If some areas are dropping by 2" per year, is it the elevation relative to the also sinking benchmark that is used? For underground utilities, the real NAVD88 elevations might be less important than the relative elevation to the existing system, but then what datum do you report?
For flood insurance certifications, I am guessing that OPUS is used?
Just curious.
dmyhill, post: 373653, member: 1137 wrote: http://www.foxnews.com/science/2016/05/23/new-orleans-is-sinking-faster-than-previously-thought.html?intcmp=hphz07
If some areas are dropping by 2" per year, is it the elevation relative to the also sinking benchmark that is used? For underground utilities, the real NAVD88 elevations might be less important than the relative elevation to the existing system, but then what datum do you report?
For flood insurance certifications, I am guessing that OPUS is used?
Just curious.
I'm not from the NO area, but I have heard a lot of discussion about the subsidence issue. I think the short answer is "gps is a great thing".
[USER=505]@Cliff Mugnier[/USER]
I remember, a long time ago, maybe even on the "old board"; Professor Mugnier giving us a detailed explanation and example of how the benchmarks were setup, maintained and monitored. Something about long stainless steel rods driven to refusal; a very good discussion ensued.
Might be worth a search, if you are interested.
Dougie
Robert Hill needs to answer this question as he is in that area.
[USER=378]@Robert Hill[/USER]
Mr. Hill,
Tell about the Magic and Blessed Marks. Is it true that there is an ordnance call for use of GulfNet?
DDSM:beer::beer::beer:
An alternate hypothesis to that presented in the scientific paper the FoxNews site links is here: http://www.nogs.org/Content/pdf/2006_october_RDokka.pdf
I have just downloaded the FoxNews linked paper and not read beyond the abstract at this point. Their attribution of dewatering and groundwater withdrawal are discussed in the Dokka paper I linked above.
The late Dr Roy Dokka of LSU and the LSRC did a lot of thinking (and writing) on the issues and challenged the dewatering and groundwater withdrawal hypothesis as complete explanations. Dr Dokka supervised the doctoral work of (now) Dr. Kurt Shinkle whose dissertation is available on the NGS web site: http://www.ngs.noaa.gov/heightmod/NOAANOSNGSTR50.pdf
NOAA TECHNICAL REPORT NOS/NGS 50
RATES OF VERTICAL DISPLACEMENT AT BENCHMARKS IN THE LOWER MISSISSIPPI VALLEY AND THE NORTHERN GULF COAST
Kurt D. Shinkle
National Geodetic Survey
Dr. Roy K. Dokka Louisiana State University
July, 2004
I am not involved with either LSRC nor NGS but recollect their work on a velocity model to account for temporal changes in their BM network. GPS os pf course the only way to do the monitoring. Mr Mugnier will hopefully pipe in the update, clarify and correct my remarks.
Cheers,
DMM
dmyhill, post: 373653, member: 1137 wrote: http://www.foxnews.com/science/2016/05/23/new-orleans-is-sinking-faster-than-previously-thought.html?intcmp=hphz07
If some areas are dropping by 2" per year, is it the elevation relative to the also sinking benchmark that is used? For underground utilities, the real NAVD88 elevations might be less important than the relative elevation to the existing system, but then what datum do you report?
For flood insurance certifications, I am guessing that OPUS is used?
Just curious.
metadata, all of it ...
===
for Flood, consider the how and why. You are expected to show the relationship with the flood levels estimated by FEMA.
Do whatever it takes to get on that datum, as it existed at the time of the study.
publish the metadata, all of it ...
===
For fixed works get on the existing published monuments, a few of them, flip a coin, publish the metadata ... all of it ...
the tools you use to do that with, and the extent of the research, and the degree of redundancy, is all up to the Surveyor's Professional Judgment.
...be cautious, and get Paid for your work...
Second Thought: what is the client requesting? to that. and whatever else it takes to comply with the above.
Survey 101
Looking at the LSRC web page I no longer see their bench mark update page.
BTW, I see a note in the presentation linked below that the LSU CORS network is the legal source for elevations in Louisiana.
http://www.ngs.noaa.gov/heightmod/2014Partner/18_201404LSRCHeightModernizationEffortsInLouisiana.pdf
copied from the presentation:
Center for GeoInformatics (C4G) and the LA Spatial Reference Center (LSRC) Û¢ 2006 ÛÒ LSU CORS network becomes a Legal Source for elevations in LA (R.S. 50:173.1) Û¢ 2007 ÛÒ Established Real-Time Network (RTN) Û¢ 2009 ÛÒ GNSS Integration within the RTN
High-accuracy GPS-derived ellipsoid heights combined with a high-resolution geoid model yields accurate heights!
From memory... In late 2005, not long after Hurricane Katrina, the NGS released the results of an extensive vertical survey that had been undertaken in south Louisiana; at that time all but about 100 vertical benchmarks in South Louisiana (mainly south of I-10) were invalidated, and I believe that it was stated that those remaining would no longer be maintained. Coincidentally with this, the revised Geoid03 Region 7 file was released; it is dramatically different than the original Geoid03. Some of the aforementioned benchmarks that were updated were revised by a foot or more; some of the most drastic changes were actually in Acadiana, I distinctly remember that the Crowley area in particular had marks that were moved by as much as 1.5'.
It was because Dr. Dokka had installed the beginnings of what is now Gulfnet that the true nature and scope of the subsidence problem began to be understood. It's my understanding that the issue first came to light when it was observed that existing CORS stations at sites like English Turn, Galveston Bay, and Winnsboro were moving at different rates. The first (I want to say 12) Gulfnet stations were purchased and installed in the mid to late '90's; at the time of Katrina Gulfnet was not yet a VRS but was a fairly dense network, and the data it provided to the surveyors who went in after was invaluable. As an aside, many people thought - mistakenly - that the updates to and invalidation of the benchmarks, as well as the release of the revised geoid, were a result of Katrina. This was not the case; it was, however, epically bad timing.
By statute the Gulfnet RTN is the standard for vertical surveys in Louisiana. It's still a complicated situation; the Gulfnet stations are aligned with the CORS stations on the 2010.00 epoch of NAD83 (2011). If you download enough data from the NGS CORS stations in the network (and believe me, I have) you will see that there is still a lot of slop in the vertical - as much as 0.2' or more, depending on what stations you include in a network. I have access to the data from all of the Gulfnet stations, not just the ones in the NGS, and this is also true of those, although some are better than others. Compounding the problem, station HOUM, which was 1) only about a mile or so from my office, and 2) a good "best fit" of the vertical on the surrounding stations, was taken down a couple months back because the building it was on is being renovated and repurposed. They say it may be back up in a year or so, but in the interim that doesn't help the situation.
On a personal note, I knew Roy Dokka and had a good working relationship with him. He was one hell of a guy, and we were all shocked and devastated at the news of his untimely passing.
NAVD88 is still the legal datum in the USA and in Louisiana. Of course local "passive" benchmarks are moving, some faster than others dependent on a whole lot of reasons. We see that in the movement of our antennas everywhere in the State, as well as the entire northern rim of the Gulf of Mexico since our CORS sites include Corpus Christi, Texas to the East coast of Florida (Jacksonville).
Just because there is surface movement does not imply that there is a different datum. NAVD88 can be considered as being fixed in 3D space, with the current legal geoid being used as the tool to get to that vertical datum. Slop? Sure, there's slop, but NGS is currently working on minimizing that amount of uncertainty; it's called GRAV-D. NGS is "hoping" to publish a new 3D datum in 2022, which will be the first continental 3D datum, ever. The hope is that the envelope of uncertainty for the vertical will be about a centimeter ... we hope.
In regard to velocities of Benchmarks, Professor Roy Dokka and Rick Shinkle found that different Benchmarks have moved at different rates at different times, remained stable for a while, and then started moving down again. Which ones and why? Some we know did, others we just don't know. The hurrier we go, the behinder we get ... ain't Science wonderful? The unknowns exist because the tool used to find those subsidence rates was the NGS database of First-Order leveling in the 20th century. Some Benchmarks were leveled to many times, many were not. The work involved an enormous amount of detailed examinations that only a Ph.D. graduate student (Shinkle) would do under the iron fist of a major professor (Dokka).
I gave my first all-day seminar on subsidence in Louisiana back in 1979 and proposed measuring relative gravity at local Benchmarks. By the middle of the next decade, my graduate students did just that (Lacoste & Romberg G-meters - $50K each - good to a milligal) at over 450 Benchmarks in metro New Orleans and select places including the Tide Gauge at Grand Isle, Louisiana. Since then, we have had Absolute Gravity observed at numerous locations throughout the State of Louisiana, including multiple re-observations at many of our LSU Gulfnet (C4GNet) CORS sites. We now have a $100k Scintek relative gravity meter (good to a microgal) involved in gravity surveys with another Scintek on order.
Is there a Louisiana Law about the LSU CORS being the official reference source? Yup, it's LA REV Stat å¤ 50:173.1.
Dr. Josh Kent ([email protected]) of our C4G Research Center is currently involved with our GPS observation historical database and processing observations with JPL's "GIPSY" software in conjunction with a JPL/NASA project for InSAR observations of metro New Orleans subsidence. ( http://www.jpl.nasa.gov/news/news.php?feature=6513 ).
Yes, Louisiana Benchmarks are moving. So is LSU research into the geodesy of subsidence.
Cliff Mugnier, post: 373878, member: 505 wrote: Slop? Sure, there's slop, but NGS is currently working on minimizing that amount of uncertainty; it's called GRAV-D. NGS is "hoping" to publish a new 3D datum in 2022, which will be the first continental 3D datum, ever. The hope is that the envelope of uncertainty for the vertical will be about a centimeter ... we hope.
Cliff -
Good stuff, there's no doubt LSU has done and continues to do great work on this subject.
A clarification though - the 0.2' "slop" I refer to is in the ellipsoid heights of the published coordinates for the Louisiana CORS stations. As you know, the decision was made very early on to align the VRS CORS coordinates with the published NGS values - to do anything else would create chaos. But there is little doubt in my mind that the VRS software COULD produce ellipsoid heights for the stations that fit together a whole lot better than the published values. For instance, station LMCN is all jacked up - and I have it on fairly good authority that NGS KNOWS it's all jacked up. I've run countless network adjustments using several days of long sessions of data from HOUM, AWES, LWES, DSTR, LMCN, FSHS, AMER, GRIS, and ENG5 & 6 (not all at once, typically, but in some combination depending on the project site). The results are consistent and predictable. This is why I was dismayed when HOUM went down... it seemed to split the differences rather nicely. It also makes it nearly impossible to faithfully re-create most of the control work we've done in South LA that relied heavily on it.
This is what will drive you crazy with this stuff... what's "right"? All of them? None of them?? Obviously, nothing is absolute and I understand that, but most folks don't...
Original questions:
Q1: If some areas are dropping by 2" per year, is it the elevation relative to the also sinking benchmark that is used?
A1: No. If the surveyor learns the benchmark has moved, he is obliged to NOT use the previous wrong reference values for that mark. He must find a good BM or re-establish good values before reporting.
Q2: For underground utilities, the real NAVD88 elevations might be less important than the relative elevation to the existing system, but then what datum do you report?
A2: Report relative to the datum required by one's client. If none is explicitly specified, then the standards would call for using the current datum in use by the National Spatial Reference System (NSRS). That is currently the North American Vertical Datum of 1988 (NAVD88). Necessary precision would also be prescribed by a qualified client or must be determined by the surveyor in the best interest of the client and/or the public.
Q3: For flood insurance certifications, I am guessing that OPUS is used?
A3: OPUS is one means of establishing a reference height for the NFIP elevation certificate. A good real-time network solution is usually also adequate. It is up to the surveyor to verify!
The expectation of FEMA of the precision/accuracy of the reference position is 1/10 ft. The GNSS (GPS is one part of the GNSS) results usually achieve that rather easily for ellipsoid height. Unfortunately, GNSS has no reference for Up relative to determining elevation.The satellites orbit around and around the earth, so Up could be in any direction. The GNSS give a very good location relative to the center of mass of Earth (X, Y, Z). However, how water flows or how a plumb bob hangs depends upon the direction of gravitational attraction at any point. It is rarely toward the center of the earth and varies as one moves around from place to place with different densities.
"Old fashioned" leveling was set and repeatedly re-set relative to the direction of gravity's "down" as the level survey progressed. With GNSS we make BIG jumps from base to rover. The GNSS ellipsoid height must be combined with a model of the shape of a surface within Earth's gravity field, its geoid. Only very recently have geoid models become good enough to be useful. The models themselves contain residual doubt.
All of this combines to limit the level of precision we may expect when making determination of elevations (orthometric heights) using GNSS.
JAC