GPS only provides ellipsoid height – very accurate and very precise, but that is not elevation. To obtain elevation information from an ellipsoid height we must utilize a theoretical model of the earth’s gravity field called “the geoid.” Our knowledge of the geoid is only approximate, and it is very faulty in Louisiana because of the constant surface and subterranean movement due to consolidation, faulting, salt domes, and subsidence – the result being largely responsible for “coastal erosion” and the inability of benchmarks to remain reliable for more than a couple of years or so. Therefore, the primary thrust of research must be with regard to a better understanding of the geoid as it exists in Louisiana coupled with the systematic processing of the decade of GPS observations archived in LSU’s server banks. 
We have a research plan that is in physical geodesy which involves GPS observations, coupled with absolute gravity field observations at the same locations, and with zenith camera observations to determine the slope of the geoid at those same locations. Eventually, many of those locations will need to be observed in precise geodetic leveling campaigns (a very expensive proposition also).
In the 1980s, benchmark densification projects were undertaken in Orleans, Jefferson, St. Bernard, and Plaquemines Parishes. These projects consisted of 1st- and 2nd-Order Geodetic Leveling observations accompanied by 2nd-Order Relative Gravity (double ladder) observation surveys. The total number of benchmarks surveyed was over 500 monuments, and all observations and descriptions were Blue Booked, accepted, and published by the National Geodetic Survey. This was later followed by FG-5 Absolute Gravity observations first at the University of New Orleans which was observed multiple times and then at numerous GPS Continuously Operating Reference Station (CORS) sites throughout the State of Louisiana, some more than once by either the National Geodetic Survey or the National Geospatial-Intelligence Agency. The density of these observations were at approx. 100 mile distances, and are not sufficient to provide significant data points to improve the geoid model. The primary purpose for these observations was to provide an independent check on changes in ellipsoid heights observed at the LSU CORS sites.
Equipment needed is an A-10 Absolute Gravity Meter and a CODIAC Zenith Camera to supplement GPS observations in South Louisiana (and eventually North Louisiana). The National Geodetic Survey recommends an observational density grid of 40 kilometers which will occupy field researchers for many years. That data combined with our existing database will insure that the LSU GPS Real Time Network will provide centimeter-level accuracy for elevations throughout the state; a prime example of the direct impact that research has on practical applications.
NOTICE: NGS Update, August 17, 2012
The National Geodetic Survey has determined that the GEOID12 model was developed using erroneous input control points, primarily in the Gulf Coast region. Click here for more information. NGS immediately corrected these errors and developed the GEOID12A replacement model. GEOID12A is now available for public analysis and comment on the NGS Beta website for GEOID12A and will soon be released for production.
C4G will analyze GEOID12A and publish our findings as soon as possible. Click here to see a map of GEOID12A minus GEOID12, changes in the data used for GEOID12A are also noted on this map.
You can keep up with this and other C4G related topics by following us on Twitter, Facebook, YouTube, C4Gnet RSS feed and the C4G website.
C4Gnet.XYZ GNSS Online Post Processing - powered by Trimble Online Processing App
Works throughout Louisiana and some parts of Mississippi and Texas.
Get a Free C4Gnet.XYZ GNSS Online Post Processing Service Account, user accounts are available for the C4Gnet.XYZ GNSS Online Post Processing Service which requires a yearly free subscription that is available simply by asking one of the C4Gnet.XYZ administrators. To get a free user account, first Register Here then send an email to
Here is a list of other free online GPS post-processing services:
- OPUS: (Currently Under Government Shutdown) NGS GPS Data Processing Online Positioning User Service
- CSRS-PPP: Canadian Spatial Reference System, Natural Resources Canada
- AUSPOS: Geoscience Australia
- GAPS: University of New Brunswick
- APPS: Jet Propulsion Laboratory
- SCOUT: Scripps Orbit and Permanent Array Center (SOPAC). University of California, San Diego
- magicGNSS: GMV
- CenterPoint RTX: Trimble Navigation
UPDATED 1/8/2020 Due to the Multi-Year CORS Solution 2 (MYCS2) transformation of NAD 83(2011) epoch 2010.00
The Center for GeoInformatics (C4G) at Louisiana State University (LSU) is the appointed Louisiana Spatial Reference Center (LSRC) and as such developed a network of Continually Operating Reference Stations (CORS) aligned with the National Spatial Reference System (NSRS) defined by the United States National Geodetic Survey (NGS).
The mission of NGS is to define, maintain, and provide access to the NSRS, which is the official reference system for latitude, longitude, height, scale, gravity, and orientation throughout the United States and its territories. On June 30, 2012, NGS completed a nationwide adjustment of NGS "passive" control (physical marks, such as brass disk benchmarks) positioned using Global Navigation Satellite System (GNSS) technology. The adjustment was constrained to current North American Datum of 1983 (NAD 83) of NGS Continuously Operating Reference Stations (CORS), an "active" control system consisting of permanently mounted GNSS antennas that are the geometric foundation of the NSRS.
Over the 2019 holiday break we adopted the new realization of NAD83 2011 that came in the wake of the NGS Multi-Year CORS Solution 2. NGS updated the coordinates for all of the NOAA CORS that form the NSRS including the 31 C4G owned and operated NOAA CORS here in Louisiana. The NGS Datasheets for each of these 31 NOAA CORS now incorporate IGS14 (realization of ITRF2014 at epoch 2010.0 which replaces IGS08) for each of these stations.
NGS has said that datasheets for NOAA CORS that are part of this realization will show this message:
Due to the release of the International GNSS Service (IGS) 2014 realization of the International Terrestrial Reference Frame of 2014 (ITRF2014), NGS reprocessed all NOAA CORS Network and some IGS stations using data collected between 1/1/1996 and 1/30/2017. The resulting ITRF2014 epoch 2010.00 coordinates, referred to as Multi-Year CORS Solution 2 (MYCS2), were transformed to NAD 83 (2011/PA11/MA11) maintaining the currently published epoch of 2010.00.
Note that when we checked a few of the datasheets for C4G owned and operated CORS, we didn’t find this message, however there is a note under the coordinate header stating “NAD_83 (2011) POSITION (EPOCH 2010.0), Transformed from IGS14 (epoch 2010.0) position in Jun 2019.“
These adjusted coordinates hit the datasheets without much fanfare back in September of 2019 and here at C4G we immediately started analyzing the new realization of NAD83 in one of our test systems, we monitored the new coordinates for a few months and we found that all but two of the 31 new NGS published NOAA CORS in Louisiana were within the NGS tolerance (2 cm horizontal and 4 cm vertical). So 4 of the 6 NOAA CORS (AMER, CALC, MCNE and SBCH) that C4G had adjusted back in December of 2017 have been changed to the new coordinates published on the current NGS datasheets. However the remaining two sites (GRIS and LMCN) that C4G adjusted back in 2017, were found to still be out of tolerance so we opted to retain the C4G adjusted coordinates to maintain a better fit to the NSRS.
| SITE | Latitude | Longitude | Elipsoid Height | |
| 1 | AMER | 29°26’58.49765” N | 91°20’17.21198” W | -14.403 m |
| 2 | CALC | 29°46’05.28102” N | 93°20’34.37130” W | -13.860 m |
| 3 | GRIS | 29°15’55.88294” N | 89°57’26.26208” W | -15.688 m |
| 4 | LMCN | 29°15’17.90439” N | 90°39’40.65134” W | -14.791 m |
| 5 | MCNE | 30°10’50.02279” N | 93°13’03.84340” W | -8.769 m |
| 6 | SBCH | 29°52’05.20564” N | 89°40’23.63833” W | -14.860 m |
| These coordinate adjustments were made in December of 2017 | ||||
The new coordinates can be found on the NGS datasheets for each of the NOAA CORS or you can use the C4G Sensor Map to get the current coordinates for each of the our CORS. Simply select a site from the righthand list or one of the pushpins on the map and you will get a popup bubble, click on the info tab and you will see the coordinates we are currently using for the site. Currently 29 stations are fixed to the NOAA CORS coordinates, which constrain the Real Time Network to the NSRS. Data collected in Real Time should agree with raw data post processed in tools like OPUS or C4G’s free C4Gnet.XYZ Online Post Processing application. (You will need a free account to access this tool, if you don’t have one already contact C4G to get one setup.)
Additional information on MYCS2 is available at https://geodesy.noaa.gov/CORS/coords.shtml
The prior NAD 83 CORS coordinates were determined by re-processing all CORS data collected in the NGS initial Multi-Year CORS Solution (MYCS1) project. The resulting CORS coordinates were published by NGS in September 2011, and constitute a new realization referred to as NAD 83(2011), NAD 83(PA11), and NAD 83(MA11) Epoch 2010.00.
Read more about The National Adjustment of 2011 Project
The Louisiana Spatial Reference Center (LSRC) was established in 2002 at Louisiana State University in response to users’ and public safety needs. The LSRC operates in conjunction with NOAA to develop and provide height modernization procedures in Louisiana as well as to share technology development with others.
Read more about NOAA05-R499-03 - IMPORTANCE OF USING NEW ELEVATIONS IN LOUISIANA
NGS performs a daily coordinate quality check for each National CORS site. If the results of these daily solutions indicate a change in the current position of more than 2 cm horizontally, or 4 cm vertically, the posted coordinates for the site in error will be revised.
Read more about the National CORS system
However, there has been great difficulty in keeping the revisions of published coordinates, particularly heights, up-to-date in areas of rapid subsidence, e.g., South Louisiana and the northern coast of the Gulf of Mexico. Datasheets for these stations will lack a published elevation and contain this warning.
** This station is in an area of known vertical motion. If an
** orthometric height was ever established but is not available
** in the current survey control section, the orthometric height
** is considered suspect. Suspect heights are available in the
** superseded section only if requested.
In the standard publication model CORS’ published coordinates are updated if they have changed by greater than 2 cm horizontally or greater than 4 cm vertically. Therefore, the initial coordinates used for reference in the real-time-network (RTN) may differ from the more precisely resolved coordinates fitted by the active software. In the subsidence area that is South Louisiana, the RTN struggled with using some stations’ published coordinates. Some were seeing a difference of as much as 13 cm! The staff of LSU C4G performed a careful network adjustment for those errant stations to determine the most probably correct values for them in the NSRS. Some advanced users of the C4Gnet may study the vectors in their RTN solutions and notice a difference from the coordinate of a CORS from its published coordinates. This is to be expected if the station is one that was brought more closely into line with the NSRS by the adjustment.
The issue may be restated this way. LSU C4G constrains C4Gnet to the NSRS by holding the stations to the published values of the thirty-one National CORS within the network. When the published coordinates are within the expected error budget allowed by NGS the network performs as expected. If a station is determined to have a position outside the expected error budget, the network software then struggles as it tries to make that errant position fit, and will discontinue its use until the source of the difference is resolved. Six stations of the thirty-one exhibited excessive deviation and updated positions were determined for them as follows.
| SITE | Latitude | Longitude | Elipsoid Height | |
| 1 | AMER | 29°26’58.49765” N | 91°20’17.21198” W | -14.403 m |
| 2 | CALC | 29°46’05.28102” N | 93°20’34.37130” W | -13.860 m |
| 3 | GRIS | 29°15’55.88294” N | 89°57’26.26208” W | -15.688 m |
| 4 | LMCN | 29°15’17.90439” N | 90°39’40.65134” W | -14.791 m |
| 5 | MCNE | 30°10’50.02279” N | 93°13’03.84340” W | -8.769 m |
| 6 | SBCH | 29°52’05.20564” N | 89°40’23.63833” W | -14.860 m |
| These coordinate adjustments were made in December of 2017 | ||||
If a user of the C4Gnet is working near one of these stations and takes the trouble to determine the position of the station on the reference side of a vector, he will see these as the coordinates in lieu of the out-of-spec published coordinates
The whole of the LSU C4G RTN, called C4Gnet, is carefully aligned with the NSRS. NGS specifies that to make that claim 10% of a network’s CORS be National CORS. Thirty-one of the fifty-seven GULFnet CORS in Louisiana are National CORS, 54%. Subscribers to C4Gnet RTN may have very high confidence that it is well aligned to and represents the NSRS, and results from using the C4Gnet compliant with Louisiana Revised Statute 50:173.1.
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Louisiana Revised Statute 50:173.1.
Vertical controls for all surveys shall be determined in the North American Vertical Datum of 1988 (NAVD88). All measurements shall be referenced to local control stations of the National Spatial Reference System, specifically the public domain Louisiana State University Continuously Operating Reference Stations network or other currently National Oceanographic and Atmospheric Administration National Geodetic Survey approved reference stations, such as benchmarks, monuments, or continually operating reference stations.
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- JAC 2018
Urgent: Critical Change to GEOID18 Grid Files
NOAA's National Ocean Service sent this bulletin at 12/09/2019 04:00 PM EST
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These .ggf files were created at 12/09/2019 04:55 PM CST with the corrected NGS GEOID18 binary files using Trimble's free Grid Factory tool. The NGS GEOID18 data download page can be found HERE.
GEOID18 Grid 6 & 7 Combined |
GEOID18 Grid 7 |
GEOID18 Grid 6 |
 GEOID18 Grid 6 & 7 combined .ggf file |
 GEOID18 Grid 7 .ggf file |
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GEOID18 Full ConusNote: This file is not yet corrected!!! |
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