http://www.amerisurv.com/PDF/TheAmericanSurveyor_SnayPearson-CopingWithTectonicMotion_Vol7No9.pdf
The article was written by Drs. Richard Snay and Chris Pearson.
It gives a very good overview of the plate boundary dynamics along the West Coast, and it also touches on some of the vertical issues in North and South Central CONUS.
There is also a brief discussion about where the NGS is taking HTDP (and later TDP) in the near future.
Well worth the read for anyone.
Loyal
Thanks for posting that link, Loyal. What many probably do not realize is that there is discussion about going with a dynamic datum like ITRF in the future for the NSRS, then we would all have to deal with the time dependent component of the reference system. Not saying it is definitely coming, but it is under consideration.
John,
I think going to the ITRF for the U.S. is a bad idea. I think we should have our national datum tied to an epoch and stick to that. NGS is the proper venue to keep track of where the ITRFxx is from year-to-year for those practioners that need to use the international system in a transformation. Local surveyors and engineers practicing within the U.S. should not have to bother with that "extra" step.
North Island and South Island of New Zealand are moving at a pretty fast clip, and I think they have the right idea. Each island has its own local system that is maintained by their national government, and local surveys don't need to bother with the ITRFxx.
Block movements within a country are another thing. I'd say they should each have their own local block system. The only way around that would be for monthly updates to the ITRFxx (and of course, backwards to any prior epoch). Should make for happy civil servant geodesists ... job security in spades.
John, You're absolutely correct about that possibility.
I have “argued” BOTH sides of the issue (fully-dynamic ITRF v. pseudo-Stable NAD), and I see advantages and disadvantages in both directions.
It ALL comes down to the ACCURACY of the velocity field MODEL at your point of interest!
IF the model is accurate, then an ITRF2008 epoch 2010.0000 coordinate on that point, can be duplicated at ANY TIME in the past or Future. It (the Epoch 2010.0000 coordinate value) in effect remains STABLE! (except of course when it doesn't due to un-modeled episodic events).
IF the model is NOT accurate, then the NAD83(xxxxx) Epoch 2002.0000 coordinate on that point can NOT be duplicated ANY [other] TIME! So it is [in effect] Unstable!
I think that all of this just flies over the heads of most folks.
The whole point (IMO) of using a “stable North American Datum,” is so that folks situate on the “stable” portion of CONUS, can IGNORE velocities all together. That does NOT mean that the coordinates (or “points”) themselves are truly “stable.”
East of the Rockies most points are pretty stable (at least in the Horizontal), but West of the Rockies we start getting into nontrivial velocities (relative to NAD83) fairly fast. Granted, you have to get into Western Nevada (in the Great Basin) to start seeing big numbers, but it doesn't take all that much when you are sliding things around 8-10 years one way or the other.
Loyal
Edit: Am I [sometimes] agree with Cliff's stance on the issue too (when I'm not arguing for the "other side")!
I have always felt that a line should be drawn between surveying and geodesy. GPS makes it easy for surveyors to think they know (or need to know) something about geodesy. I know something about a lot of things, but I am not foolish enough to think I am a specialist in more than one field. Geodesy and geodetic surveying should be left to those who are knowledgeable and have more than a computer that spits out numbers. I would venture to say that HTDP's are of little value to the vast majority of boundary and construction surveyors, and that they add another level of unneeded complexity to many. Just because something can be done doesn't mean it should be...
I agree, Cliff. I wasn't advocating, merely pointing out that it IS being considered. I believe it would be a disaster as many would not be able to properly deal with it.
I work in both stable areas and not-so-stable areas, so I deal with it both ways. HTDP is a tremendous tool, thanks to Mr Snay and the NGS.
How others may cope with Tectonic Motion and other issues
Good ideas have been presented. However, there is one issue which has not been addressed. It is the one I've been taught to repeat as "assume." It makes an ASS of U and ME. Too often I hear surveyors assume and use the statement "Everybody does it that way." Yet, I know surveyors, including myself, who do not "do it that way." More importantly, it probably should not be done "that way."
As a consequence, in order to get around "assume," I think it is imperative to state on your document, be it a report, plat, map or description, the time and date of your work and also to make a statements about the assumptions you've made, the methods employed, the instruments used and the computer programs and algorithms used. Imagine yourself as a surveyor 100 years from now reading the document you have just prepared. Will you have enough information to accurately, for the intended purposes, position, locate or restore monuments and accessories? Look back a 100 years and ask: what could surveyors have done to make my project eaiser to complete? When you view your work from this or a similar perspective, it can suggest appropriate measures to take.
Use the system which is appropriate for your project. Don't forget to tell future document viewers when, what, how and why things were done and presented in the manner that they were.
How others may cope with Tectonic Motion and other issues
Excellent!
NO:
...EXCELLENT!!!
Loyal
How others may cope with Tectonic Motion and other issues
Excellent point - a complete narrative with ALL of the factors that went into the formulation of the project is imperative. We can not expect anyone to "follow the footsteps" if we don't leave any!!
