HIGH ACCURACY REFERENCE NETWORK FOR NEBRASKA
David R. Doyle, Senior Geodesist
National Geodetic Survey
The National Geodetic Survey (NGS) has recently completed the final adjustment of the Federal and Cooperative Base Networks (FBN/CBN) for Nebraska. Consisting of 220 stations, 103 new and 117 existing National Spatial Reference System control stations spaced at approximately 40 kilometer (25 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 Nebraska Department of Roads (NEDR). Field operations were conducted between September and November, 1995, by NGS/NEDR surveyors using Trimble 4000SST and 4000SSE dual frequency Global Positioning System (GPS) receivers. Additional co-observations for the CBN were also provided by the United States Forest Service, Lancaster County Engineering Department, and the Colorado Department of Transportation. 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 A-Order FBN stations in Colorado, Kansas, Minnesota, Missouri, South Dakota and Wyoming, all existing horizontal control in the State will be readjusted to provide consistency between the HARN 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 Nebraska 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 1995, and are designated NAD 83 (1995). 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 0.50 meter (1.6 feet). Positions and velocities relative to the International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF) will also be published for all HARN stations.
Orthometric heights for the HARN were determined by occupying 82 bench marks and 4 stations with previously determined heights by GPS, referenced to the North American Vertical Datum of 1988 (NAVD 88). NAD 83 ellipsoidal heights were determined by holding the values published for 8 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 (1995) using version 2.10 of the NADCON
software supplied by NGS, with special transformation grids for the Nebraska adjustment
(NEHPGN.LAS and NEHPGN.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 HARN and state-wide readjustment or coordinate transformations should be directed to Jerry Odum, NGS Nebraska Geodetic Advisor, telephone (402) 479-4379, or email firstname.lastname@example.org, or Dave Doyle, NGS Observation and Analysis Division, telephone (301) 713-3178, or email daved@ ngs.noaa.gov.
Also send to: Nebraska GIS Steering Committee Newsletter, James W. Merchant, Conservation and Survey Division, University of Nebraska-Lincoln, 113 Nebraska Hall, Lincoln, NE 68588-0517, (402) 472-2410
Professional Surveyors Association of Nebraska, Mr. Ron Way, Douglass County Engineers Office, 15505 West Maple Rd., Omaha, NE 68116, (402) 444-6479