Editing Geodetic datum (section)

== Datum transformation ==
{{main|Datum transformation}}

The difference in co-ordinates between datums is commonly referred to as ''datum shift''. The datum shift between two particular datums can vary from one place to another within one country or region, and can be anything from zero to hundreds of meters (or several kilometers for some remote islands). The [[North Pole]], [[South Pole]] and [[Equator]] will be in different positions on different datums, so [[True North]] will be slightly different. Different datums use different interpolations for the precise shape and size of Earth ([[reference ellipsoid]]s). For example, in Sydney there is a 200 metres (700&nbsp;feet) difference between GPS coordinates configured in GDA (based on global standard WGS{{nbsp}}84) and AGD (used for most local maps), which is an unacceptably large error for some applications, such as [[surveying]] or site location for [[scuba diving]].<ref>{{cite web |last=McFadyen |url-status=live |url=http://www.michaelmcfadyenscuba.info/viewpage.php?page_id=80 |title=GPS - An Explanation of How it Works |archive-url=https://web.archive.org/web/20060819064902/http://michaelmcfadyenscuba.info/viewpage.php?page_id=80 |archive-date=2006-08-19 |website=Michael McFadyen's Scuba Diving Web Site }}</ref>

Datum conversion is the process of converting the coordinates of a point from one datum system to another. Because the survey networks upon which datums were traditionally based are irregular, and the error in early surveys is not evenly distributed, datum conversion cannot be performed using a simple parametric function. For example, converting from [[North American Datum#North American Datum of 1927|NAD 27]] to [[NAD 83]] is performed using NADCON (later improved as HARN), a raster grid covering North America, with the value of each cell being the average adjustment distance for that area in latitude and longitude. Datum conversion may frequently be accompanied by a change of [[map projection]].