Only 28 of 7500 heights in area considered valid.
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while this notice on the NGS homepage indicates local groups ??work to? update heights it does not provide a link to any of these groups.?ÿ
Does anyone know anyone/any group currently involved?
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Whoa.....
Heck, subsidence was already a known, big problem when I started working in Houston in the mid-2000s. I fielded a lot of flood certs around that time period, using the shiny new benchmarks and floodplain maps established in response to Tropical Storm Allison. The only place I have found with worse sidewalks than Houston was New Orleans...
Yikes, from Indiana, no, I know of no such groups. ?ÿ
How much movement is happening?
What vertical delta threshold of movement kicks 7472 monuments into the suppressed category?
How do local dumb dirt surveyors (like me) continue to function and operate under these conditions with normal site topo etc. day to day activities?
And here I always thought that subsidence stopped at the Louisiana - Texas border!
As Rover83 notes, subsidence is a long-standing problem in the areas mentioned in the NGS notice. I recollect discussions about this decision to ??repress? these monuments. The decision is long overdue. Providing datasheets with heights known to be suspect is wrongheaded.
There are too many who see a value on a data sheet and assume it must be valid. Too few even make the effort to make check observations to other monuments.
The solution to the problem is more observations either as projects to be submitted to NGS or OPUS solutions to be shared with NGS.?ÿ
As LeeD notes, the phenomena causing land surface changes are not limited to political boundaries.
My note was to inquire whether there is an organized effort by surveyors in east Texas to work toward solving this problem.?ÿ
BTW, remember that the geoid-ellipsoid separation is impacted by subsidence and other effects.?ÿ
For those interested in some magnitudes as well as some sources of additional information see:
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https://hgsubsidence.org/science-research/what-is-subsidence/
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https://www.ngs.noaa.gov/heightmod/NOAANOSNGSTR50.pdf
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Reading about the Houston area always makes me homesick. This has been an issue for a long time. I remember working for a firm in the mid 1980s that did weekly subsidence checks in the Medical Center area because someone obviously had concerns about it. I was actually the chainman on the crew that did these checks every Monday morning for probably six months and it was really a pretty interesting project.
For the City of Houston, we rarely used any benchmarks other than those set by the City. Thinking back on it, I guess that might be unusual since I haven't encountered anything similar anywhere else. The City was always updating their benchmarks. They rarely went two years without getting an updated elevation. I don't know if this practice is still in place, or if the City still has their system of benchmarks, but that's the way it was back then.
I managed to find a newspaper article from 1982 (in the New York Times) that talks about Houston's subsidence. Wow, Dan Jones is a blast from the past. They mention in the article that Houston could sink 14 feet by the year 2020! I doubt that happened. I remember most projects having elevations in the low to mid 70s, so losing 14 feet would put it down to around 60. I don't know how much further inland the problem would continue, but subsidence was an issue all the way down to and including Galveston.
When they say Seventy Five Hundred monuments, they are including all the second and third order vertical marks ?
The few they consider acceptable seem to be mostly CORS stations, and one PAC in Conroe.
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@rj-schneider acceptable CORS seems to be good news for the local dumb dirt surveyors like me...?
You might also enjoy this 2005 article from Texas Monthly: https://www.texasmonthly.com/articles/that-sinking-feeling-2/
Lots of interesting papers on faulting as a possible cause as groundwater withdrawal has greatly diminished.
In the now many years since my departure from Texas (and retirement), I no longer keep track of these issues.
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It was more just thinking out loud. There probably could be seventy five hundred first-order monuments between here and Beaumont. I have no idea.
Most of these elevations are on DOT monuments or related to them and most of them are simply not certified as being approved CORS locations.
