Contour lines and cross-sections – reading a topographic map in 2D and 3D
GeographyMaps & MappingAges 11–12
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Sign in to playA computer-generated landscape 4 km × 2.5 km (hills, a saddle, a valley, a ridge, a cliff) is drawn as a topographic map with contour lines, labelled index contours and spot heights; switch to 3D to see the same land as a block model. Drag a line on the map to draw the cross-section A–B, and move the pointer to read height, gradient and landform. Turn on the visible area to find out which directions an observer standing at A can and cannot see.
Lesson: Relief and landforms – reading large-scale contour maps and drawing cross-sections
What it shows
A contour line joins points of equal height above sea level, so each one is a horizontal slice through the land. The contour interval is the height difference between neighbouring lines; every fifth line is a thicker index contour with its height written on it. Where contours crowd together the slope is steep, and where they are far apart it is gentle. In a valley the contours bend in a V pointing uphill; on a ridge or spur they bend pointing downhill. A cross-section along a line shows the shape of the land, and a straight sight line shows whether one point can be seen from another.
How to use
Drag across the map to draw the cross-section A–B, or drag the dots A and B to adjust it; the observer stands at A. Move the pointer over the map to read height, gradient and landform, and over the profile to trace each point. Change Landscape, press New terrain or change the Contour interval. Raise the Water level to see a shoreline follow a contour. Tick Visible from A and change Eye height to find the hidden ground. Press 3D to see the block model.
Parameters you can change
- Type of landscape Mixed (hills, valley, cliff), Two hills and a saddle, River valley between two ridges, Plateau with a cliff
- Contour interval 10 m, 20 m, 25 m, 50 m
- Height of the highest summit 200–1500 m
- Colour by height (layer tints)
- Show spot heights
- Show the area visible from A
- Observer's eye height above the ground 2–50 m
- Vertical exaggeration (3D) 1–5 ×
Questions to explore
- Why does a place where the contour lines crowd together have a steep slope?
- How do contours bend differently in a valley and on a ridge?
- Why can someone on a rounded summit sometimes not see the foot of the hill, while someone at the slope's edge can?
