The National Geodetic Survey (NGS) has recently completed the final adjustment of the Federal Base and Cooperative Base Networks (FBN/CBN) for Utah. Consisting of 126 stations, 58 new, and 68 existing horizontal and vertical control stations spaced at approximately 50 kilometer (31 mile) intervals, the network was observed to A and B-Order horizontal accuracy standards (5 mm + 1:10,000,000 and 8 mm + 1:1,000,000) as defined by the Federal Geodetic Control Subcommittee. This network is often referred to as the Utah High Accuracy Reference Network (HARN).

Field operations were conducted between August and November, 1994, by NGS, using Trimble 40000SSE dual-frequency GPS receivers. Co-observations for the CBN were provided by Salt Lake, Uintah, Utah, and Washington Counties, as well as Clark County, Nevada, and the University of Utah in Salt Lake City. A connection was also completed to the Federal Aviation Administrations' (FAA) Wide Area Augmentation System (WAAS) GPS site in Salt Lake City. In addition, B-order control points were established at 25 airports in support of the FAAs' Area Navigation Approach (ANA) program. The data from the ANA program will allow the FAA to develop GPS approaches for air navigation. Most observations far exceeded the 1:1,000,000 proportional accuracy required for the B-Order adjustment.

In addition to adjusting the GPS data to fiducial stations of the International GPS and Geodynamics Service (IGS), and existing FBN/CBN stations in Arizona, Colorado, Idaho and Nevada, all existing horizontal control in the State will be readjusted to provide consistency between the FBN/CBN and the existing horizontal network. The readjustment will extend into the bordering states to the extent necessary to maintain consistency of the National Spatial Reference System (NSRS). Until the completion of the state-wide readjustment, HARN stations will be designated as "SPECIAL STATUS" on NGS data sheets to indicate their positional differences with the existing lower order NSRS stations. Given the current back log of other HARN state-wide readjustments, the Utah readjustment could require as much as 2 years to complete. The new coordinate values are referred to as North American Datum of 1983 (NAD 83), Adjustment of 1994, and are designated NAD 83 (1994). This designation is necessary to distinguish between the original NAD 83 Adjustment of 1986, or NAD 83 (1986). Coordinate values should be properly labeled to eliminate confusion. Positional changes due to the network improvement vary across the State, but are generally less than 0.7 meter (2.3 feet). Positions and velocities relative to the International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF) will also be published for all FBN/CBN stations.

Orthometric heights for the FBN/CBN were determined by occupying 40 bench marks and 5 stations with previously determined heights by GPS referenced to the North American Vertical Datum of 1988 (NAVD 88). Ellipsoidal heights were determined by holding the values published for 6 existing A-Order quality stations in and around the State. Accuracy of ellipsoidal heights determined by these observations vary, and are sometimes less than third-order. Orthometric heights are considered to be equivalent to those obtained by conventional vertical angle observations (0.1 meter/0.3 feet).

All GPS surveys performed prior to the HARN, and not submitted to NGS ("Blue Booked") for inclusion in NSRS, should be readjusted from original observations to maintain consistency with NSRS. Lower order coordinate information (e.g. cadastral survey, photogrammetry,) can be transformed from NAD 83 (1986) to

NAD 83 (1994) using version 2.10 of the NADCON software supplied by NGS, with special transformation grids for the Utah adjustment (UTHPGN.LAS and UTHPGN.LOS). The transformation grids will be developed by NGS following the state-wide readjustment, and should provide transformation values accurate to an average of 0.06 meter +/- 0.02 meter (0.20 +/- 0.06 feet) across the State. Updated coordinate information, and the NADCON software can be obtained from the NGS Information Services Section at (301) 713-3242, the NGS Electronic Bulletin Board Service at (301) 713-4181/4182, and the NGS World Wide Web site at http://www.ngs.noaa.gov.

Questions concerning the FBN/CBN, state-wide readjustment, or coordinate transformations should be directed to Dave Doyle, NGS Observation and Analysis Division at (301) 713-3178, or email daved@dancer.ngs.noaa.gov.