Editing Contour line (section)

===== Interpretation =====

There are several rules to note when interpreting terrain contour lines:
* '''The rule of Vs''': sharp-pointed vees usually are in stream valleys, with the drainage channel passing through the point of the vee, with the vee pointing upstream. This is a consequence of [[erosion]].
* '''The rule of Os''': closed loops are normally uphill on the inside and downhill on the outside, and the innermost loop is the highest area. If a loop instead represents a depression, some maps note this by short lines called hachures which are perpendicular to the contour and point in the direction of the low.<ref>{{cite web|last =Leveson|first= David J.|title = Depression Contours – Getting Into and Out of a Hole |url= http://academic.brooklyn.cuny.edu/geology/leveson/core/linksa/depression.html |publisher= [[City University of New York]] |date= 2002}}</ref> (The concept is similar to but distinct from hachures used in [[hachure map]]s.)
* '''Spacing of contours''': close contours indicate a steep slope; distant contours a shallow slope. Two or more contour lines merging indicates a cliff. By counting the number of contours that cross a segment of a [[stream]], the [[stream gradient]] can be approximated.

Of course, to determine differences in elevation between two points, the contour interval, or distance in altitude between two adjacent contour lines, must be known, and this is normally stated in the map key. Usually contour intervals are consistent throughout a map, but there are exceptions. Sometimes intermediate contours are present in flatter areas; these can be dashed or dotted lines at half the noted contour interval. When contours are used with [[hypsometric tints]] on a small-scale map that includes mountains and flatter low-lying areas, it is common to have smaller intervals at lower elevations so that detail is shown in all areas. Conversely, for an island which consists of a plateau surrounded by steep cliffs, it is possible to use smaller intervals as the height increases.<ref>''[[Sark]] (Sercq)'', D Survey, Ministry of Defence, Series M 824, Sheet Sark, Edition 4 GSGS, 1965, [[Online Computer Library Center|OCLC]] {{OCLC|27636277}}. Scale 1:10,560. Contour intervals: 50&nbsp;feet up to 200, 20&nbsp;feet from 200 to 300, and 10&nbsp;feet above 300.</ref>