What is a Datum?

A datum provides a reference for the geographic coordinates we use to reference data to the earth. For hundreds of years, datums were established in each area for the data to be referenced to within that area. However, the datums we use today are based on ellipsoids for the entire earth rather than a single area.

A datum includes an ellipsoid, that's it.

Note that when we talk about a "datum" we are typically talking about "horizontal datums". There are also vertical datums which are critical for applications involving the height of the ocean and tides. These are often still established at a local level.

A modern datum specifies:

You can see the contents of a datum by opening a ".prj" file for a shapefile in "Notepad" or another text editing application. Use the ".prj" from a shapefile containing data in geographic coordinates and you'll see the ellipsoid, units, and prime meridian for that data set.

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