‘…the geoid is used to describe the unique and irregular shape of the earth. However, only recently have the more substantial irregularities in the surface created by the global mean sea level (MSL) been observed.’

NGS is developing models and working with other agencies to use the GPS data from the CORS network to make high spatial and temporal resolution maps of the ionosphere on a continental scale

Recently purchased by Google, Keyhole is a subscription satellite/aerial imagery service, much like WorldWind.

‘This process is designed to provide the datum shift between the NAVD 88 and NGVD 29 vertical datums at a specified geographic position’

old (2001) slideshow with some good information on NGVD, &c.

GIS gain much of their power from being able to collate and cross-reference many types of data by location.

This document describes an RDF vocabulary for representing WGS84 (world geodesic survey) lat/long/alt positioning.

“World Wind allows any user to zoom from satellite altitude into any place on Earth, leveraging high resolution LandSat imagery and SRTM elevation data to experience Earth in visually rich 3D, just as if they were really there.”

good definition with coordinate systems and figure of the earth

Schematic diagram showing the relationship between local mean sea level, the geoid, and local ocean dynamic topography

Schematic diagram showing the relationship between the geoid, orthometric heights and ellipsoid. Note that the ellipsoid is drawn above the geoid. This is the actual case for all points in the conterminous United States

Scematic diagram showing some of the “level surfaces” of the Earth, including the geoid, and relation to the Earth’s crust and local mean sea level