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 INTRODUCTION
 The series of GPS surveys called High Accuracy
Reference Networks (HARNS) were completed in 1998 to establish the
Federal Base Network (FBN) and the
Cooperative Base Network (CBN). These surveys were carried out on a
state-by-state basis, leading to some inconsistencies at the borders between
states. With the advent of the Continuously Operating
Reference Stations (CORS) network in the mid-1990's, additional
inconsistences were discovered between the CORS and HARN (FBN/CBN) coordinates.
From September 1997 through July 2003 NGS re-observed the Federal Base Network
and in some cases assisted in re-observing and/or densifying the Cooperative
Base Network (CBN). The purpose of this project was to improve the accuracy
of the horizontal and ellipsoid height components of FBN stations. Accuracy
requirements for these surveys were set to ensure 2-centimeter local accuracy
in the horizontal component, as well as 2-centimeter local accuracy in the
ellipsoid heights. In each FBN/CBN survey, ties were made between the CORS
and FBN/CBN stations, and the latest PAGES software was used to reduce the
data. The availability of all this high accuracy data suggests that the time
is right for a new National Readjustment.
On September 24, 2003, NGS's Executive Steering Committee approved a
plan for the readjustment of horizontal positions and ellipsoid heights
for GPS stations in the contiguous United States. Some of the key
points are:
- Only GPS will be adjusted. Classical geodetic observations will not be
included.
- The CORS stations will serve as control, ie, CORS positional coordinates
will be held fixed.
- The ADJUST software will be used.
- The FBN/CBN surveys are a key element since these are high accuracy
(2 cm) surveys that tie the traditional geodetic marks from the HARN network
to the CORS throughout the contiguous United States.
- A layered approach will be taken. The Readjustment of the NSRS will be
divided into national layers (higher order projects) and regional layers
consisting of User Densification Network (UDN) projects.
- Both NAD 83 and ITRF coordinates will be produced and published. The former
will be designated NAD 83 (NSRS).
- User densification projects will be included if observed with GPS and
tied to the HARNs.
- Network and local accuracies will be implemented with the Readjustment of
the NSRS.
- In the event of a delay in software development, testing, and
implementation of the new network and local accuracies, a contingency
option was adopted. This option endorses immediate statewide GPS
readjustments of both horizontal positions and ellipsoid heights.
The National Readjustment is scheduled to commence in June 2005.
Adjustment of the national layers will proceed sequentially and are
scheduled for completion in October 2006. The regional layers will be
adjusted from November 2006 through September 2007.
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