GULFNET & C4GNET ALIGNED WITH THE NSRS
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.
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.
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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
Surveyors Historical Society Annual Rendezvous
In New Orleans, During Its Tricentennial
September 12-15, 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.
Click here to visit the Surveyors’ Historical Society website. Click here to download the event brochure.
SPACE WEATHER AND GPS SYSTEMS
The use of single and dual frequency satellite radio navigation systems, like the Global Positioning System (GPS), has grown dramatically in the last decade. GPS receivers are now in nearly every cell phone and in many automobiles, trucks, and any equipment that moves and needs precision location measurements. High precision dual frequency GPS systems are used for farming, construction, exploration, surveying, snow removal and many other applications critical to a functional society. Other satellite navigation systems in orbit include the European Galileo system and the Russian GLONASS system.
There are several ways in which space weather impacts GPS function. GPS radio signals travel from the satellite to the receiver on the ground, passing through the Earth’s ionosphere. The charged plasma of the ionosphere bends the path of the GPS radio signal similar to the way a lens bends the path of light. In the absence of space weather, GPS systems compensate for the “average” or “quiet” ionosphere, using a model to calculate its effect on the accuracy of the positioning information. But when the ionosphere is disturbed by a space weather event, the models are no longer accurate and the receivers are unable to calculate an accurate position based on the satellites overhead.
In calm conditions, single frequency GPS systems can provide position information with an accuracy of a meter or less. During a severe space weather storm, these errors can increase to tens of meters or more. Dual frequency GPS systems can provide position information accurate to a few centimeters. In this case the two different GPS signals are used to better characterize the ionosphere and remove its impact on the position calculation. But when the ionosphere becomes highly disturbed, the GPS receiver cannot lock on the satellite signal and position information becomes inaccurate.
Geomagnetic storms create large disturbances in the ionosphere. The currents and energy introduced by a geomagnetic storm enhance the ionosphere and increase the total height-integrated number of ionospheric electrons, or the Total Electron Count (TEC). GPS systems cannot correctly model this dynamic enhancement and errors are introduced into the position calculations. This usually occurs at high latitudes, though major storms can produce large TEC enhancements at mid-latitudes as well.
Near the Earth’s magnetic equator there are current systems and electric fields that create instabilities in the ionosphere. The instabilities are most severe just after sunset. These smaller scale (tens of kilometers) instabilities, or bubbles, cause GPS signals to “scintillate”, much like waves on the surface of a body of water will disrupt and scatter the path of light as it passes through them. Near the equator, dual frequency GPS systems often lose their lock due to “ionospheric scintallation”. Ionospheric scintallations are not associated with any sort of space weather storm, but are simply part of the natural day-night cycle of the equatorial ionosphere.
LSU C4G International Partnership to Advance Geodesy
LSU Center for GeoInformatics(C4G) in partnership with the Space & Earth Geodetic Analysis Laboratory (SEGAL) at the Universidade de Beira Interior (UBI) and Instituto Dom Luis (IDL), Portugal will provides a framework for advancing geodetic analysis and modeling. Combining the skills and experiences from these of capable and motivated partners will lead to new opportunities that were previously inaccessible.
The partners will collaboratively pursue research and support for the precise point positioning of GPS/GNSS data, gravimetric geoid modeling, and the application of emerging geo-informatics technologies and services. C4G and SEGAL will develop new tools for analyzing Global Navigation and Satellite Systems (GNSS) data. Long term collaboration will also advance the development of a new gravimetric geoid model for Louisiana.
“Subsidence is a leading cause, if not the principal driver of wetlands losses in Louisiana,”
- C4G researcher Joshua Kent.
Louisiana’s coastal wetlands are lost at nearly one football field every hour.”
- U.S. Geological Survey
“Gravimetric surveys conducted by the C4G will ultimately allow us to re-connect with the geoid to calculate better elevations" and “knowing how high we are above sea level.”
- C4G geodesist Cliff Mugnier.
“We have applied our in-house developed software in the computation of regional and national geoid projects in North Mozambique, Madeira and more recently in Bhutan.”
- Machiel Bos, SEGAL researcher
“This collaboration is a unique opportunity to partner with a very enthusiastic team that combines pure and applied research in several areas of geosciences with enormous societal implications,”
- Rui Fernandes, of SEGAL.
“I anticipate significant innovation from our relationship.”
- George Z. Voyiadjis, Boyd Professor and Director of C4G
Download the Official Press Release for detailed information HERE
C4G Acquires Geodetic Instruments to Better Understand Subsidence Rates in Louisiana
The Center for GeoInformatics (C4G) in the Department of Civil and Environmental Engineering (CEE) recently received new geodetic instruments to model the Earth’s gravity field. A Scintrex CG-5 Relative Gravity Meter, Leica T60 Total Station, and Trimble R10 GPS Rover Kit were acquired as part of an enhancement grant sponsored by the Louisiana Board of Regents. Drs. George Voyiadjis (PI) and Joshua Kent (Co-PI) led the one-year project, which ended in June, 2017. The instruments are acquired to address the needs of three objectives: First, to develop a novel, high-resolution gravity model of sea level (i.e., geoid); second, to augment knowledge of existing subsidence rates and the driving mechanisms; and finally, promote advanced geodetic research at the University.
Here, as in many river deltas around the world, land surfaces are sinking due to subsidence. On average, southern Louisiana experiences ~10 millimeters per year of subsidence. Understanding the mechanisms that drive subsidence is essential for mitigating risk and promoting sustainability. The CG-5 relative gravity meter supports these goals by measuring the relative differences in the Earth’s gravity across southern Louisiana. 
Surveys using the total station and R10 rover kit are currently underway to geodetically correlate the CG-5 data with absolute gravity readings collected in the early 2000s by the National Geospatial-Intelligence Agency and the National Geodetic Survey. The updated gravimetric surveys conducted by C4G researchers and staff will deliver much needed insight into the variety of geophysical processes driving the spatially and temporally heterogeneous rates of subsidence measured across the state.
In addition to the subsidence research, this enhancement grant will directly and indirectly benefit Louisiana’s geodetic stakeholder and consumer communities. For nearly a decade, the C4G has provided tools, services, and other geodetic resources dedicated to precise positioning throughout the state and across the region. 
Central to these resources is the C4GNet real-time reference network. The network includes more than 50 continuously operating GPS reference stations (CORS) installed across Louisiana.
Over the next five years, the C4G plans to geodetically correlate the gravity measurements with antenna heights at each station. Extended surveys will include CORS in neighboring states. When completed, the data will contribute to the creation of a novel, high-resolution geoid model that will allow the geodetic community to accurately and precisely measure elevations above sea level.
The instruments acquired by this grant represent an investment into the geodetic research capacity at the C4G and CEE. In addition to the above goals and objectives, these resources have already been selected for use by investigators in two external funding proposals, both of which will rely on the precision of these instruments to deliver meaningful geodetic solutions. These instruments not only promote research activities, they have galvanized national and international collaborations with partners across the US Gulf Coast and western Europe. More information about these instruments and geodetic models is available at the C4G website www.c4g.lsu.edu or gravity.c4g.lsu.edu.
