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Figure 1.3 Location of P is shown at three parallels north of the Equator and four
meridians west of the Prime Meridian; it might equally be described, by doubling
the number of parallels and meridians, as six parallels north and eight meridians
west.
the diametral equatorial plane about the polar axis of the Earth.
These lines are called meridians: Meri + dies = half day, the sit-
uation of the Earth at the equinoxes. The unique line through
Greenwich is called the Prime Meridian; other halves of great cir-
cles are simply meridians.
• This particular reference system for the Earth is not unique; an ini-
nite number is possible. There is abstract similarity between this par-
ticular geometric arrangement of a coordinate system on a sphere and
the geometric pattern of Cartesian coordinates in the plane.
• To use this arrangement, one might describe the location of a
point, P , on the Earth-sphere as being at the 3rd parallel north
of the Equator and at the 4th meridian to the west of the Prime
Meridian. This description locates P according to one reference
system; however, someone else might employ a different reference
system with a finer mesh (halving the distances between succes-
sive lines). For that person, a correct description of the location
of P would be at the 6th parallel north of the Equator and at the
8th meridian to the west of the Prime Meridian. Indeed, an infi-
nite number of locally correct designations might be given for the
single point, P : An unsatisfactory situation in terms of being able
to replicate results. The problem lies in the use of a relative, rather
than an absolute, locational system (Figure 1.3).
 
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