A datum

Datums define the spheroid and fix the zeros for the coordinate system on that spheroid.

A reference ellipsoid must have a zero point somewhere and the mean sea level must be established from the geoid.  These are fixed for a particular UTM zone, for example, or standard parallel.

Satellite data are currently being gathered and fitted to world-centered ellipsoids (e.g., WRS80, Google earth uses WGS84) that do a better job for the entire surface of the earth as a whole, but less well than the hundred year old regional ellipsoids at any given location.

And so there’s a problem.  Some USGS data come from NAD27 maps (Clarke66 ellipsoid) while most digital data come in NAD83 (WRS80 ellipsoid)…..
look at datum shift (for lat long) for NAD27 to NAD83 on Lexington quad 

Want to see it?
open demo\introPro\introPro project, projections_and_datums map tab

So where are you????

The NAD 27 to 83 datum shift includes

  • initial point change (to center of earth from reference ellipsoid matched to earth’s surface)
  • change in length of ellipsoid axes
  • ground survey network changes (local deviations, distortions, errors)
  • Plate tectonics?? (opening is 15 cm/year * 60 years….)