Contour Lines and Topographic Map Reading

A contour line is an isoline connecting all points on a map that share the same elevation, and the constant vertical distance between one contour and the next is the contour interval, chosen according to map scale and the ruggedness of the terrain being depicted — a mountainous 1:24,000-scale United States Geological Survey (USGS) quadrangle might use a 40-foot interval, while a comparable map of flat coastal terrain might use 5 or 10 feet to preserve any visible relief at all. To keep dense contour lines legible, every fifth line is typically drawn heavier and labeled with its elevation as an index contour, while the intermediate contours between index lines are left thinner and unlabeled, so a reader can determine any unlabeled contour's value by counting intervals from the nearest index line.