The US Geologic Survey is the main producer of topographical maps for use in the United States. On the most common maps, contour lines are drawn at intervals of 200 feet above sea level.
Contour lines are most commonly used to determine the grade of a slope without seeing it. Lines that are close together indicate very steep terrain, while lines that are further apart denote gradual slopes and meadows.
Topographical maps and contour intervals are of particular use in hiking, backpacking and orienteering, or in any activity requiring land navigation.
Because contour lines can be difficult to distinguish on maps of particularly varied terrain, most maps make every fifth line an index line, which appears in bold and has the elevation printed on it.
Contour lines can be used to connect points of reference for many subjects other than topography. Meteorology (pictured above), sociology and geology are just a few of the topics in which scientists might use contour intervals to illustrate features of a particular region.