OK, what thos of you who use RTN do in California when the earth shifts during middle of your boundary survey??ÿ How about 10 years later?
What happens to accuracy of GNSS monumentation on either sides of these faults??ÿ?ÿ
just wondering what i would do.
For those in California, My wife and her Sister have been out there for the last 10 days visiting her Sisters Son in the Marines. They are on the way back and are currently in Arizona so the shaking should stop. I have no way on knowing for sure but I would guess that my wife got out there and put her foot down. She tends to do that from time to time. ?????ÿ
I'm glad it's not me surveying out there. Carry on.
James
The California guys are lucky. Velocities make it obvious that coordinates relative to distant control should not control local boundaries.
via the Orange County CLSA Facebook page:
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http://sopac-csrc.ucsd.edu/index.php/csrc/
Stay tuned for an update about the impact the recent earthquakes did (and did not) have on the published positions of static and real-time stations across the state and be aware the impact local private networks may have also suffered. Check in with the CSRC website for updates.
via the Orange County CLSA Facebook page:
---http://sopac-csrc.ucsd.edu/index.php/csrc/
Stay tuned for an update about the impact the recent earthquakes did (and did not) have on the published positions of static and real-time stations across the state and be aware the impact local private networks may have also suffered. Check in with the CSRC website for updates.
not sure how to publish links here:
https://mtouch.facebook.com/pg/OC.CLSA/posts/?ref=page_internal&mt_nav=0
?ÿ
The closest station running in the California Real Time Network (CRTN), BFSH, is about 60 miles from the epicenters of the earthquakes near Ridgecrest last week. Initial analysis shows this station displaced approximately 2 cm west and almost nothing north/south (25 mm south). The closest Continuous GPS station, P595, which is not streaming real-time, appears to have displaced 0.49m east by 0.25m west (1.83 feet total). Once things settle down, the impacted stations will be readjusted on the 2017.50 epoch (and ITRF) and the results will be posted on the CSRC website:
Effing Impressive!
Looking at some other analysis this morning indicates that P595 may have shifted (total displacement after the two major events) around 2.3' southeasterly from where it was before the events. It will be a while before things settle down.
If I read this correctly,?ÿ the daily routine of those who depend on CRTN in certain areas has been turned upside down.?ÿ?ÿ
should the CRTN turn off corrections from those stations with 'large' positional shifts??ÿ And, over a period of time, how much are these stations shifting?
Tangently on topic...
<Sarcasm ON> Good thing we adopted BLMs Direct Point Positioning Survey (DPPS) methods to "establish" cadastral corners in the area. DPPS sure saved everyone lots of time and money by eliminating the need for monumentation. The real bonus is:?ÿ when you don't have monuments, you don't have to worry about them moving!?ÿ <Sarcasm OFF>
I dredge this up because DPPS is still smoldering at BLM. These earthquakes remind us just how much boundaries rely on markers in/on the ground to protect bona fide rights. Condsider the types and magnitude of boundary problems there would be over time if boundaries were based solely on coordinates. The 2.3' at P595 that SPMPLS has identified would simply be smeared throughout your cadastre when it was adjusted - yes there would be a scheme that weighted control but the adjustment of the cadastre coordinates would be a crapshoot - the fact is, coordinates could be derived in a variety of ways and there would be a ton of boundary litigation with no case law to reference - demand for real estate attorneys would skyrocket. This is more evidence that DPPS continues to be a bad idea.
?ÿ
I've surveyed for decades in earthquake country and boundaries do not change except if the rift actually passes through a given parcel.?ÿ Then the monuments control and the parcel is distorted by the displacement.?ÿ The vertical is of much more importance concerning engineering activities, I've seen 2' displacements that upended gravity flow infrastructure (and broke pipes) which required millions to repair.
Thinking GPS derived geodetic coordinates matter in earthquake episodes is short sighted; what's happened on the ground locally is what matters.
Scott described how the infrastructure issues are handled, but for the end user it's a matter of knowing your RTN base stations and checking for movement if you suspect one or more have been affected.?ÿ There are stations in my area that I know aren't reliable for vertical, and I rely on a handful that I've found to be stable over time.?ÿ ("Stable" in this case refers to vertical; they're all moving horizontally in this part of the world.)?ÿ It's a dynamic system, but as long as you know what's moving in which direction and how fast it's not a big deal.?ÿ Most routine positioning tasks only require relative values anyway.
Jim is spot on. Knowing the positional integrity of the control in the areas where you work is the gospel I preach every chance I get. I have presented to surveyors from the USGS and USF&WS who work in other areas of the country but can occasionally be deployed to California. When I show them examples of time series plots from Continuous GPS/GNSS stations here, they are shocked by how fast things are moving horizontally. Then I show them plots of stations that are moving rapidly downward, while seasonally pumping up and down along the way. Because many of their surveys are for water features where vertical is the most critical component, they are quite alarmed. I know of at least one NGS CORS in California that has been as much as 29 cm lower than the 2010.00 height reported on the NGS datasheet. It has rebounded a bit and now is only about 25 cm lower.
Large earthquakes result in sudden displacements and surveyors working in those areas are, or should be, alerted by the event to be cautious and perhaps utilize more distance control outside of the disruption zone to derive accurate absolute positions. But the reality is, if you work in California, things are always moving. The rates at which they are moving vary drastically depending on location and can be quite disparate over relatively small areas/distances. My presentation at the NGS 2019 Geospatial Summit showed specific examples.
As several have stated, this issue has little to no impact on boundary determination, except if one is trying to reset a corner using a previously established coordinate (poor practice anywhere, in most cases) using a control system that is dynamic. Epoch dates are of great significance here and using HTDP or SECTOR to model positions over time is not just some obtuse concept.
The only item that I would add to Jim's comment is that you should have a decent idea of where the plates are and make an attempt to measure your site from the same tectonic plate.?ÿ Since I have been moving bases that the crews seemingly randomly select, including work done many years ago, we have minimized the effects of the general movement.?ÿ We still do sanity locations of old control just to verify the assumptions as well.?ÿ I have a static session for every base in our constellation for each year from the early 2000's which allows me to have a very close approximation of the station geometry in any specific year.?ÿ The more "avultive" movements are certainly minimized this way, but, you really should know dates and locations of the major ones.?ÿ Easy for me to say, as we do not have major ones all the time as I remember growing up in Alaska, but, that is a different story.?ÿ We have stations that I will not use as well because of their vertical movement, but, really it is just one.?ÿ Our bases were attempted to be set on each of the tectonic plates from what I am told and I am trying to get a map of them from the university that played a part in the location decisions to no avail so far.?ÿ I really want that map to see how it compares with what I know of the area.?ÿ As far as the crew comment, I know more goes into it than their breakfast selection, but, sometimes it does not appear that way as I have seen bases from two counties away.
Map of displacements, each color cycle represents 12 cm of displacement (in the radar line of sight).
https://www.jpl.nasa.gov/spaceimages/details.php?id=PIA23150
