Relief (Contours, Spot Heights, Cross Sections)

Contours

Contours are lines drawn on maps that connect points of equal height above sea level. They show the shape and height of the land, helping to represent relief (the variation in height across a landscape) on a flat map.

Purpose of contours:

• Show the height of the land at different points.
• Indicate the steepness or flatness of slopes.
• Help identify landforms such as hills, valleys, ridges, and depressions.

Close vs spaced contours:

• Close contours mean the land is steep because the height changes rapidly over a short horizontal distance.
• Spaced contours indicate gentle slopes or flat land where height changes slowly.

Index contours: These are thicker contour lines usually labelled with the height (in metres) to make reading the map easier. They appear at regular intervals (e.g., every 50 metres), with thinner contours in between.

Interpreting slope steepness:

By looking at how close the contour lines are, you can tell how steep the land is. For example, a hill with contours close together on one side and far apart on the other has a steep slope on the close side and a gentle slope on the far side.

For instance, if contour lines are 10 metres apart vertically and 5 metres apart horizontally, the slope is steeper than if they were 10 metres apart vertically but 20 metres apart horizontally.

Spot Heights

Spot heights are exact points on a map marked with a dot and a number showing the precise height above sea level at that point. They provide specific height information where contours might not be detailed enough.

Use of spot heights:

• To give accurate height measurements at key locations such as hilltops, road junctions, or mountain peaks.
• To supplement contour lines for better understanding of relief.

Identifying spot heights on maps: Look for small black dots with numbers next to them, for example, "245" meaning 245 metres above sea level.

Relation to contours: Spot heights often lie between contour lines and help confirm the height of the land where contours are widely spaced or irregular.

For example, if contour lines show heights of 240 m and 250 m, a spot height of 245 m between them confirms the land’s height at that exact point.

Cross Sections

A cross section is a side-on view of the landscape along a line drawn on a map, showing how the height changes across that line. It helps visualise the relief in a way that is easier to understand than just contour lines.

Drawing cross sections from contours:

1. Choose a straight line on the map (called the transect) along which you want to show the relief.
2. Mark where this line crosses each contour line and note the height at these points.
3. On graph paper, draw a horizontal baseline representing the distance along the transect.
4. Plot the heights at the correct distances along this baseline.
5. Connect the points smoothly to show the shape of the land.

Interpreting landform shapes:

• Steep slopes appear as sharp rises or falls in the cross section.
• Gentle slopes show gradual rises or falls.
• Peaks and valleys are clearly visible as high and low points.

Using cross sections to show relief: Cross sections provide a clear visual of how the land rises and falls, making it easier to understand the terrain for activities like planning routes, construction, or environmental studies.

For example, a cross section across a hill will show a peak where the height is highest and slopes on either side.

Example: Imagine a transect line crosses contour lines at 100 m, 120 m, 140 m, and 160 m over a horizontal distance of 4 km. Plotting these heights at the correct distances on graph paper and connecting them will show the slope of the land along that line.