The contours on a topographic map follow strict rules.
Contour maps are used in many sciences to help show and analyze the distribution of data over a geographic area. Some well-known contour map types are weather maps showing rainfall or barometric pressure patterns and topographic maps that help visualize the shape of landforms. No matter what sort of data a map shows, contour lines must always follow some simple rules to be valid.
Crossing
A contour line connects points with the same value. For example a line may represent all places where total rainfall was 1 inch or where the elevation is 250 feet above sea level. If two contours cross, that would mean that it rained both 1 inch and 2 inches there yesterday or the point is both 250 and 300 feet above sea level. Both are impossible, so contours never cross. They can get very close, but cannot touch.
Splits
Every contour line divides a map into two spaces. On one side of the line, every point has a higher value than the contour while on the other side, every point has a lower value. This means that contour lines cannot split to form a Y, because a Y shape divides the map into three areas. If there are three areas, then two must be higher than the contour line and one lower, so a split is unnecessary.
Closure
Every contour line eventually closes somewhere and forms a circle or oval shape. Many lines do not close within the map area, but would close if the map were big enough: On a map of rainfall in Denver, the 0.50-inch contour might disappear off the edge of the Denver map, but could close on a map of rainfall in the entire state of Colorado.
Repeating
Single contour lines do not occur between two contours of a higher or lower value. Where the trend of the contoured data changes, such as the crest of a ridge, contours repeat on the other side of the highest or lowest point.
Spacing
Closely spaced contours show areas where the mapped data change rapidly, such as on steep hills. Widely-spaced contours depict gentle changes, or large areas with nearly the same value.
Tags: contour line, contour lines, above level, divides into, Every contour
