A **coordinate system** is a method for
identifying the location of a point on the earth. Most
coordinate systems use two numbers, a **coordinate**, to identify the location of a point. Each of
these numbers indicates the distance between the point and some
fixed reference point, called the **origin**.
The first number, known as the X value, indicates how far left
or right the point is from the origin. The second number, known
as the Y value, indicates how far above or below the point is
from the origin. The origin has a coordinate of 0, 0.

Longitude and latitude are a special kind of coordinate
system, called a **spherical coordinate system**,
since they identify points on a sphere or globe. However, there
are hundreds of other coordinate systems used in different
places around the world to identify locations on the earth. All
of these coordinate systems place a grid of vertical and
horizontal lines over a flat map of a portion of the earth.

A complete definition of a coordinate system requires the following:

- The projection that is used to draw the earth on a flat map
- The location of the origin
- The units that are used to measure the distance from the origin