HIGH ACCURACY REFERENCE NETWORK
David R. Doyle
National Geodetic Survey
The National Geodetic Survey (NGS) has recently completed the final adjustment of the Federal and Cooperative Base Networks (FBN/CBN) for Ohio. Consisting of 244 stations, 151 new and 93 existing National Spatial Reference System (NSRS) control stations spaced at approximately 30 kilometer (23 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 Ohio
Department of Transportation (OHDOT). Field operations were conducted between September
and November, 1995, by NGS/OHDOT surveyors using Trimble 4000SST and 4000SSE,
Ashtech Z XII3, and Leica CR333 dual frequency Global Positioning System (GPS) receivers.
Additional co-observations for the CBN were also provided by the Franklin County Engineers
office. 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 stations in Indiana, Kentucky, Michigan,
Pennsylvania, and West Virginia 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 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 Ohio 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 0.40 meter (1.3 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 51 bench marks and 7 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 3 Continuously Operating Reference Stations (CORS) and 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 generally 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 Ohio adjustment (OHHPGN.LAS and OHHPGN.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
Questions concerning the HARN and state-wide readjustment or coordinate transformations
should be directed to David Conner, NGS Ohio Geodetic Advisor, telephone (614) 292-1619, or
email firstname.lastname@example.org, or Dave Doyle, NGS Observation and Analysis Division,
telephone (301) 713-3178, or email email@example.com.
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