Editing Geographic information system (section)

===Projections, coordinate systems, and registration===
{{Main|Spatial reference system}}
The earth can be represented by various models, each of which may provide a different set of coordinates (e.g., latitude, longitude, elevation) for any given point on the Earth's surface. The simplest model is to assume the earth is a perfect sphere. As more measurements of the earth have accumulated, the models of the earth have become more sophisticated and more accurate. In fact, there are models called [[datum (geodesy)|datums]] that apply to different areas of the earth to provide increased accuracy, like [[NAD83|North American Datum of 1983]] for U.S. measurements, and the [[World Geodetic System]] for worldwide measurements.

The latitude and longitude on a map made against a local datum may not be the same as one obtained from a [[GPS receiver]]. Converting coordinates from one datum to another requires a [[Geographic coordinate conversion#Datum transformations|datum transformation]] such as a [[Helmert transformation]], although in certain situations a simple [[Translation (geometry)|translation]] may be sufficient.<ref name=Irish>{{cite web |url = http://www.osi.ie/GetAttachment.aspx?id=25113681-c086-485a-b113-bab7c75de6fa |title=Making maps compatible with GPS |publisher=Government of Ireland 1999 |access-date=15 April 2008 |archive-url = https://web.archive.org/web/20110721130505/http://www.osi.ie/GetAttachment.aspx?id=25113681-c086-485a-b113-bab7c75de6fa |archive-date=21 July 2011 |url-status=dead }}</ref>

In popular GIS software, data projected in latitude/longitude is often represented as a [[Geographic coordinate system]]. For example, data in latitude/longitude if the datum is the '[[North American Datum]] of 1983' is denoted by 'GCS North American 1983'.