Understanding Map Projections

equator
The parallel of reference that defines the origin of latitude
values, 0° north or south.
equatorial aspect
A planar projection that has its central point at the
equator.
equidistant projection
A projection that maintains scale along one or more lines
or from one or two points to all other points on the map.
equivalent projection
A projection on which the areas of all regions are shown
in the same proportion to their true areas. Shapes may be
greatly distorted (Snyder and Voxland, 1989). Also known
as an equal-area projection.
false easting
A linear value added to the x-coordinate values, usually to
ensure that all map coordinates are positive. See false
northing.
false northing
A linear value added to the y-coordinate values, usually to
ensure that all map coordinates are positive. See false
easting.
flattening
A measure of how much a spheroid differs from a sphere.
The flattening is the ratio of the semimajor axis minus the
semiminor axis to the semimajor axis. Known as ‘f’ and
often expressed as a ratio. Example: 1/298.3. Also known
as the ellipticity.
Gauss–Krüger
A projected coordinate system used in Europe and Asia
that divides the area into six degreewide zones. Very
similar to the UTM coordinate system.
geocentric latitude
Defined as the angle between the equatorial plane and a
line from a point on the surface to the center of the
sphere or spheroid.
geodesic
The shortest distance between any two points on the
surface of a spheroid. Any two points along a meridian
form a geodesic.
geodetic latitude
Defined as the angle formed by the perpendicular to the
surface at a point and the equatorial plane. On a spheroid,
the perpendicular doesn’t hit the center of the spheroid in
the equatorial plane except at the equator and the poles.
geographic coordinate system
A reference system that uses latitude and longitude to
define the locations of points on the surface of a sphere
or spheroid.
geographic transformation
A method that converts data between two geographic
coordinate systems (datums). Also known as a datum
transformation.
Global Positioning System
A set of satellites operated by the U.S. Department of
Defense. Ground receivers can calculate their location
using information broadcast by the satellites.
GPS
See Global Positioning System.
graticule
A network of lines representing a selection of the earth’s
parallels and meridians (Snyder and Voxland, 1989).
great circle
Any circle on the surface of a sphere formed by the
intersection of the surface with a plane passing through
the center of the sphere. The shortest path between any
two points lies on a great circle and is therefore important
to navigation. All meridians and the equator are great
circles on the earth defined as a sphere (Snyder and
Voxland, 1989).
Greenwich prime meridian
The prime meridian located in Greenwich, England.
grid
A network of lines representing a selection of a projected
coordinate system’s coordinates.
HARN
See High Accuracy Reference Network.
102 • Understanding Map Projections