For most global data sets when storing data for general purposes, we use spherical/topocentric/lat-lon coordinate systems. Sometimes geocentric/cartesian coordinates are used for certain specialized uses. Each projection is also a coordinate system (or getting really specific, each projection, when combined with a datum, a unit of measure, and an axis ordering) These may be used for specific regions or for special purposes. Various equal area, gnomonic, and conformal projections are used for specialized forms of storage/analysis.
A common coordinate system is UTM/UPS which is based on a combination of secant transverse mercator zones in north-south strips with the poles covered by polar stereographic zones. The coordinates then combine the zone you are in along with a position within that zone in metres. This can be combined with various datums, with WGS84 being the most common.
A datum is a model of the planet. Real cartography is generally based on a spheroidal datum (flattened sphere) used to approximate the geoid (an equipotential surface of the earth's gravitational field) The most common spheroid for modern geography is called GRS80. This has an equatorial radius of 6,378,137 m and a polar radius of 6,356,752.314,140,347 m. Different datums then move the centre or sometimes rotate it very slightly, and select a central meridian. WGS84 tries to position it so it matches the geoid over the whole surface while using greenwitch as the central meridian. NAD83 tries to match the geoid over North America. The vertical and horizontal components of a datum are often handled separately. GRS80 based datums generally have an error of 2 metres relative to one another. Other datums, particularly spherical ones can produce significant errors in comparison.
The European Petroleum Survey Group maintains a database of tens of thousands of coordinate systems which are frequently identified by their "EPSG Code". Anyone involved in GIS has memorized several of the important ones like EPSG:4326 (WGS84) and EPGS:3857 (Web Spherical Mercator). UTM involves a separate code for each zone, for each datum. EPSG:3157 is UTM Zone 10 North in a Canadian specific update of the NAD83 Datum.
The datums also receive periodic updates to account for improvements in the measurement of earth's gravity and for continental drift. New Zealand has special coordinate systems that take a time parameter to account for the comparatively rapid rotation of the Zealandia microcontinent.
If you want to do distance or geodetic bearing calculations globally, you have to use spheroidal mathematics. There's no coordinate system which will make it work as if euclidean that works outside of a restricted extent, and even then it's only approximate.
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Originally Posted by Hai-Etlik
