Contours, Cross-Sections and Relief Mapping

Relief indicates the variation in the nature of the land surface ie. "the lie of the land". It thus shows the broad features and relative heights of highlands and lowlands such as flood plains, spurs, hills, plateaux etc.

Contouring is the standard method of representing relief on topographic maps. Contour lines are lines joining points of equal elevation on the surface of the ground. They can also be thought of as the lines of intersection between a series of horizontal planes and the ground surface.

For a given map the vertical distance between adjacent contour lines or the contour interval is fixed. By examining the horizontal distance between successive contours it is possible to visually estimate the variation in slope of parts of the terrain. As the vertical distance between contours is the same, the closer the contour lines are together the steeper the slope.

A more precise method determining the variation in slope is to construct a profile or cross section through the topography. By marking off the horizontal distance between contour lines along a base line and then plotting the elevation or altitude of these points, a profile can be drawn showing the gradient of slope and shape of the topography along the line of section (see Figure 2).

Figure 2: Representation of topography by contouring contour map (above) and profile along AB (below).

Click this button to open a map which gives you the opportunity to compare a contour map with its profile by clicking either to the left or right.

Some basic relationships between slope and the arrangement of contours:

• Evenly spaced contours represent a uniform slope;
• Contours that are widely spaced indicate a gentle slope;
• Contours that are close together near the top of a hill and widely spaced at the bottom indicate a concave slope
• Contours that are widely spaced at the top of a hill and close together at the bottom indicate a convex slope.

Question 9 : Click this button to open a series of contour maps from which you can draw cross sections for a range of different terrains by aligning the vertical blue line and clicking the mouse button on the correct elevation.