Editing Contour line (section)

=== Physical geography and oceanography ===

==== Elevation and depth ====
[[File:Topographic map example.png|thumb|right|[[Topographic map]] of [[Stowe, Vermont|Stowe]], [[Vermont]]. The brown contour lines represent the [[elevation]]. The contour interval is 20 [[foot (length)|feet]].]]

Contours are one of several [[Cartographic relief depiction|common methods]] used to denote [[elevation]] or [[altitude]] and depth on [[map]]s. From these contours, a sense of the general [[terrain]] can be determined. They are used at a variety of scales, from large-scale engineering drawings and architectural plans, through [[topographic maps]] and [[bathymetric charts]], up to continental-scale maps.

"Contour line" is the most common usage in [[cartography]], but [[isobath]] for underwater depths on [[bathymetric]] maps and '''isohypse''' for elevations are also used.

In cartography, the '''contour interval''' is the elevation difference between adjacent contour lines. The contour interval should be the same over a single map. When calculated as a ratio against the map scale, a sense of the hilliness of the terrain can be derived.

===== 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>

==== Electrostatics ====
An '''[[isopotential map]]''' is a measure of electrostatic potential in space, often depicted in two dimensions with the electrostatic charges inducing that [[electric potential]]. The term '''[[equipotential]] line''' or '''isopotential line'''  refers to a curve of constant [[electric potential]]. Whether crossing an equipotential line represents ascending or descending the potential is inferred from the labels on the charges. In three dimensions, '''[[equipotential]] surfaces''' may be depicted with a two dimensional cross-section, showing [[equipotential]] lines at the intersection of the surfaces and the cross-section.

The general mathematical term [[level set]] is often used to describe the full collection of points having a particular potential, especially in higher dimensional space.

==== Magnetism ====<!-- [[Isogon (geomagnetism)]], [[Isogonic line]], [[Isogonic lines]], [[Agonic line]], [[Agonic lines]], [[Isoclinic line]], [[Aclinic line]] and [[Isodynamic line]] redirect here -->
[[Image:IGRF 2000 magnetic declination.gif|thumb|left|Isogonic lines for the year 2000. The agonic lines are thicker and labeled with "0".]]
In the study of the [[Earth's magnetic field]], the term '''isogon''' or '''isogonic line''' refers to a line of constant [[magnetic declination]], the variation of magnetic north from geographic north. An '''agonic line''' is drawn through points of zero magnetic declination. An '''isoporic line''' refers to a line of constant annual variation of magnetic declination
.<ref>{{cite web
|url=http://historicalcharts.noaa.gov/historicals/preview/image/3077-00-1946
|access-date=2015-07-20
|title=isoporic line
|website=historicalcharts.noaa.gov
|date=1946
|archive-date=2015-07-21
|archive-url=https://web.archive.org/web/20150721172841/http://historicalcharts.noaa.gov/historicals/preview/image/3077-00-1946
|url-status=dead
}}</ref>

An '''isoclinic line''' connects points of equal [[magnetic dip]], and an '''aclinic line''' is the isoclinic line of magnetic dip zero.

An '''isodynamic line''' (from {{lang|grc|δύναμις}} or ''dynamis'' meaning 'power') connects points with the same intensity of magnetic force.

==== Oceanography ====
Besides ocean depth, [[oceanography|oceanographers]] use contour to describe diffuse variable phenomena much as meteorologists do with atmospheric phenomena. In particular, '''isobathytherms''' are lines showing depths of water with equal temperature, '''isohalines''' show lines of equal ocean salinity, and '''[[isopycnal]]s''' are surfaces of equal water density.