Contour 3D Lab

Understand contour topographic maps

Contour lines are closed curves connecting points of equal elevation. Projecting a 3D landscape onto a map reveals heights, slopes and terrain patterns on a sheet of paper.

8 km × 8 km virtual mountain regionThree northern peaks form an east–west range. Two streams start south of the saddles and meet in the south-central area. A fault creates a cliff southeast of the eastern peak; a basin lake lies southwest and an isolated hill lies south. The floating map below shows the same terrain.

Drag to rotate, scroll to zoom, and right-drag to pan.

Move the pointer to hover over the terrain and read elevation and slope. The lower-left map marks the same position.

On the lower-left map, drag to draw a profile line or click to fly the camera to that location.

Use ← → to change chapters during classroom demonstrations.

The formation of contour lines

Slice the terrain with equally spaced horizontal planes. Each plane intersects the surface along a contour line. Project these lines vertically onto one plane to form a contour map.

Equal elevation on a line: all points on one contour have the same elevation.

Equal interval on a map: adjacent contours share the same elevation difference.

Closed curves: every contour closes, sometimes beyond the map boundary.

Usually no intersections: contours overlap only at cliffs.

Altitude and relative altitude

Elevation is vertical height above sea level; relative height is the elevation difference between two locations.

Relative height = higher elevation − lower elevation

Contour distance and slope

The height difference between adjacent contour lines is the contour interval. On the same map, closely spaced lines indicate steeper slopes; widely spaced lines indicate gentler slopes.

Close: steep · Wide: gentleFor the same elevation gain, a shorter horizontal distance means a steeper slope.

Mountain tops and basins

Both appear as sets of closed contours. A higher interior indicates a peak; a lower interior indicates a basin or depression.

Higher inside → PeakLower inside → Basin

Short lines perpendicular to contours point toward lower elevation.

At peaks, marks lie outside the closed contour; in basins they lie inside. See the lower-left map.

Ridges and Valleys

Where contours bend in the same direction, a bend toward lower elevations indicates a ridge; a bend toward higher elevations indicates a valley.

Bend downhill: ridge · Bend uphill: valley

Ridge lines (gold) are watersheds: rainfall drains away on both sides.

Valley lines are drainage lines, where rivers commonly form (blue streams).

Rivers flow opposite to the uphill bend of valley contours. Both streams here begin near the northern saddles, flow north to south and merge.

Saddle

A saddle is a low point between neighboring peaks. On a contour map it lies between two groups of closed contours.

A low point between two peaksLower than the peaks on either side, but higher than the adjacent valleys.

steep cliff

A cliff occurs where contour lines overlap. Cliff symbols have short teeth pointing downhill. The southeastern side of the east peak is cut by a NE–SW fault. The downthrown side leaves a tall fault scarp whose height peaks in the middle and tapers at either end. Contours overlap only near its middle.

Terrain profile

A terrain profile shows elevation along a straight line. Choose the profile line, find its intersections with contours, plot their elevations to scale and connect them with a smooth curve.

01Choose line

02Find intersections

03Plot elevations

04Draw curve

In-class testing