The advancement of GPS technology and methodologies has reached the point that, without much effort, relative positioning of points with respect to the National CORS sites can be accomplished with subcentimeter-level accuracies. Accordingly, the National Geodetic Survey (NGS) has upgraded NAD 83 positions and velocities for all CORS sites, except those located on Pacific Islands, so that they equal the transformed values of recently computed ITRF00 positions and velocities. The applied transformation equations are given at http://www.ngs.noaa.gov/CORS/metadata1/.

This upgrade will remove inconsistencies among previously published NAD 83 positions and velocities which are detectable with modern high accuracy GPS surveys. These inconsistencies would have corrupted results obtained with the OPUS (Online Positioning User Service) utility that uses CORS positions and velocities to compute accurate positions for other points.

Moreover, for each CORS site, NGS will now provide NAD 83 positional coordinates that are referenced to an epoch date of 2002.00. That is, the published coordinates for a site's position will correspond to this site's location on January 1, 2002. The site's velocity needs to be applied to compute the site's position on any other date.

Previously, NAD 83 positions for the CORS sites were published for an epoch date of 1997.00 (January 1, 1997). The use of the more current epoch date will reduce those systematic errors occurring when points are positioned relative to CORS sites without applying appropriate site velocities. This more current epoch date will especially benefit those involved in positioning activities in areas of active crustal motion, like western CONUS and Alaska.

Published ITRF00 positional coordinates will continue to be referenced to an epoch date of 1997.00 to remain consistent with international convention.