Geographic Coordinate Systems uses a three-dimensional spherical surface to define locations on the earth. Points on the earth's surface are referenced by longitude (north-south vertical lines) and latitude (east-west horizontal lines) measured in degrees (or in grads) as angles from the earth's center. Although longitude and latitude can locate an exact position on the earth's surface, they are not uniform units of measure. For example, latitude gets gradually smaller as one leaves the equator and approaches the poles.
Datums define the position of the spheroid relative to the center of the earth by defining the origin and orientation of latitude and longitude lines. Because local datums are aligned with a particular area of the earth's surface, a datum for Europe (ED 1950) can't be used to reference locations in North America (NAD) and vice versa. There are numerous datums and dates of datums primarily due to the improvements of satellite data with WGS 1984 serving as the framework for locational measurements worldwide.
