HIGH ACCURACY REFERENCE NETWORK
David R. Doyle
National Geodetic Survey
The National Geodetic Survey (NGS) has recently completed the final adjustment of the Federal Base Network/Cooperative Base Network (FBN/CBN) for Oklahoma. Consisting of 129 stations, 55 new and 74 existing horizontal and vertical control stations spaced at approximately 50 kilometer (30 mile) intervals (Diagram), the network was observed to A and B-Order 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 High Accuracy Reference Network (HARN).
Project implementation and coordination were directed by NGS, with the assistance of the Oklahoma Department of Transportation (OKDOT). Field operations were conducted between June and September, 1993, by NGS/OKDOT surveyors using Trimble 4000SST and Ashtech LM-XII OC dual frequency GPS receivers. 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 NGS Eastern Strain Network, International GPS and Geodynamics Service (IGS), Very Long Baseline Interferometry (VLBI) stations, and existing FBN/CBN stations in Colorado, New Mexico and Texas, all existing horizontal control in the State has been readjusted to provide consistency between the FBN/CBN and the existing horizontal network. The readjustment extended into the bordering states to the extent required to maintain consistency of the National Spatial Reference System (NSRS). The new coordinate values are referred to as North American Datum of 1983 (NAD 83), Adjustment of 1993, and are designated NAD 83 (1993). 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 1 meter (3.2 feet).
Orthometric heights for FBN/CBN stations were determined by occupying 36 published bench marks and 4 stations with previously determined heights by GPS referenced to the North American Vertical Datum of 1988 (NAVD 88). Orthometric heights are considered to be equivalent to those obtained by conventional vertical angle observations (0.1 meter/0.3 feet). Ellipsoidal heights related to NAD 83 were also determined for all HARN stations. Accuracy of ellipsoidal heights vary, and are sometimes less than third-order.
All GPS surveys performed prior to this readjustment, and not submitted to NGS ("Blue Booked") 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 (1993) using version 2.10 of the NADCON software supplied by NGS, with special transformation grids for the Oklahoma adjustment (OKHPGN.LAS and OKHPGN.LOS). The transformation grids are being tested by NGS and should provide transformation values accurate to an average of 0.06 meter +/- 0.02 meter (0.20 +/- 0.06 feet) across the State. These grids should be available by early summer, 1995. Updated coordinate information, and the NADCON software can be obtained from the NGS Information Services Branch at (301) 713-3242, or from the NGS Bulletin Board Service at (301) 713-4181/4182.
Questions concerning the FBN/CBN and state-wide readjustment or coordinate transformations should be directed to Dave Doyle, NGS Observation and Analysis Division at (301) 713-3178.