Velocities

Finding Velocities For Yourself

I would do it the same way I get OPUS solutions, pick 3 CORS that surround me and do a weighted mean of their velocities.

Paul in PA

Local Subsidence

(Courtesy MassDOT)

FoggyIdea and I are in the Red areas. The nearest CORS are in the Blue. We are seeing localized subsidence that amounts to ~0.5 - 0.7' per century. That's 0.10' in 14-20 years.

It's all relative. How much does it matter that the elevation of your house is a 0.10' lower than it was 15 years ago if everything around you is also 0.10' lower as well? I'm not suggesting it doesn't matter, only thinking about proper perspective. Now, you're very near the coast, it could make a difference perhaps, but local flooding will still be localized. $#!+ will still run down the same hill because the whole hill subsided.

I think it becomes an issue of metadata. It also becomes important perhaps, to use local benchmarks for local needs - don't try to transfer elevations using GPS from a bench mark that is 30-50 miles removed from your project.

Also, for the sake of the velocities, you can use NGS's HTDP. Which should not to be confused the HGTV - as Mrs. Spledeus will probably find HTDP much less entertaining and you will find HTDP to be much cheaper than HGTV. You can enter the coordinates of a specific location and see what velocities are for that point. This is where epochs get kind of important.

This is how I understand it. (Someone more knowledgeable is free to correct me). You observed this point in 2009 which meant that it would be related to NAD83,CORS96, Epoch 2002. You observed today, which is related to NAD83,2011, Epoch 2010. You might think that you would use the velocities from based on 5 years elapsed time (2014-2009), but actually you would use 8 years (2010-2002).

I hope to see your follow up.

Not So Fast Shawn

When you are so close to the ocean, subsidence is a big deal.

Another problem I believe is that our elevations are not currently based on sea level, because if the sea is indeed rising then we are all sinking. Southeast Cape Cod is relatively sinking faster.

It will be necessary to rely on local benchmarks for elevation. I do that quite a lot, hold the CORS or OPUS position horizontal and a local benchmark for vertical.

Paul in PA

Not So Fast Shawn

> When you are so close to the ocean, subsidence is a big deal.
>
Now, [if] you're very near the coast, it could make a difference perhaps, but local flooding will still be localized.

Cape Cod Bay To Atlantic Side Bays 2-3 Miles

Close enough Shawn?

Paul in PA

Velocities - VTDP

There's Zilkoski's VTDP, and yup we're seeing that in South Louisiana.

I like the idea of holding the nearby benchmarks. Seems like a good idea that is the easy path with a big but.

I know the ground is sinking. Am I not obliged to adjust based on the sinking earth? (Especially with elevations in or near flood plains...) Really, NAVD is based on MSL of a gauge in Quebec.

The map above is some GIS made by the DOT that compares all their level runs. I have no idea if anyone there has really analyzed the runs relative to time, but most of the NGVD level runs were performed in the 40's and 50's where most of the NAVD level runs were performed in the 90's. We had a CORS in Chatham until the early 00's and the computed velocity was dropping in kind to the results on that map.

As far as Stuff flowing downhill, I have worked on some sewer and those big pipes are flat. Look at the map at mid Cape where the blue turns yellow then red. If the subsidence continues at similar rates and if a big pipe were laid from the blue to the red, how long would it take for the stuff to flow the wrong way? (A far fetched but possible example and nobody wants that stuff to flow the wrong way)

I will have to look at the HDTP a little closer and when I catch up with other things (next winter) I may plot out the whole Cape to see how it compares to the MassDOT map.

It disturbs me that the world is moving; it would comfort me to know it was more stable.

> I know the ground is sinking. Am I not obliged to adjust based on the sinking earth? (Especially with elevations in or near flood plains...)

In my opinion, yes. Local bench marks may accurately reflect local relative conditions, but if the risk is from a non-local source, the positional values of the local marks can become dangerously unreliable.

The region I'm in is has large subsiding areas due to groundwater withdrawal. We have millions of acres of land protected from flooding by levees. As portions of those levees subside, they become more susceptible to overtopping, which can have very expensive consequences. Similarly, California's Central Valley has hundreds of miles of canals that carry water from north to south. Due to subsidence, the capacity of some canals has been seriously reduced, and some are threatened with failure. Checking the canal heights against local bench marks won't reveal the truth of the situation; you have to go outside the area.

I agree with Shawn that trying to bring an elevation via GPS from direct observations from stations 50 miles away is probably not a good idea. The standard practice is described in NGS-58 and NGS-59, which describe reaching out to multiple stable marks -- marks that are believed to have reliable datum values -- and bridging into the area of interest in 40 km hops using 5-hour sessions on three different days.

As I recall the whole LSU CORS program came about as a result of surveyors using subsided bench marks with inaccurate values to assess flood risk. Cliff Mugnier (and probably Robert Hill) can shed more light on this.

Excellent post. I was considering coastal flooding in my post above but didn't think about canals. That's fascinating.

Regarding my statement about benchmarks from 50 miles away, I was thinking more about context, which seems like it will become increasingly important. What is the context of the job? Is it to evaluate a project site's relationship to local infrastructure? Even in the presence of subsidence, better use a local benchmark. Is it to evaluate a project site's relationship to regional flooding or infrastructure? Better consider a stable distant benchmark. Are both local and regional contexts a concern? Probably better be related to both.

Thank you all.
Most of my work is in the area of more rapid subsidence. All my local NGS benches are low by about 0.10'. I have been developing a large network solution and now I may be revising my elevations accordingly. That task seems overwhelming at the moment. I am adding miles of level runs that have been performed over the past 30 years, so that may help if enough points remain.
I will also need to add some legalese to the proposals to account for the dynamic nature of the area.
I believe southern LA. is sustaining 3-5 feet/century subsidence so it would be something more important. I can say men about the tenth and adopt an adjustment at some point this year...