HIGH ACCURACY REFERENCE NETWORK FOR MICHIGAN
David R. Doyle,
Observation and Analysis Division, National Geodetic Survey
The National Geodetic Survey (NGS) has recently completed the final adjustment of the Federal Base and Cooperative Base Networks (FBN/CBN) for Michigan. Consisting of 119 stations, 40 new, and 79 existing horizontal and vertical control stations spaced at approximately 30 kilometer (19 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 Michigan High Accuracy Reference Network (HARN).
Project implementation and coordination were directed by NGS, with the assistance of the Michigan Department of Commerce (MIDOC). Field operations were conducted between October, 1994 and February, 1995, by NGS/MIDOC surveyors using Trimble 4000SSE 4000SST, and Ashtech LD-XII and MD-XII dual frequency GPS receivers. Additional assistance for GPS observations was also provided by the Minnesota Department of Transportation (Trimble 4000 SSE), Ohio Department of Transportation (Ashtech MD-XII), Orchard, Hiltz & McCliment, Inc. of Livonia, MI (Ashtech Z-12), and Wade-Trim, Inc. of Flint, MI (Trimble 4000SSE). 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 Iowa, Minnesota, Ohio, and Wisconsin, 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. Given the current back log of other HARN state-wide readjustments, the Michigan 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 0.8 meter (2.6 feet). Positions and velocities relative to the International Earth Rotation Service (IERS) Terrestrial Reference Frame (ITRF) will also pe published for all FBN/CBN stations.
Orthometric heights for FBN/CBN stations were determined by occupying 74 published bench marks referenced to the North American Vertical Datum of 1988 (NAVD 88). Ellipsoidal heights were determined by holding the values published for 15 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 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 Michigan adjustment (MIHPGN.LAS and MIHPGN.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, or the NGS World Wide Web site at http://www.ngs.noaa.gov.
Questions concerning the FBN/CBN and state-wide readjustment, or coordinate transformations should be directed to Ron Ramsey, NGS Michigan Geodetic Advisor at (517) 377-1510, or email firstname.lastname@example.org, or Dave Doyle, NGS Observation and Analysis Division at (301) 713-3178, or email email@example.com.