“The International GPS Service (IGS) is a voluntary federation of more than 200 worldwide agencies that pool resources and permanent GPS station data to generate precise GPS products”

GEOID03 is a refined model of the geoid in the conterminous United States (CONUS), which supersedes the previous models GEOID90, GEOID93, GEOID96, and GEOID99.

Through the use of careful GPS survey procedures coupled with high-resolution geoid models, surveyors have obtained orthometric heights with an accuracy commensurate with that of leveling.

‘…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.’

Yellow triangle represents XYZ coordinates of latest solution; refreshes every few minutes

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

10 People. 10 Walks. 10 Stories Told.

‘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.