Contours and slopes

Contours are imaginary lines joining places of the same height above mean sea level; they are also called level lines. A map showing landforms by contours is a contour map.

• Contours were once drawn from ground surveys and levelling; today aerial photographs are used.
• The contour interval or vertical interval (V.I.) is the height difference between two neighbouring contours, such as 20, 50 or 100 m. It is usually constant on a map.
• The horizontal equivalent (H.E.) is the ground distance between them. It is large on gentle slopes and small on steep slopes.

Rules for reading contours:

• Closely spaced contours: steep slope.
• Widely spaced contours: gentle slope.
• Two or more contours merging: a vertical face, such as a cliff or waterfall.
• Contours of different heights do not usually cross.

Types of slope:

• Gentle: low angle, contours far apart.
• Steep: high angle, contours close together.
• Concave: gentle in the lower part, steep in the upper part: contours wide apart below, close together above.
• Convex: gentle in the upper part, steep in the lower part: contours wide apart above, close together below.
• Irregular or undulating slopes also occur.

Example (new): a hill path where the contours on the map are evenly spaced all the way up rises at the same steepness throughout; if the contours bunch together only near the summit, the last part of the climb is the hardest, a concave slope.