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What Happens When The Earth Moves?


bicknell

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This is an offshoot of the NAD 83 - WGS 84 topic.

 

As we all know, the earth moves. Earthquakes happen, continental plates drift, and all that other good stuff. I'm going to phrase this to make it what I hope is a fairly simple case.

 

When a slip-fault (eg, sliding) earthquake occurs, and two plates move several feet in each direction, what does the NGS do with the coordinates of triangulation stations? Obviously measurements across the fault line have changed, what method of correcting / updating the data is followed?

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Bicknell,

 

Though NGS is a National Agency, they often work in unison with the many State agencies which, quite obviously find themselves at ground zero when these things happen, and since there are similar yet different approaches these links should give you the gist of it.

 

http://www.profsurv.com/ps_scripts/article.idc?id=1149 A Nice Overview.

 

http://csrc.ucsd.edu/projects/hpgn2000.html Specific to California

 

http://ees2.geo.rpi.edu/rob/www/gps/g0026.html A Washington State Study

 

I am fairly sure that from reading those three links you will se how they continue to watch and study, as well as how they react to the changes they observe this and correct for the movements over time. Regional adjustments can happen and do.

 

Hope that helps.

 

Rob

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I guess I was thinking about it from a more mechanical point of view. If CA moves 5 feet to the north, and the NGS has a data sheet for a monument set in 1950 and not visited since, what do they do with the data sheet? Do they leave it the same? Do they updated it via the computer model of what happened? If it is updated, is there some way you can tell it was adjusted for such an event?

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Funny, I was thinking about the same thing last night. Earthquakes, continental drift, etc., certainly have to be considered when you are working on a sub-centimeter scale.

 

Perhaps this question should be moved to the NGS forum?

 

-ArtMan-

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Bicknell,

 

I had hoped the answer was in the links I posted, and in many ways it is.

 

Let me help sort it out.

 

NGS is the overseer of Geodesy and Survey accuracy standards in the country.

 

Each state has it's own program in most cases to help sort it out on the state level. They generally follow NGS guidelines but have their own paid staff, as well as their own Standards and methodologies. Often they come under the "umbrella" of NGS standards. (Many counties and Cities carry this further to their own areas of influence.

 

These days we are using CORS and HARN networks, They are real time stations and the record geodetic movements in 3 axis's 24/7 and report movements and their directions as "velocities". Though these CORS Stations are in the NGS database and NGS can look at the data real time, they are often not sponsored by NGS, in fact Many are owned by States, Counties, Cities, Other Government Agencies, Universities, Private Survey Companies, etc. You see, It is networked in a lot of different ways and used locally by locals to establish and maintain the accuracy of local control networks as well. It really is multi function in it's roles.

 

In spite of and recent or not so recent adjustment to a Station Mark, not visited, we can apply velocities to the data to help us understand what may be happening in any given area.

 

As for the datasheet. Well some have been adjusted since a recent earth moving event in their area, some not. Like we have discussed in other threads, look at the datasheet for the NAD83 (XXXX) XXXX will represent the year of the latest adjustment recorded on the datasheet. If you need newer positioning, then you can include the data with local realtime observations and apply the changes, then run a least squares for it in the network. One way you could anyway. If we use it and don't do that, the published data has the date, so they know when the value we are using was last updated and they know to use that.

 

Some Stations will not be adjusted any more at the NGS Level because it has been determined that the quality is simply not high enough to bother any longer given the resources available. It is still important as a geodetic position and good to know it is still there. We just see that it is an older adjustment er, Realization if you will. We know what reference frames were used to determine the position. There are uses for lower order survey on different jobs and so it is still of value.

 

Funding and manpower have us using the 80/20 rule in regards to survey control a lot. In other words we are investing the most energy into our highest quality. BUT! the datasheet for the station is still the published data and it is usable. This maintains the value of even the lowliest of station marks. The rule is to go with the data on the datasheet. It will tell you how old it is and if need be the changes that have occurred in a locality can be applied to the data to bring it in line with current situation if that is needed. The thing is, it isn't always necessary in fact, usually not.

 

Some survey markers have a different role than others. Just like people. Third order Landmarks are no longer included in adjustments. A and B order stations are High priority. CORS and HARN adjust themselves 24/7. The realtime data can be applied trough the network if we need , if not, the date tells us when the survey was last adjusted and we either hold it there or we can apply the observed changes to it and go with that. Ultimately, it could be resurveyed too. It depends if it is chosen or needed to be.

 

The link from what California does to handle the Geodetic movement is a pretty good example of what is done in that State, and that area of the country is where the lion's share of the action is. That is the agency watching the situation. As you read, they have chosen the stations they choose to use in order to monitor the earth movements they have. It is enough to give them a good picture yet at the same time remain manageable. I think if that website outlines how they are handling the movement of geodetic control, and the are in partnership with NGS, (both are true btw) then I think you can take what they are saying as to how they deal as the truth. That is the answer. If they submit the latest observations to NGS, then perhaps the stations in question will be included in the NSRS adjustment which will be commencing this June. Well, that is if they meet the criteria for the adjustment. If you like, the California site has people there who you can write to if you need further details. It is their baby. The same would go with any respective State or other localities' site.

 

Hope that helps.

 

Rob

Edited by evenfall
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This is an offshoot of the NAD 83 - WGS 84 topic.

 

As we all know, the earth moves.  Earthquakes happen, continental plates drift, and all that other good stuff.  I'm going to phrase this to make it what I hope is a fairly simple case.

 

When a slip-fault (eg, sliding) earthquake occurs, and two plates move several feet in each direction, what does the NGS do with the coordinates of triangulation stations?  Obviously measurements across the fault line have changed, what method of correcting / updating the data is followed?

 

Here's an answer to your question from a California surveyor. earthquake movement

 

This is how the surveyors handle the problem of earth movement in the field.

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Unless it is a truely massive earthquake, when the earth moves along a fault line it does not affect the entire state. The reason that earthquakes occur is because as the two sides of a fault move past one another (at geological speeds), the friction causes the earth near the fault to compress and distort. When the stress builds up enough to overcome friction the compressed earth moves and catches up with the rest. So the actual movement is restricted to the area around the fault.

 

If a marker is near the fault, then it can move. But otherwise earthquakes don't cause widespread changes in lat/long.

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