Topographic maps provide accurate information about features on the ground whether they are natural or man-made. They are so detailed that it is almost as if one is using a three-dimensional format, rather than a two-dimensional map. These maps can show water features such as rivers and streams as well as land reliefs including mountains, valleys, slopes and land depressions. Contour lines are used to denote changes in elevation on topographic maps.
By connecting several points of equal elevation, contour lines can be used to show the terrain, or the relief, of an area on a map. If one is mapping out a mountainous terrain, then these lines would be closer together, indicating a rapid change in elevation. As the mountains ease into foothills and then valleys, the contour lines become farther and farther apart.
The vertical distance between one contour line and the next is called the contour interval. Contour lines are drawn at specific intervals, such as every 10 feet, 20 feet or 30 feet. Each line represents a multiple of the scale chosen. Using the 10-foot scale as an example, each contour interval would represent a 10 feet change in elevation. The contour interval is printed on the map, usually in the margin.
Most topographical maps have so many contour lines that it is not possible to label all of them. In this case every fifth contour line is highlighted and marked and is known as an index contour line. Since the two highlighted lines are labeled, the contour interval for the unlabeled lines can be calculated by dividing the distance between the two highlighted lines by five. As an example: one index contour line is labeled 100 feet, the next is labeled 200 feet. Each contour interval would be 20 feet.
Map makers have to be careful they use a contour interval that accurately represents the terrain and that is easy to read. In an area with many changes in elevations, such as a mountain range, map makers will use a large contour interval. This prevents all the contour lines from being too close together and unreadable. In areas of little elevational contrast, map makers will use a small contour interval so that existing features show up on the map. In some cases different contour intervals will be used for different parts of a map. This is done when part of an area is mountainous and the rest is relatively flat.