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Global Coordinate Systems
Coordinate
systems to specify locations on the surface of the earth have been used
for centuries. In western geodesy the equator, the tropics of Cancer and
Capricorn, and then lines of latitude and longitude were used to locate
positions on the earth. Eastern cartographers like Phei Hsiu used other
rectangular grid systems as early as 270 A. D.
Various
units of length and angular distance have been used over history. The meter
is related to both linear and angular distance, having been defined in the
late 18th century as one ten-millionth of the distance from the pole to
the equator.
Latitude, Longitude, and Height
The
most commonly used coordinate system today is the latitude, longitude, and
height system.
The
Prime Meridian and the Equator are the reference planes used to define latitude
and longitude.
Sample Image: primem.gif
The
geodetic latitude (there are many other defined latitudes) of a point is
the angle from the equatorial plane to the vertical direction of a line
normal to the reference ellipsoid.
The
geodetic longitude of a point is the angle between a reference plane and
a plane passing through the point, both planes being perpendicular to the
equatorial plane.
The
geodetic height at a point is the distance from the reference ellipsoid
to the point in a direction normal to the ellipsoid.
Earth Centered, Earth Fixed X, Y, and Z
Earth
Centered, Earth Fixed Cartesian coordinates are also used to define three
dimensional positions.
Earth
centered, earth-fixed, X, Y, and Z, Cartesian coordinates (XYZ) define three
dimensional positions with respect to the center of mass of the reference
ellipsoid.
The
Z-axis points toward the North Pole.
The
X-axis is defined by the intersection of the plane define by the prime meridian
and the equatorial plane.
The
Y-axis completes a right handed orthogonal system by a plane 90° east
of the X-axis and its intersection with the equator.
Geodetic Datums
Datum Types
Datum
types include horizontal, vertical and complete datums.
Datums in Use
Hundreds
of geodetic datums are in use around the world.
The
Global Positioning system is based on the World Geodetic System 1984 (WGS-84).
Parameters
for simple XYZ conversion between many datums and WGS-84 are published by
the Defense mapping Agency.
all about global positioning system page: 1,2,3,4,5,6,7,8.9.10,11,12