Continuously Operating Reference Station Coordinate Update Policy

June 23, 1999

To meet the needs of the varied groups that use data from the National Continuously Operating Reference Station (CORS) system, the National Geodetic Survey (NGS) publishes coordinates for CORS in several reference frames including the North American Datum of 1983 (NAD 83) and other reference frames that are consistent with the International Earth Rotation Service Terrestrial Reference Frame (ITRF).

NGS may revise its adopted NAD 83 coordinates for a particular CORS if recent computations consistently indicate that the site's adopted horizontal coordinates differ by more than 2 cm and/or its adopted vertical coordinate differs by more than 4 cm.

Similarly, for the latest ITRF version, NGS may revise a site's adopted ITRF coordinates if recent computations consistently indicate that the adopted horizontal coordinates differ by more than 1 cm and/or the adopted vertical coordinate differs by more than 2 cm.

In the above cases, NGS will revise both the horizontal and the vertical coordinates, even if only one set differs by more than the allowable coordinate change tolerance.

NGS generally does not revise coordinates for earlier ITRF versions.

If NGS adopts a new ITRF version, then NGS will compute and publish new ITRF coordinates consistent with that reference frame. NGS will transform these new ITRF coordinates into provisional NAD 83 positional coordinates. NGS, however, will replace the currently adopted NAD 83 coordinates of a particular CORS with its corresponding provisional coordinates only if these two positions differ by more than 2 cm horizontally and/or 4 cm vertically.

If the physical location of the "antenna-reference-point" of a CORS is significantly altered, NGS will rename the site and adopt new coordinates for it as soon as practicable.

NGS routinely conducts network analysis in areas to support the National Spatial Reference System. Such analysis includes CORS coordinates and new GPS survey data, and might indicate the need for area readjustments. If it is ascertained that the published CORS coordinates have a negative influence on an area readjustment, then the CORS coordinates expressed in the ITRF and/or the NAD 83 frame may be revised irrespective of any coordinate change tolerances.