Contour lines on topographic maps

Contour lines are the lines drawn on the map connecting all points on the earth’s surface with equal elevations above a fixed datum line (usually the mean sea level).
The act of using contour lines in order to represent the relief of an area on the map is known as contouring. Contouring is the most common method of showing relief on topographic maps because it has the ability to show all five (5) aspects of relief (shape, orientation, size, elevation and slope) on the map.

Characteristics of contour lines.
Generally, the contour lines have the following characteristics:

1. They connect points of equal elevations above mean sea level
2. They are numbered in either feet or meters, above sea level. Usually these numbers represent the exact elevation of land above sea level for all points along that line.
3. Contour lines never cross or intersect each other. This is because it is impossible to have a point on earth’s surface that can be at two or more different elevations above sea level at the same time. However, the only exception for this is ‘overhanging cliffs’ where many contour lines merge to form one contour line.
4. The spacing or gap between the contour lines represents the degree of slope. Usually, closely spaced contour lines indicate a steep slope while contour lines far apart indicate gentle slope.
5. Contour lines lie perpendicular to streams and rivers.

This figure shows how contour lines are shown on map

Contour interval
Contour lines can be drawn for relatively any elevation. However, for simplification, only the lines for certain elevations are drawn on the map. These lines are drawn in such a way that they have to be evenly spaced vertically.
This vertical spacing between the contour lines is what is known as contour interval or vertical interval.
In other words, contour interval is the difference in height between any two successive contours on map.
Information regarding the contour interval used in the map is usually shown in the marginal information on that given map. When supplementary contours are used, their interval is also indicated on the map.

This figure shows how information regarding contour intervals is shown on the map, as taken from one of the US Army’s topographic map series 2201 (Dar Es Salaam 1:2,000,000 topographic maps)

The Index, intermediate and supplementary contours.
Sometimes a map may have several contour lines such that it becomes difficult for the cartographer to label the elevations of each contour line.
Thus, in order to make the map easier to read and less cumbersome, every fifth (5th) contour line from the mean sea level is usually labelled with its elevation and sometimes bolded a little bit heavier than the rest. These bolded and labeled contour lines are usually referred to as the index contour lines.
The remaining contours are called intermediate contours. These usually fall in between the index contour lines. The Intermediate contour lines are finer than index contours and do not have their elevations labelled on them. There are normally four intermediate contour lines between any two successive index contour lines.

This figure shows how contours are shown on maps. Notice how only some of the contour lines are labelled with their elevations above mean sea level, while the others are not. The bold labeled contours lines are the index contours whereas the finer and un-labelled contours are the intermediate contours.

However, in some certain areas on a map, the normal contour interval is sometimes too large to represent a significant change in elevation, such as on level terrain, and thus, additional half-interval contours are added.
These kinds of contours are called supplementary contours or form lines. They usually appear on the map as dashed or thin continuous lines with no representative value of elevation on them.  They are used when the overall change in elevation is very gradual. When supplementary contours appear on the map, their interval is usually specified.

Recommended: Methods of representing relief on topographic maps


Advantages of contouring
Contouring as a way of representing relief on the map has the following merits

1. Contouring is the most suitable method of representing relief on large scale topographic maps
2. Contours provide the basis for other methods of representing relief such as layer tinting, hachuring and relief shading.
3. Contours can indicate the exact elevation of a point on the map
4. The distance between the contours may help to determine the slope of the land
5. The shape and patterns that the Contour lines make on the map can portray distinctive landforms such as hills, valleys, spurs and cliffs.

The limitations of contouring include;

1. They are not suitable for representing relief on small-scale maps.
2. Not all physical features and landforms can be portrayed by the contour lines. E.g. coral reefs and outcrop rocks.
3. The contour lines may fail to indicate some of the highest and important elevations on the map due to limitations of the vertical interval used.

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