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.
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.
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.
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.
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.
|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.
- JAC 2018
The Surveyors Historical Society is dedicated to exploring, preserving and teaching the accomplishments of surveyors. Their annual Rendezvous has grown to become a premier event in the United States, educational, affordable and fun and anyone may attend! The Louisiana Society of professional Surveyors is hosting the event with Ralph Gipson’s leadership as chairman.
The Surveyors Historical Society erects a monument or researches some important survey corner as part of each year’s Rendezvous. The New Orleans Rendezvous in the fall of 2018, will see the preservation of the first Initial Point in the Public Land Survey System. The Government Land Office surveys started in the Mississippi Territory in 1803 with a 40-mile experimental line run from Natchez Mississippi to the 31st Parallel set by Andrew Ellicott known as the “Line of Demarcation”, or “Ellicott’s Line”.
Deputy Surveyor Charles Defrance ran East 6 miles and 12 perches Commencing at a mound (# 18) set by Ellicott, where on November 27th, 2018 he set the Initial Point of the Washington Meridian. Thomas Freeman returned to this Initial Point in 1819 and ran a Meridian South that became the St. Helena’s Meridian, which controlled GLO surveys in Louisiana.
An iron pipe as shown in Albert White’s book on Initial Points of the United States currently marks the point (30-59-56.0 N, 91-09-36.8 W). The line of demarcation has been researched by Milton Denny, PLS and Larry Crowley PE, PhD. Additional work needed to prove the correct location including looking for original witness tree ties will be performed in the field research in May.
Once the correct location is proven, the plan is to have a stone monument cut about 4 feet in height and 12 inches square with Washington Meridian inscribed on the North side, the South side will say St. Helena Meridian, the East side will say Line of Demarcation and the West will say Ellicott Line.
The LSU Center for GeoInformatics (C4G) supports efforts to preserve survey controls and their histories. Mr. J. Anthony Cavell, resident surveyor for the LSU C4G will join in the proving survey May 11 & 12. Mr. Cavell, is also one of the featured speakers at the Rendezvous in September. Watch this space for updates.
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.
The NGS has issued a notice on their website regarding observed discrepancies between the 2009 and 2012 GEOID models. The notice expansion on August 8, 2012, came in the wake of analysis performed by the LSU Center 4 GeoInformatics back in the week of July 15th where we identified issues in Louisiana, Mississippi, Texas, and other states within the CONUS. C4G notified the NGS of the issues found during the ESRI conference in San Diego, CA during the week of July 25th and we are working with them to resolve the problems as soon as possible.
You may find it useful to do your own testing of GEOID12 in your project area but C4G is recommending that everyone in Louisiana stick with GEOID09 until we learn more about what caused the issues we are seeing in GEOID12 and have confidence that NGS has resolved or justified the differences we are seeing to our satisfaction.
GEOID12 errors found in northern Gulf Coast region
The National Geodetic Survey (NGS) has determined that the new hybrid geoid model, GEOID12, contains localized errors in Louisiana, Mississippi, Texas, and Alabama. Some of these errors are in addition to those identified in the previous notice. Click here for more information.
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.