As you probably know, locations on the Earth's surface are measured
and represented in terms of **coordinates.** A coordinate
is a set of two or more numbers that specifies the position of a point,
line, or other geometric figure in relation to some reference system.
The simplest system of this kind is a Cartesian coordinate system (named
for the 17th century mathematician and philosopher René Descartes). A
Cartesian coordinate system is simply a grid formed by juxtaposing two
measurement scales, one horizontal (x) and one vertical (y). The point
at which both x and y equal zero is called the **origin**
of the coordinate system. In Figure 2.10.1, above, the origin (0,0) is
located at the center of the grid. All other positions are specified
relative to the origin. The coordinate of upper right-hand corner of the
grid is (6,3). The lower left-hand corner is (-6,-3). If this is not
clear, please ask for clarification!

Cartesian and other two-dimensional (plane) coordinate systems are
handy due to their simplicity. For obvious reasons, they are not
perfectly suited to specifying geospatial positions, however. The
**geographic coordinate system** is designed specifically
to define positions on the Earth's roughly-spherical surface. Instead of
the two linear measurement scales, x and y, the geographic coordinate
systems juxtaposes two curved measurement scales. The east-west scale,
called **longitude** (conventionally designated by the
Greek symbol *lambda*), ranges from +180° to -180°. Because the
Earth is round, +180° (or 180° E) and -180° (or 180° W) are the same
grid line. That grid line is roughly the International Date Line, which
has diversions that pass around some territories and island groups.
Opposite the International Date Line is the **prime
meridian**, the line of longitude defined by international treaty
as 0°. The north-south scale, called **latitude**
(designated by the Greek symbol *phi*), ranges from +90° (or 90°
N) at the North pole to -90° (or 90° S) at the South pole. We'll take a
closer look at the geographic coordinate system next.