Owner's manual

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GPS Basics -1.0.0en
Geodetic Aspects
4.2. The GPS coordinate system
Although the earth may appear to be a
uniform sphere when viewed from
space, the surface is far from uniform.
Due to the fact that GPS has to give
coordinates at any point on the earths
surface, it uses a geodetic coordinate
system based on an ellipsoid. An
ellipsoid (also known as a spheroid) is a
sphere that has been flattened or
squashed.
An ellipsoid is chosen that most accu-
rately approximates to the shape of the
earth. This ellipsoid has no physical
surface but is a mathematically defined
surface.
There are actually many different ellip-
soids or mathematical definitions of the
earths surface, as will be discussed
later. The ellipsoid used by GPS is
known as WGS84 or World Geodetic
System 1984.
A point on the surface of the earth (note
that this is not the surface of the ellip-
soid), can be defined by using Latitude,
Longitude and ellipsoidal height.
An alternative method for defining the
position of a point is the Cartesian
Coordinate system, using distances in
the X, Y, and Z axes from the origin or
centre of the spheroid. This is the
method primarily used by GPS for
defining the location of a point in space.
An Ellipsoid
Earth's Surface
Height
Latitude
Longitude
Defining coordinates of P by
Geodetic and Cartesian coordinates