NEW COORDINATE ADJUSTMENT
David R. Doyle, Senior Geodesist
Observation and Analysis Branch, National Geodetic Survey
The National Geodetic Survey (NGS) has recently completed the final adjustment of the
Mississippi High Accuracy Reference Network (HARN) and state-wide readjustment. Consisting
of 64 stations, 19 new and 45 existing horizontal control stations spaced at approximately 50
kilometer (31 mile) intervals (Diagram), the HARN 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 Committee. In addition, First-Order control points were set for the National Aeronautics
and Space Administration - Stennis Space Flight Center (NASA-SSC), Mississippi Department of
Environmental Quality (MSDEP), and Mississippi State University (MSU).
Project implementation and coordination were directed by Mr. Don Rexrode, NGS Geodetic Advisor to Mississippi, with the assistance of the Mississippi Department of Transportation (MSDOT), and in cooperation with the NASA-SSC. Additional field support was also provided by the U.S. Army Corps of Engineers, U.S. Geological Survey, U.S. Naval Oceanographic Office, MSDEP, MSU, Louisiana Department of Transportation, Louisiana State University, EMC Inc., and Navigation Electronics Inc. Field operations were conducted between May 3 - 26, 1993, under the direction of Mr. Claude Stevens, Chief of NGS Field Party G-03, using Trimble 4000SST, 4000SSE, 4000ST, Ashtech XII, Ashtech PXII and Mini-Mac 2816 dual and single frequency GPS receivers. Most observations far exceeded the 1:1,000,000 proportional accuracy required for the B-Order adjustment. A majority of lines exceed 1:10,000,000!
In addition to adjusting the GPS data to fiducial stations of the International GPS and Geodynamics Service (IGS), Very Long Baseline Interferometry (VLBI) stations, and existing HARN stations in Louisiana, Alabama, and Tennessee, all existing horizontal control in the State has been readjusted to provide consistency between the HARN and the existing horizontal network, and improve the internal consistency of the existing 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, 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 typically less than 1 meter (3.2 feet).
Orthometric heights for the HARN were determined by occupying 32 published bench marks and six stations with previously determined heights by GPS, referenced to the North American Vertical Datum of 1988 (NAVD 88). Accuracy of ellipsoidal heights determined by the GPS observations are less than third-order, and 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 this readjustment, and not submitted to NGS ("Blue Booked") should be readjusted from original observations to maintain consistency with NGRS. 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 Mississippi adjustment (MSHPGN.LAS and MSHPGN.LOS). The transformation grids have been tested by NGS and should provide transformation values accurate to an average of approximately 0.05 meter across the State. Updated coordinate information, and the NADCON software can be obtained from the NGS Information Services Section, telephone (301) 713-3242. Questions concerning the HARN and state-wide readjustment or coordinate transformations should be directed to Don Rexrode, (601) 944-9098 or Dave Doyle, (301) 713-3178.