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.