Reading Topographic Maps and Contour Lines

A topographic map shows the shape and height of the land using contour lines, each of which joins points of equal elevation above sea level, and the contour interval is the vertical distance between neighboring lines. In brief: contours that are close together mean a steep slope, and widely spaced contours mean a gentle slope. Closed circles show hills (or depressions, if marked with hachures), V-shapes pointing uphill show valleys and V-shapes pointing downhill show ridges. Elevation is read from labeled index contours, and a cross-section (profile) can be drawn by plotting the heights along a line.

Contour lines turn a flat sheet of paper into a three-dimensional picture of the land. This guide explains topographic maps, contour lines, contour interval and elevation, how to read the landforms and how to draw a cross-section, compares the contour patterns in a table, works through a gradient calculation and a profile, outlines the history of contour mapping and corrects common mistakes.

What is a topographic map?

A topographic map is a detailed, large-scale map that shows natural and human features together: relief (the shape of the land), water, vegetation, roads, buildings and boundaries. National mapping agencies produce them at scales such as 1:25,000 and 1:50,000. They are widely used for walking, planning, construction and military and emergency work. The relief is shown with contour lines, supported by spot heights (single points with their elevation) and triangulation points.

What are contour lines and intervals?

A contour line joins all points at the same elevation above sea level. Imagine slicing a hill horizontally at regular heights: each cut, seen from above, is a contour. The contour interval is the vertical difference between neighboring contours, and is constant on a map (for example 10 m); it is stated in the map key. Every fifth line is usually drawn thicker and labeled with its height: these are index contours. The elevation of a point between two contours is estimated from the neighboring values. Key rules: contours never cross (except at an overhanging cliff, which is rare) and never split; each contour closes on itself, even if beyond the edge of the map; the contours of a steep slope are close together and of a gentle slope are far apart; and the height numbers are written so that the top of the number points uphill.

How do you read landforms from contours?

• Hill or peak: concentric closed contours with the highest value inside.
• Depression: closed contours with short ticks (hachures) pointing downhill, and the lowest value inside.
• Ridge: contours form U- or V-shapes pointing downhill (away from the high ground).
• Valley: contours form V-shapes pointing uphill (toward the high ground), and a stream runs along the bottom of the V.
• Saddle or col: a low point between two peaks, shown by two pairs of contours facing each other.
• Cliff: contours that merge or almost touch.
• Plateau or plain: large areas with few or no contours.
• Spur: a narrow ridge extending from higher ground.

Rivers flow downhill, so the direction of flow is perpendicular to the contours, from the higher to the lower value.

How do you draw a cross-section?

1. Draw a straight line A–B across the map where the profile is needed.
2. Place the edge of a strip of paper along the line, and mark each point at which a contour crosses it, with the height of that contour.
3. Draw a horizontal axis on graph paper equal to the length of the line, and a vertical axis with a height scale that covers the range of heights.
4. Transfer the marks to the horizontal axis and plot the heights above them.
5. Join the points with a smooth curve to give the profile. Note that the vertical scale is often exaggerated to make the relief visible, and that fact should be stated (vertical exaggeration = horizontal scale ÷ vertical scale).

Contour patterns compared

Worked example: gradient and profile

1. Map data: a 1:25,000 map with a contour interval of 10 m. Two points P and Q are on the 100 m and 150 m contours and are 2 cm apart on the map.
2. Ground distance: 2 cm × 25,000 = 50,000 cm = 500 m.
3. Height difference: 150 − 100 = 50 m. This is 5 contour intervals.
4. Gradient: 50 ÷ 500 = 0.10, which is a gradient of 1 in 10 (10 percent). The angle of slope is arctan(0.10) = 5.7°.
5. Comparison: on another part of the map, the same 50 m rise occurs over 1 cm (250 m): the gradient is 50 ÷ 250 = 0.20, or 1 in 5, a slope of 11.3°, so the contours there are twice as close, which means a slope twice as steep.
6. Profile: along the line P to Q, the six contours at 100, 110, 120, 130, 140 and 150 m are plotted at distances along the line, and the vertical scale is chosen as 1 cm to 10 m; with a horizontal scale of 1 cm to 250 m this exaggerates the relief by 25 times (250 m ÷ 10 m).

How did contour mapping develop?

Early maps showed hills by drawing small pictures, then by hachures (short lines in the direction of slope). Lines of equal depth were used on charts of rivers and the sea floor from the 16th to 18th centuries, and contour lines for land relief became established in the 18th and 19th centuries. National surveys, such as the Ordnance Survey (founded 1791) and the U.S. Geological Survey (founded 1879), made contour maps of entire countries during the 19th and 20th centuries. Since the late 20th century, heights have been measured by aerial photography, radar and lidar, and contours can be generated automatically from digital elevation models.

Which mistakes are common?

Mistake 1: Misreading the contour interval

Always check the key. A 5 m interval and a 20 m interval give very different slopes for the same spacing.

Mistake 2: Mixing up ridges and valleys

V-shapes pointing toward higher ground mark valleys. V-shapes pointing toward lower ground mark ridges.

Mistake 3: Thinking close contours mean a high place

Close contours mean a steep slope, not a high altitude. Read the numbers to know the height.

Mistake 4: Forgetting vertical exaggeration in a cross-section

Different horizontal and vertical scales make slopes look steeper than they are. State the exaggeration.