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August 2019
PHOTOGRAMMETRIC ENGINEERING & REMOTE SENSING
The first observation of absolute gravity in Louisiana at the one micro gal level of precision was at the
University of New Orleans in 1989 by the National Geodetic Survey. Since then, four additional obser-
vations through 2018 have shown a cumulative apparent subsidence of 147mm in 29 years (-5mm/yr).
In 2002, the Commander, New Orleans District Corps of Engineers requested assistance of what is now
the National Geospatial-Intelligence Agency (NGA) to perform an absolute gravity observation campaign
throughout the State of Louisiana at many of the LSU Center for GeoInformatics (C4G) GPS Continuous-
ly Operating Reference Station (CORS) sites. A second observation campaign was completed by NGA this
year. Cumulative apparent elevation changes are:
Baton Rouge negligible
Old River
-34 mm
Thibodeaux
+7 mm
Oakdale
negligible
Lafayette
negligible
Lake Charles
-16 mm
Sicily Island
+8 mm
LUMCON
-20 mm
Rayville
+13 mm
Boothville
-13 mm
Ruston
-9 mm
Hammond
negligible
Shreveport
negligible
Natchitoches
+17mm
Alexandria
-49 mm
Changes in the absolute value of gravity at a location can be a result of uplift/subsidence as well as
variations in ground water and tectonic motion. In a generally homogenous sedimentary basin such
as Louisiana, it’s likely some combination of subsidence and ground water. LSU now has a three-per-
son permanent gravity survey crew that is traveling to all C4G CORS sites state-wide as well as to tide
gauges collocated with CORS sites throughout the northern rim of the Gulf of Mexico for the observation
of absolute gravity and for deflection of the vertical. These observations are expected to contribute to the
knowledge of the surface motions of the State as well as to form the basis of a new quasi-geoid model for
Louisiana in collaboration among NGA, NGS, and LSU.
This is a follow-up to
“Are We Sinking?— National Geospatial-Intelligence Agency Visits LSU Campus to
Measure Subsidence”
which was originally published in the January 2019 issue of
PE&RS
.
