Leica Geosystems GPS Basics - Manual (Page 28)

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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
P
0
X
Y
Z
,Y
,Z
,X
Earth's Surface
Height
Latitude
Longitude
Defining coordinates of P by
Geodetic and Cartesian coordinates