This does not mean that its a valley. It can be a lower section between two peaks on a mountain.
6. Ending of Contour Lines
A contour line cannot simple end anywhere. It must ultimately close on itself ( sometimes not necessarily within the limits of the map). This can be understood by understanding the water marks.
7. Cliff
Cliff are steep rock faces along the sea coast that are usually vertical or overhanging. The contour lines of vertical cliff coincide with each other while the contour lines intersect each other in the case of overhanging cliff.
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| Vertical Cliff and Overhanging Cliff |
8. Contour Lines With Same Elevation
Contour lines with same elevation cannot intersect each other nor can one split into two lines.Â
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