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).

GULFnet & C4Gnet CORS
LSU C4G's GULFnet & C4Gnet CORS (Yellow indicates NGS CORS)

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.

Current 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

 

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.

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.

- JAC 2018