ETL 1110-1-184
01 Oct 1998
Appendix B
DETAILED INFORMATION ON THE CORS SYSTEM
B-1. General. The CORS system provides an additional GPS
reference station for Geographic Information System (GIS) data
collection and geodetic survey applications that do not require
real time positioning.
a. GIS Data Collection (Code Phase). GIS data collection
refers to applications where 1-3 meter accuracy is desired and
code phase GPS receivers are used. For these applications, the
CORS system can be used as the primary reference station,
depending on the distance from the reference station to the
project site. Keeping the distance between the reference station
and the project site less than 250 km (~150 miles) should meet
the 1-3 meter desired accuracy.
b. Geodetic Surveying (Carrier Phase). Geodetic surveying
refers to applications where centimeter accuracy is desired and
carrier phase GPS receivers are used. Using the CORS system for
geodetic surveying provides an inexpensive way to tie a local
network to the national network. It can also be used to provide
redundancy or checks for a control survey. However, it should be
noted that the NAD83 positions for the CORS sites are based on a
different adjustment than most state High Accuracy Reference
Networks (HARNs) or High Precision Geodetic Networks (HPGNs).
B-2. Coordinates and Datum Issues. The positions of each CORS
site are of the L1 phase center of the GPS antenna. When using
CORS as a primary reference or base station for code or carrier
phase positioning, it is important to identify the datum that the
coordinates are based on for those sites.
a. Coordinates. For each CORS site, the positions are
listed in X,Y,Z Cartesian coordinates and latitude, longitude,
and ellipsoid height. The latitude, longitude and ellipsoid
height are computed from the Cartesian coordinates using the
Geographic Reference System of 1980 (GRS80) ellipsoid parameters.
There are also two positions listed for each CORS site. One of
the positions is based on the International Terrestrial Reference
Frame (ITRF) and the other is based on NAD83. The differences
between these reference systems can be ~1.5 meters.
b. NAD83. The NAD83 position for the CORS sites are based
on the 1996 adjustment of the CORS network noted as NAD83 (1996).
In states where a HARN or HPGN has been established, the relative
accuracy between a CORS site (NAD83-1996) and a HARN or HPGN site
(NAD83-199x) is usually better then 5 cm, but can be up to 10 cm.
For states where no HARN has been established, the relative
B-1
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