HIGH ACCURACY REFERENCE NETWORK FOR NEVADA

David R. Doyle

Senior Geodesist

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 Nevada. Consisting of 102 stations, 43 new and 59 existing horizontal and vertical control stations spaced at approximately 50 kilometer (31 mile) intervals, 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, under a cooperative agreement with the Nevada Association of Land Surveyors. This survey was in consortium with a variety of in-state Federal, State, county and municipal agencies. Field operations were conducted between May and July, 1994, by NGS, Bureau of Land Management, U. S. Forest Service, and Clark County surveyors using Trimble 4000SSE, 4000SST, and Ashtech LM-XIIOC dual frequency Global Positioning System (GPS) receivers. Special recognition is given to Mr. Bill DeSantis of the Clark County Surveyors Office for his professional dedication to this survey and valuable liaison assistance to the NGS field unit.

In addition to adjusting the GPS data to fiducial stations of the NGS Eastern Strain Network, International GPS and Geodynamics Service (IGS), and Very Long Baseline Interferometry (VLBI) stations, the network will also be adjusted to existing FBN/CBN stations in Arizona, California, Idaho, and Oregon. As resources become available, 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 required 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. 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 0.5 meter (1.6 feet).

Orthometric heights for the FBN/CBN were determined by occupying 45 bench marks and 7 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. Ellipsoidal height accuracies vary, and are sometimes less than third-order.

All GPS surveys performed prior to the HARN, and not submitted to NGS ("Blue Booked") for inclusion in the 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 Nevada adjustment (NVHPGN.LAS and NVHPGN.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 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.