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What is an index contour?

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An index contour is one of the ways that vertical dimension, or vertical scale, is demonstrated on a topographical map. The index contour represents the vertical scale on a map region by a thick solid line with the various elevations printed on it. Thinner solid lines, called intermediate contours, are used to depict lesser elevations that are referenced to the height information provided by the map's legend.

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Most topographical maps contain only the index contour and intermediate contour lines, but some also contain supplementary contour lines. These are broken, or dashed, lines that are used to display lesser elevations. As in the case of the intermediate contour lines, the map legend provides information regarding the distances involved.

Another form of closed line, usually circular, may be used to display a negative elevation such as a sinkhole or crater. These lines display closed depressions by perpendicular inner tic marks that point in the direction of the depression.

Hachure marking represents an older method of displaying vertical dimension that is not as accurate in depicting elevations as the index, intermediate and supplementary contour lines are. Generally not found in maps designed as part of scientific survey, older hachure maps often display the slope of a terrain through lines in which the steeper slopes are shown by the relative thickness of the line.

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