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.
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
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.
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.
Big picture ideas to consider from the presentation
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: