Cartography Over Time (in different eras)

[Gabriel S]

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.

Thank you! This is a perfect place for me to start!

New Zealand has special coordinate systems that take a time parameter to account for the comparatively rapid rotation of the Zealandia microcontinent.

That is fascinating! I am off to bury myself in reading, thank you!