Index contour lines are accented with a heavier mark, so that these lines will be the first thing to catch your eye when you look at a topographical map. Like all contour lines, they form in concentric circles or shapes and each index contour line is evenly spaced from one line to the next. Typical intervals between contour lines might be 100 or 200 feet although greater or lesser numbers are possible. Index contour lines are marked with the elevation above sea level and they are usually figured in intervals, such as every 100 or 200 feet. By looking at all three types of contour lines, it is possible to determine the relief of the land that the topographical map represents.
Between each pair of contour lines, there exists a set of intermediate contour lines. The intermediate contour lines usually come in sets and each intermediate contour line represents an equal amount of elevation change between each line. Also important is the fact that the elevation change between one index contour line and an adjacent intermediate contour line will also be the same value as the change between two intermediate contour lines that are located next to each other.
For example, if on a map, you have your index lines placed 100 feet apart, the likely scenario is that there will be four intermediate contour lines placed between each pair of index lines. Four lines would create five spaces and to make the change equal, the gap between each line would have to represent 20 feet of elevation change. It is important to remember that intermediate and supplementary contour lines are not marked with their elevation above sea level.
Supplementary contour lines are expressed as a dashed line. These lines are drawn at all one elevation, but they differ from the previous two types of lines in that their spacing or change in elevation that they represent is different. They almost always represent half the elevation change that is found between intermediate and index contour lines. Therefore, these lines are only used on topography maps where the overall change in elevation is very gradual or slight.
Article Written By Henri Bauholz
Henri Bauholz is a professional writer covering a variety of topics, including hiking, camping, foreign travel and nature. He has written travel articles for several online publications and his travels have taken him all over the world, from Mexico to Latin America and across the Atlantic to Europe.