Observations on Positions: Louisiana Is Subsiding
Professional Surveyor Magazine - January 2004
Clifford Mugnier, CP, CMS
Recent results of research by Dr. Roy K. Dokka and Mr. Kurt Shinkle of Louisiana State University have demonstrated irrefutable proof of subsidence rates in the Lower Mississippi Valley. The forthcoming official report to be published by the United States National Geodetic Survey (NGS) is the first regional analysis of a major continental area that is based on nearly a century of First-Order Geodetic Leveling observations tied to a common datum. Professor Dokka has been performing geodetic-quality GPS surveys in Louisiana and the northern rim of the Gulf of Mexico for a number of years and he has established the GULFNET CORS sites. Initial funding for that was made possible by the National Science Foundation and the LSU Board of Regents.
On January 31, 2001, the Lower Mississippi Valley Division approved a proposal of the New Orleans District (NOD) to convert all project elevations and gauge elevations to the North American Vertical Datum of 1988. To implement that plan, the NOD has assisted in the establishment of a number of GPS Continuously Operating Reference Stations throughout the district by the LSU Center for GeoInformatics (C4G). Later that year, the NGS recommended to Congress that a local arm of the NGS be established and funded within C4G. That new arm is the Louisiana Spatial Reference Center (LSRC) at LSU, and is responsible for the establishment, monitoring, and maintenance of the CORS sites throughout Louisiana and the NOD. This is of great importance to the Corps of Engineers because the LSRC represents the local implementation of the National Height Modernization Program. Professor Dokka is the Director of C4G and Chief of the LSRC.
The establishment of elevation bench-marks has been performed with differential leveling (also termed "spirit leveling") techniques for perhaps 3,000 years. The ancient Babylonians and Romans utilized the same principles by sighting along the level surface of water contained in small portable wooden troughs. Elevation benchmarks established in the United States in the 19th and 20th centuries by the Corps of Engineers and the NGS and its predecessor agencies were done with the spirit leveling of optical instruments. Spirit leveling is still performed for local construction and for metropolitan benchmark densification projects, but "long-line leveling" for hundreds and thousands of miles is too expensive in the 21st century. For instance, a rule-of-thumb nowadays for geodetic leveling is $1,000/kilometer or one mile of geodetic levels to observe two benchmarks = $1,600! (That cost includes the reconnaissance, descriptions, formatting, observation corrections, etc.).
The Lower Mississippi Valley is a sedimentary basin, and the analysis by Dokka and Shinkle has documented hard scientific evidence of subsidence from St. Louis, Missouri to Venice, Louisiana. Actual subsidence rates have been computed upstream as far as Memphis, Tennes-see (4 mm/year), and this will serve as the basis for the planning of updating elevation benchmarks due to subsidence. Some areas along the Louisiana coast appear to be subsiding at rates approaching one inch per year!
I have been involved in the research of subsidence of Louisiana since the late 1970s, and have persuaded the NGS to observe Absolute Gravity at the University of New Orleans periodically since the early 1980s. Geodetic leveling in metropolitan New Orleans has shown subsidence in the metro area of approximately 9 millimeters/year, and re-observations of Absolute Gravity have shown an increase in the value corresponding to an annual subsidence of 9.1 millimeters/year.
With the enormous cost of "long-line leveling," the solution to the problem of updating Louisiana's benchmarks and gauges that have subsided is being solved with the implementation of the National Height Modernization Program. A reference system is needed, and that is realized by the North American Vertical Datum of 1988 (NAVD88). Although many elevation benchmarks in the United States have been published on the NAVD88, some of the published data is no longer valid since the marks have physically sunk away from their previous positions.
That generalization is true for the majority of benchmarks in the state of Louisiana and for every benchmark in the NOD. The NAVD88 is still correct, but the published values in Louisiana are incorrect. Therefore, how is the National Height Modernization Program going to help the New Orleans District, the Vicksburg District, the Memphis District, etc.? (Note that VERTCON is inappropriate for use in Louisiana).
The GPS Continuously Operating Stations of the LSRC are now monitoring the movement of the buildings that the antennas are bolted to, and as Louisiana moves, so do the CORS sites. As an independent check of vertical movement of the CORS buildings, the National Imagery and Mapping Agency (NIMA) and NGS have observed Absolute Gravity at approximately two dozen present and planned CORS sites in Louisiana and Mississippi. Repeat observations at those locations several years from now may indicate that Absolute Gravity has changed in correspondence to individual subsidence evidenced by the continuous GPS observations (I hope so, anyway, since I'm the one that has convinced a number of agencies to spend the money for the observations).
The current plan is to observe a line of GPS Leveled points from Pensacola, Florida to the Mississippi-Louisiana State line along the Gulf Coast. There's a newly-surveyed line of GPS leveling observations at benchmarks that have been observed by a number of firms from the Memphis area down through the mouth of the Mississippi River that is now under scrutiny and adjustment by NGS. Connections in the coming year are expected to allow the establishment of NAVD88 current elevations at the GULFNET CORS sites of the LSRC. There are also plans afoot to begin benchmark densification observations in some of the Louisiana Parishes (counties) this year that will include First-Order Invar Leveling as well as GPS Leveling observations. Some of the work is being partly funded by the Corps of Engineers in support of environmental concerns, and may offer substantial business opportunities for firms qualified to do geodetic-quality observations with GPS equipment as well as the current barcode invar leveling gear offered by several instrument manufacturers.
CLIFF MUGNIER is a Board Certified Photogrammetrist and Mapping Scientist (GIS/LIS) and teaches Surveying, Geodesy, and Photogrammetry at Louisiana State University. He is also a Contributing Writer for the magazine.
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