Definition

name :ahmed wazer                                                                     Nov 2012

category: Important Definitin.

To better understand what the WGS84 datum means, it helps to think about how we define a location (i.e. a latitude and a longitude) on the Earth's surface:

The Earth is not flat, and the Earth is not a sphere. So scientists and geographers use an ellipsoid, which is a simplified model of Earth’s shape. A datum is a set of reference locations, including elevations, describing more precisely the surface of the Earth.

There are several common datums still in use:

• NAD27 (North American Datum of 1927): A system derived from land-based surveys, using the Clarke (1886) ellipsoid.
• NAD83: A satellite-based system using the center of the earth as a reference point using measurements taken in 1980, and adopted internationally as GRS80 (geodetic reference system 1980).
• WGS84 (World Geodetic System 1984): A refinement of GRS80 used by the US military (and therefore by all GPS systems).

Each of these reference systems uses a different ellipsoid. The differences between these models, considering the size of the earth, are very small (no more than about 70 meters). But if you are trying to find a specific location, and you are using two different datums, or don't know what datum you're using, it will impact your success.