GISDataDepot - Data Projections
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Some Key Terms (supplied by USGS)
Azimuth—The angle measured in degrees between a base line radiating
from a center point and another line radiating from the same point.
Normally, the base line points North, and degrees are measured clockwise
from the base line.
Aspect—Individual azimuthal map projections are divided into three aspects:
the polar aspect which is tangent at the pole, the equatorial aspect which
is tangent at the Equator, and the oblique aspect which is tangent anywhere else.
(The word "aspect" has replaced the word "case" in the modern cartographic literature.)
Conformality—A map projection is conformal when at any point the scale is the
same in every direction. Therefore, meridians and parallels intersect at right
angles and the shapes of very small areas and angles with very short sides are
preserved. The size of most areas, however, is distorted.
Developable surface—A developable surface is a simple geometric form capable of
being flattened without stretching. Many map projections can then be grouped by
a particular developable surface: cylinder, cone, or plane.
Equal areas—A map projection is equal area if every part, as well as the whole,
has the same area as the corresponding part on the Earth, at the same reduced scale.
No flat map can be both equal area and conformal.
Equidistant—Equidistant maps show true distances only from the center of the
projection or along a special set of lines. For example, an Azimuthal Equidistant
map centered at Washington shows the correct distance between Washington and any
other point on the projection. It shows the correct distance between Washington and
San Diego and between Washington and Seattle. But it does not show the correct
distance between San Diego and Seattle. No flat map can be both equidistant and
Graticule—The graticule is the spherical coordinate system based on lines of latitude
Great circle—A circle formed on the surface of a sphere by a plane that passes through
the center of the sphere. The Equator, each meridian, and each other full
circumference of the Earth forms a great circle. The arc of a great circle shows
the shortest distance between points on the surface of the Earth.
Linear scale—Linear scale is the relation between a distance on a map and the
corresponding distance on the Earth. Scale varies from place to place on every map.
The degree of variation depends on the projection used in making the map.
Map projection—A map projection is a systematic representation of a round body
such as the Earth or a flat (plane) surface. Each map projection has specific
properties that make it useful for specific purposes.
Rhumb line—A rhumb line is a line on the surface of the Earth cutting all meridians
at the same angle. A rhumb line shows true direction. Parallels and meridians, which
also maintain constant true directions, may be considered special cases of the rhumb
line. A rhumb line is a straight line on a Mercator projection. A straight rhumb line
does not show the shorter distance between points unless the points are on the Equator
or on the same meridian.