Physical Geodesy is the science of studying the variations of the Earth's gravity field. The Geodesy of Subsidence has been an active topic of research in the LSU System since the middle 1980s. The intensity of the Earth's gravity at any spot on the surface is a function of the density of the Earth's crust at that spot and the distance to the center of mass. In a homogeneous sedimentary basin (Lower Mississippi Valley and the Delta), the density is practically constant except for occasional salt domes; therefore any variation in the gravity field may be attributed directly to changes in elevation, which usually relates to subsidence in Louisiana. Some "noise" can result from significant changes in groundwater.

Relative gravity surveys have determined the magnitudes of change from one elevation benchmark to another in the Gulf Coast Region, including Louisiana but the initial starting point has been necessarily assumed until the advent of Absolute Gravity observations made in the early 1990s. Re-observation of Absolute Gravity in New Orleans has been performed six times since then, and each new observation has shown a constant subsidence rate of 9.1 millimeters per year.

In the early 2000s, the LSU Center for Geolnformatics began installing GPS receivers at public buildings throughout the State of Louisiana and has continuously recorded the heights of the receivers as they have changed through the years. Most of these sites have shown varying rates of subsidence from place to place, based solely on GPS observations. A series of Absolute Gravity observation campaigns commenced in 2002 at some of the C4G GPS CORS sites by the National Geodetic Survey, and in 2006 was observed by the National Geospatial-lntelligence Agency (military).

No further observations of Absolute Gravity were performed in the State of Louisiana until 2018 when the National Geospatial-lntelligence Agency returned through the official requests of C4G and the Commander, New Orleans District Corps of Engineers. Some of the C4G CORS sites will be visited based on personnel availability and scheduling, but their generosity cannot be extended to over 100 of our CORS sites!  Our own FG-5X Absolute Gravity meter is now a necessity to support our two CG-5 Relative Gravity meters in our continuing, never-ending campaign to observe, record, and map the ever-changing gravity field of the State of Louisiana.

The significance of this research is that traditional surveying methods used to determine elevations of benchmarks have become inordinately expensive in the 21st century, and GPS is the only technology available nowadays and for the future to provide observations for that purpose. The fly in the ointment, however, is that GPS does NOT provide "elevations" related to the concept of "mean sea level." GPS only provides a geometric "height" with respect to the center of mass of the Earth which the satellites orbit about. A mathematical model of the Earth's gravity field is necessary to translate from "height" to "elevation," and that model is called the "geoid." Where does the geoid come from? It comes from knowledge of the Earth's gravity field.

The more exquisite the knowledge we have about the gravity field, the more accurate we then can model the geoid, providing more reliable elevations for flood insurance, flood control, planning for evacuation route changes based on continuing subsidence, and variations in subsidence rates throughout the State of Louisiana. The LSU GPS Continuously Operating Reference System (CORS) network is a new public utility offered by C4G, but that network is dependent on maintaining and improving the knowledge of the geoid for elevation control in support of Louisiana Revised Statute 50:173.1 which names C4G as the State Reference Standard for elevations.

Bill Henning, NGSThe LSU Center for GeoInformatics hosted a training session on Best Methods to Achieve Accurate, Repeatable Orthometric Heights Using Real Time GNSS Networks. The event was held at the Orleans Parish Levee District Offices in New Orleans, Louisiana on August 15th 2012. The event was attended by over 60 individuals including surveyors, engineers, local, state and federal government officials. 

Some of the topics covered were:

  • Vertical Geodetic Control in Southern Louisiana
  • Providing the National Spatial Reference System in dynamic regions
  • Intro to GNSS
  • Guidelines for Establishing GPS-derived Ellipsoid Heights
  • Guidelines for Establishing GPS-derived
  • Orthometric Heights
  • Improvements to the Geoid Model
  • Real-time Kinematic Surveying and Best Practices
  • Introduction to Real-time Networks

The South Louisiana Flood Protection Authority District - East has made these trainings a Requirement for Professionals Working for the Levee District. Surveyors and Engineers that attended also received 8 Continuing Education Credits. The training provided professional Land Surveyors the requisite training to properly use an RTN system such as GULFNet or C4Gnet to provide current correct elevations. The main speaker was Bill Henning of NGS, one of the premier experts in the GNSS applications field. Registration, food and refreshments were handled by LSPS District 1 and plans are currently in the works to host additional events of this type in Baton Rouge and Shreveport in the near future.

If you are interested in attending one of these training events please sign up for the C4G mailing list or follow C4G in social media on Twitter, Facebook or the C4Gnet RSS feed. You can also find Podcasts, PDF's and Videos of past events on the C4G website or the C4G YouTube channel.


GNSS Market Research and Analysis GPS/GNSS Networks and Services The Global Market for GNSS Augmentation Infrastructure and Services 2009-2013 March 2009 Abstract ©Position One Consulting Pty Ltd 2009 All rights reserved Authors Robert Lorimer & Eric Gakstatter

Global Navigation Satellite Systems (GNSS) include the United States GPS (the most commonly used today), Russia's Glonass, the EU's Galileo, China's Compass, the Indian Regional Navigation Satellite System (IRNSS) and Japan's Quasi-Zenith Satellite System (QZSS).

The signals from all these GNSS are subject to a myriad of errors and may not always tell the truth. Whereas this is not a problem for casual users or those who can tolerate such errors, there are many applications where integrity and accuracy must be improved.

C4G CORS used in C4Gnet and GULFNet Real Time Networks have been valued at over 33 Million dollars a year according to a new government report released June 15th, 2009. The report titled "Socio-Economic Benefits Study: Scoping the Value of CORS and GRAV-D" was prepared for the National Geodetic Survey by Irving Leveson. It shows billions in estimated benefits from NOAA Positioning Products and Services:

• National Spatial Reference System (NSRS): $2.4 billion per year
• CORS: $758 million per year. (C4G has 27 of the 1324 National CORS)
• C4Gnet and GULFNet have 66 CORS with a yearly valuation of $500K each.
• GRAV-D (Once completed): $4.8 billion over 15 years, including $2.2 billion in avoidance costs from improved floodplain management.

Download the entire report!

Note that the LSU Center for GeoInformatics is actively pursuing the funding necessary to do the control work needed to create better GEOID models in Louisiana. Feeding well distributed vertical control data into future GEOID models will improve the vertical accuracy of these models and is perhaps our only hope of ever creating a ±2cm GEOID in the state of Louisiana.

The NGS presentation on Friday March 15, 2013, pointed out that GEOID12a produces 95% confidence at ± 4 to 8 cm in Louisiana. The NGS conclusion states that the problem is too large for NGS to handle alone and we all need to work together to get better height results. They also recommend partnering with locals to leverage existing resources and that a plan needed to be created to move forward with improving heights.

NGS plot shows GEOID12a produces 95% confidence at ± 4 to 8 cm in Louisiana

The presentation was given by NGS geodesists / scientists, Michael Dennis and Dan Roman.

  • Michael Dennis was the recent project manager for the new adjustment to the North American Datum of 1983, NAD 83 (2011), and performed the S. LA project vertical adjustment.
  • Dan Roman is the head of research and development of geoid models at NGS.

Big picture ideas to consider from the presentation

  • Short term possibilities (through ~2015)
    • Extend 2009 MS leveling into and across LA
    • Establish GNSS infrastructure to monitor subsidence
    • New ~2015 GNSS Height Mod survey (gives 10-year delta time)
  • Medium term (~2016-2022)
    • Terrestrial gravity surveys
    • Incorporate GRAV-D aerial gravity into geoid model for region
  • Long term (~2022)
    • New vertical datum based on gravimetric geoid

Recently, NOAA’s National Geodetic Survey released updated orthometric heights for Southern Louisiana relative to the September 2010 GNSS Height Modernization project. These heights represent the most up to date heights available for the region. On March 15, 2013, NGS hosted a free, on-line webinar to present the results of this project.


Visit the NGS page for this event

To download the .mp4 file, click here.

Link to the presentation: