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This is Making Waves from NOAA's National Ocean Service. I'm Troy Kitch.
Think of the Washington Monument on the National Mall ... not what it symbolizes or its history, but think of about the monument itself -- it's a really big pillar of rock. In fact, it's the tallest stone obelisk on the planet. Now, you'd think that something that weighs over 81,000 tons would stay put. But it's moving. And it's not alone ... over time, pretty much all land-based structures move to some extent because the Earth is a restless place.
No one knows this better than Dave Doyle, chief geodetic surveyor with NOAA's National Geodetic Survey. Today, we talk with Dave about an effort to measure not only the movement of the Washington Monument, but to survey a large area of land around the National Mall. Because much of the Mall was built on reclaimed land along the banks of the Potomac, many historic structures are slowly settling and shifting over time.
Now, if you live in the DC metro area, you may have heard about this survey in the news. That's because it's part of an effort to help the National Park Service repair the Washington Monument following the earthquake that unexpectedly shook the region in August 2011.
While the earthquake is a big part of what National Geodetic Survey folks are doing on the Mall, it's only part of the story. Dave said that plans were actually in the works to survey in the Mall area even before the quake hit.
"We were in discussion with the National Park Service in looking at long-term monitoring of many of the memorials and monuments that are in the Mall area. We know that much of the Mall area -- particularly the region between the Washington Monument westward towards the Potomac River -- is built on fill, just gravel and sand. And, over time, some of those structures, because they are quite large and heavy, are settling a little bit."
Dave said that, in years past ... going all the way back to 1884 ... the National Geodetic Survey and its predecessor agency have carried out a number of high-accuracy surveys in the area, but what's different this time is the extent to which surveyors are reaching outside of the Mall area to areas that:
"we hope are a bit more stable, so that we get a big long-term picture of exactly what is going on in the Mall area in terms of settlements, subsidence, or possibly even uplift in some areas."
So that's the big picture, but what about that earthquake?
"The earthquake just kind of happened, right in the middle of all of this. And, obviously, that got everybody's attention. There was of course some damage to the Washington Monument, and the National Park Service is obviously very concerned about that, so bringing in the Geodetic Survey as a part of their overall plan for looking at reconstruction efforts is a crucial part of their program."
As part of this plan, the National Geodetic Survey is doing what’s called a vertical control survey. The goal? To get a handle on whether there's been any significant vertical motion because of the earthquake at or near the monuments close to the Potomac ... the Washington Monument, the Jefferson Memorial, the Lincoln Memorial, and parts of the Smithsonian.
Dave said that most people would recognize a vertical control survey -- commonly called a leveling survey.
"As you drive down the road, and you see a surveyor on the side of the road holding a rod that looks like a big yardstick. It's a very common surveying procedure for determining height differences. We are doing what basically looks like the same thing, with just significantly more sophisticated equipment, so that we can now determine height differences from one place to the next at the sub-millimeter level."
What you won't see, unless you watch the surveyors doing their thing for hours on end -- something that I recently did down on the Mall -- is how painstakingly slow this process is, and how much work it is.
A leveling survey basically goes like this: the team starts by placing one rod with marks on it (the thing that looks like a yardstick) on a spot marked with a geodetic control disk, better known as a bench mark -- that's a place where a very accurate height has been previously measured and recorded. Another surveyor places a second rod some distance away. In between these two rods, a surveyor looks at the first rod with an instrument set up on a sturdy tripod that's a combination of a telescope and a spirit level vial. This instrument, called a level, is used to read a height value from that first rod.
Once that measure is taken, the surveyor pivots around and takes a measurement from the second rod in the opposite direction. Once that's done, the person holding the second rod stays put, and the person holding the first rod moves to a new location past the person holding the second rod. Then the surveyor with the level moves to a point between the two rods and takes measurements again. Then repeat, repeat, and repeat. You can think of it as a game of leapfrog ... which is exactly what the surveyors call this process of taking a survey line. Oh, and once this survey line is completed, they go back and do it all over again to make sure their measurements are correct. These folks walk for miles and miles, shouldering all of their heavy equipment. So why is this necessary again?
"Everything is moving. Tectonically speaking, every place on the surface of the Earth is constantly moving. Now, by and large, we're pretty fortunate on most of the East Coast that we don't have too many significant events, like earthquakes that we see out in California and Western Washington, Oregon, Alaska, places like that. But nonetheless, the North American tectonic plate is moving laterally, and there are also issues of subsidence and uplift. Subsidence is caused locally in many areas by a number of different factors including sub-surface fluid withdrawal, sediment loading in rivers and bays and estuaries, and a number of other factors. So the surface is constantly changing, and many communities, particularly low-lying communities are affected by these minor but long-term changes in heights.
And when there's a rare event like an earthquake on the East Coast, well, that shakes things up. So the leveling survey will help the Park Service understand how the ground underneath the monuments on the Mall changed as a result of the quake.
What comes after the leveling survey? Later, this year, the National Geodetic Survey will help the Park Service take a closer look at the Washington Monument in particular. Dave said that most of the damage appears to be rather high up, so Park Service experts are figuring out the best way to repair the stone obelisk, which will likely involve putting up scaffolding around the Monument.
"Once that effort is underway, we have already engaged with the Park Service, that they will allow us to place a GPS receiver back on top of the Washington Monument. We did this back in 1999 and 2000, and it was the very first time that any GPS work had been done on top of the Washington Monument, and since we have that older data and now we'll have new contemporary data, we'll be able to not only determine has their been any shift in the Monument, although we don't think it would be significant, but more importantly is there any tilt in the Monument, and that's been a more difficult process to determine. So later in the year, we will conduct observations using the Global Positioning System on the Washington Monument as well as a number of other technically and historically important monuments in the Mall area."
The leveling survey and the data collected by GPS is needed by the Park Service because they need to know as much as possible about the stability of the monuments on the Mall, mainly because it's their job to repair the monuments from any damage sustained by the earthquake. But beyond that, it's their job to ensure the monuments on the Mall always remain safe, stable, and sound.
That's something the National Geodetic Survey can help with. Dave said that older leveling surveys conducted in the National Mall area have been high quality, but largely piecemeal ... small-scale efforts scattered around different areas and separated by many years. But he thinks that is now going to change, beginning with the larger-area survey now underway tied to the earthquake.
"The effort that we're engaged in here is to put all of these height data into what we refer to as a single epoch, that is one leveling set of observations that can be relied on in the future and, in working with the Park Service, we're in the process of creating an agreement with them where we will go down to Washington and perform this kind of a survey periodically, every two to three years depending on the needs of the Park Service."
Over time, this will give the Park Service better data to work with because the surveys will not only cover a large area, but the observations will all be made within the same time period ... during a single epoch ... that's key, remember, because the Earth is always shifting beneath our feet.
"If we make some observations, say, between the Washington Monument to the White House and we don't make observations to the Capitol -- it's not that far away, but it does take time and it does take effort -- then what we have are discrete sets of measurements between various objects, but they're not all connected together at the same point in time. And because we know that some of these areas are undergoing vertical motions, and those vertical motions are small, maybe on the order of a few tenths of a millimeter a year or maybe as large as a millimeter or so a year, it's a small change, yet very significant when you add it up over time. And since these changes are so small, if you don't have all of your changes in the same epoch, it can be very difficult to make accurate assessments of what those changes are about. If your data is parsed out over time, making decisions about, 'well, we think that something is moving at a particular rate,' becomes much more problematic."
So that's how more frequent, larger area leveling surveys will help the Park Service make better decisions about how to protect and repair the monuments on the Mall from the 2011 earthquake, and into the future.
"They will be able to rely on the data that we're providing as a part of their efforts to monitor. So, they're doing a wide range of investigations down there. They're doing some seismic studies and they've brought in a number of very specialized structural engineers to look at what's going on, so our observational data will be a part of that overall picture. With this agreement that we're working on with the Park Service to go back and repeat these observations on a periodic basis, if we do that every three years for the next 20 or 30 years, now we'll certainly get a much, much better picture of exactly what's going on that the Park Service and others can rely on. The hope is -- and we're still working on this part -- the hope is that the areas around the Capitol and up around the White House where we're also be working were less impacted by the earthquake. We won't know that until the survey is complete, but we've got pretty strong evidence that nothing much happened in those areas. So we're confident, at least right now, that we won't have to expand the range of this survey beyond that."
That was Dave Doyle, chief geodetic surveyor with NOAA's National Geodetic Survey. By the way, Dave said the agreement they’re working on with the National Park Service to survey more often and larger areas has a side benefit: it’ll also help the field staff at the National Geodetic Survey keep current on new technology and keep their leveling skills sharp. And since downtown DC is a short distance away from NOAA's campus in Silver Spring, Maryland, there won't be high travels costs involved.
And that's all for this week. If you'd like to learn more, check our show notes for the links. You’ll find these on our website at oceanservice.noaa.gov.
If you have any questions about this episode, about our oceans, or about the National Ocean Service, send us an email at firstname.lastname@example.org.
And don't forget that you can visit us online at oceanservice.noaa.gov. There you'll find an accompanying print story about the study we discussed today, and you'll links to all of the offices and programs we've mentioned in this podcast.
This is Making Waves from NOAA's National Ocean Service.