Total number of geographic coordinate systems in ArcGIS

[DasheEbra]

Hi everyone,

Recently, I’ve been exploring more about coordinates and came across the following components of a Geographic Coordinate System (GCS):

1. Spheroid (Ellipsoid): A mathematical 3D model that approximates the Earth's shape, so it can reach representing the geoid.
2. Datum: Based on a spheroid, but with adjustments to its center and orientation to better fit a specific region or spatial area.
3. Prime Meridian
4. Angular Units

While studying this, I’ve come across a few questions and would appreciate some insights:

1. Since each country or region selects a spheroid to minimize inaccuracies for its geographic area, and then adjusts by creating a datum and adjust spheroid location, why do we have multiple spheroids in the first place as long as datum will move the spheroid center? And how many spheroids exist globally in the GIS industry?

2. Similarly, how many Geographic Coordinate Systems (GCS) are currently in use across different software and standards?

[DanPatterson]

esri document listing their supported coordinate systme

Geographic and Vertical Coordinate Systems

7538 for this site

Spatial Reference List -- Spatial Reference

[DavidPike]

My take (not a geodesist):

A Spheroid for a specific regional area can better approximate the Geoid over that area - as opposed to WGS84 which is the best approximation of the World Geoid.  The Spheroid (by name) is not a sphere, so the parameters that influence its shape (major and minor axes) aren't solely the origin datum point.  

Certain spheroids will still also be available to align historical mapping etc.  Millimetre level refinements might be yet more accurate and available - but don't justify updating the current definitions in terms of practicality and effort.