Moving Mountains

August 23, 2023: A GPS antenna and receiver determine an accurate position and elevation for a geodetic control mark on the top of Mt. Blue Sky, Colorado, elevation 14,266 feet. (Photo Credit: Brian Shaw, NGS) (2023)

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HOST: This is the NOAA Ocean Podcast. I'm Marissa Anderson. Since 2007, the National Ocean Service has been working to update the National Spatial Reference System (NSRS), a massive project to modernize the system for measuring the vertical elevations of the U.S. and its territories. Through that process, scientists identified that - extending from the southeastern corner of the U.S. up into the Pacific northwest - the height of the land above sea-level was overestimated based on historical data. Today, we're joined by Derek van Westrum, a physicist with NOAA's National Geodetic Survey. Derek worked with the team of scientists who realized that our new understanding of the data in Colorado could reshape what we know about the state's iconic "Fourteeners" - 53 mountain peaks with summits of at least 14,000 feet above sea level. Let's dive right in…

HOST: So I'd like to get the record straight by confirming with you that this project was not just about measuring the heights of these Colorado mountains?

VAN WESTRUM:That's a great question. To be clear, we did not set out just to measure these fourteeners. Basically what we really are doing is testing this new aspect of the National Spatial Reference System, in particular the new heights. In 2017, the project to modernize the National Spatial Reference System, the NSRS, had been going on for almost 10 years, and in fact we finished collecting data last year, I think 2023. And we knew everything in Colorado was going to be lowered by probably a couple feet, maybe even three feet.

And what we were doing in 2017 was basically doing the heights, I'll call it the old fashioned way, using basically surveyors along the road with rulers and telescopes measuring height differences. It's a little more complicated. You also have to correct those height differences with gravity measurements. And that's kind of the last hurrah of doing it, the old fashioned way, I'll say. The future, what we'll do is all the surveyors will just be using GPS antennas and they will use a correction we give them that takes the gravity part into account. It's what we were doing in 2017 was making sure the new system was as accurate as we thought it would be. But it turns out it was. We now think we can do heights that's kind of at the three to four centimeter level. So, you know, it's just a couple inches, really, really precise stuff going forward. So fast forward a few more years. This would be 2022, 2023. We now have kind of the final beta version of those corrections for the GPS folks. And we decided to actually see like, okay, what would happen if you were surveying, like we expect people to going forward in a rugged terrain like Colorado that's very high above sea level. It turns out, again, we expected everything to drop by two to three feet. But because we're so precise now, we can get things to just a couple inches. We had some shuffling in the order of the Colorado fourteeners. And that's when we realized, oh, wow, this will be an interesting data set for the public because a lot of people are familiar with the elevations of these peaks. It will be very interesting to see how much they change and that kind of thing.

HOST: Wow. So that's definitely a sizable difference in our understanding of the elevation of these mountain peaks. And it's important to note, of course, these mountains didn't actually become shorter themselves?

VAN WESTRUM: Great, great question. So no, they look the same. You know, if you're down in the plains looking up at them, nothing has changed. In fact, if you go to the trailhead at the bottom and start hiking up one of these, you're still climbing the same amount of elevation. What we've done with the new system is we've redefined where sea level is under, for example, Colorado. We've actually done this for the whole nation, not just Colorado, but Colorado is a great place to test stuff. So if you can imagine the ocean, of course, it ends at the shoreline and say the Atlantic coast. But if you can imagine it continuing under the rocks somehow, it would be this surface that goes right under, you know, the Appalachians through the Great Plains, under the Rockies, out through California, out to the Pacific. What we've done is we've predicted where that surface would be in a way that takes into account the mass of the mountains, or the mass of the land above it. So for example, in Colorado, there's a lot of mass above sea level, and then there's even extra mass above sea level where the mountains are. So all of the mass there has a gravitational attraction, and it would, if there was water under there, it would pull the sea level up basically a few feet. That's the two to three feet we expect in Colorado. And so by changing where sea level is just mathematically and having sea level be higher than it was before, everything now above that is shorter than it used to be, if that makes sense. Again, not only under Colorado, basically all of the states and in all of the U.S. territories.

HOST:That's really fascinating. I'll be completely honest with you. I would not have wanted your job and that level of responsibility for having to determine that any of these mountains would potentially lose their fourteener status. But these mountains are beloved by a lot of people. So that must have been a little bit nerve-racking for you, right?

VAN WESTRUM: Oh, absolutely. It's funny. We, when we started, we said, oh, this is a great test bed. Yeah, we test the height of the fourteeners. That would be kind of fun. Yeah, we got to test something. You might as well test something interesting. It's kind of halfway through where you realize, like, oh, wow, like, if things really change, we might lose a fourteener, and at the time we're like, oh, you know, maybe it would be like, Pluto or something, you know, it may be annoying for people, but at least it will generate a lot of public interest. When the final list came out and we realized that, no, we, all the fourteeners remain fourteeners, and in fact, no low mountains got, you know, promoted to being a fourteener. It was kind of a relief. We love all these mountains. It's funny to see how the order shuffled. We always thought Sunshine Peak was the lowest. It turns out it's actually Huron Peak, so that's kind of interesting, but no, we didn't have to demote anybody, and that actually felt pretty good.

HOST: Can you share how you and your team measured the heights of these mountains?

VAN WESTRUM: I should start by saying the way we expect people to do this normally is actually easier to understand, so I'll just say that quickly. If a surveyor really wanted to know the height of a particular peak, not just in Colorado, this would work anywhere in the U.S., you know, Montana, Tennessee, whatever. We would have sent them to take a GPS antenna, you might have seen these things on the side of the road, they're on tripods, a little white disc with a flying saucer. If they take one of those to the top of the peak and sit there for a few hours, they can process what I'll call a raw height. They can then say, hey, for this location that I'm at, this latitude-longitude, they can go to our database and say, for that location, what is the correction due to the new sea level position? And that will give them the official height in the new system. So that's how the system is going to work going forward, and that will be the most accurate way to do that. For this current study, though, we didn't have the ability to climb all of the peaks and put GPS antennas on top of everything. So we used kind of the next best thing, which is called LiDAR, it's laser ranging.So what we did is took that raw LiDAR data, took out all of the old sea level information that they'd put in there, and put in our new model and said, okay, this isn't quite as good as GPS. GPS can do basically one to two inches. This particular study is more like four to six inches, so maybe three times worse, but still much better than it had ever been done. So that's how we did this particular study.

HOST: That's amazing to be able to get down to that level.

VAN WESTRUM: Oh yeah, and to be honest, it's funny, one of the really important things about this work we did, it's also, it seems a little bit boring, no one ever really put the uncertainty on the old values. You look up any guidebook or any website about climbing these mountains, and it would tell you the elevation is some number, but they never told you the uncertainty on it. So now we are confident that we're definitely down at a few inch level, which is pretty exciting.

HOST: I know you had said previously that none of the Colorado fourteeners lost that status. But so then with these new measurements, did that change up the rankings of these mountains? Meaning did you find that any of the mountains went higher or lower up the list in terms of height?

VAN WESTRUM: We did, for sure. So I don't think any mountains actually got higher, but they all went lower by slightly different amounts here and there. And mountains that were closer ranking before, definitely some of them swap orders. And like I said, the two lowest ones actually swapped as well. So yes, they're definitely the order has changed. I would say 10 to 12 peaks are now in a different order.

HOST: Wow, that's pretty significant. So I do want to ask you, do you personally have a favorite Colorado mountain? And if so, did its elevation change significantly?

VAN WESTRUM: I've climbed 33 of the 57 by the way, the 57 there's different ways of counting. It's anywhere between 54 and 60, but I go by 57 of them. I've climbed more than half of them. So to pick a favorite would be really tough. There's some that are really fun hikes, some that have really good views, some that were just really scary, and you felt like you've conquered something. I honestly don't know how to answer that. I'll just say Mount Snuffles is a really, a really cool one. It's a beautiful hike and spectacular scenery when you're at the top. And also you can see it from just hundreds of miles away. And it's just like a really cool looking mountain. But no, none of them changed more significantly than others. Again, they were all roughly in that three foot range, which is for sure significant, but there wasn't, you know, anything that dropped by tens of feet or anything like that.

HOST: With the results and the data of your work, does this now have greater implications for heights across the nation?

VAN WESTRUM: So, yes, it's an interesting question. Internally, of course, we were like, we basically got the answer we expected, that yes, the new height system does work extremely well. The four to six inch uncertainty we got in the mountains here, part of that's due to using the LiDAR technique. We expect kind of three to five centimeters ultimately in a place like Colorado, which is kind of the worst terrain to measure in. And what I should have said was in a place like South Texas, Louisiana, we think we can do almost a centimeter or better. You know, that's better than half an inch, which is just astonishingly precise. And that new system will have implications everywhere. The lower 48 states were surveyed pretty well, but in a place like Alaska where there's just, you know, so few roads, so little infrastructure, this technique will work the same there. And so we expect to have, you know, really good few centimeter accuracy in a place like that. So there'll be huge implications there for mapping and infrastructure development, as well as, believe it or not, you know, the Pacific islands. It's like, oh, sea level's right there. You can just look at it and measure how high you are. But knowing that undulating gravity part as you go inland is really important. So yeah, the implications will be huge for basically all of the United States when we find they go live, with this in the next year or two.

HOST: That sounds really exciting. So you previously talked about earlier in our discussion, the National Spatial Reference System. So how does this system impact the average person? How does it touch things in our everyday lives that many people may not realize?

VAN WESTRUM: So first of all, so the National Spatial Reference System is just that. It's really a single system that the entire nation, at least at the federal government level, is obligated to use. You know, the average person may not notice that, but it does mean, you know, that somebody's cell phone, the maps in there will ultimately agree with that same reference system. And it gets down to things like, you know, property disputes, you know, property surveys, like, hey, I bought some new land. Where does my property end? You know, we can now do that at the centimeter level. And, you know, if there's questions between states, the federal government has some role in saying, you know, we're the authority on how we do positioning in this country. So it's a little bit behind the scenes for sure. You won't see something in your mailbox. It says, you know, there's a new height system under your feet. Be aware. If everybody has this highly accurate centimeter level positioning system, it'll really start to change things in ways we, again, we can't even predict. You know, when GPS was first invented by the military back in the 80s, nobody thought we'd be using it to find our friends with cell phones. It's just one of those things that the technology enables all sorts of new applications.

HOST: What are some of the next steps with the study that you and your team conducted? Will other mountains throughout the U.S. be measured and potentially find that their elevations will change?

VAN WESTRUM: Yeah, so it's funny. So for us, really, the testing phase is over. We've done a few experiments, in different parts of the country, testing accuracy, and the final one was verifying these fourteener elevations. What we're doing now is actually working on that final gravity-based model of where sea level is under all of the U.S. and its territories. And then the job now is to calculate that surface. And then communicating that to all of the, you know, GPS manufacturers out there and getting all the data sets ready. So that's the next step, literally, is to calculate the model, provide it to everybody, and then go live with the new National Spatial Reference System. Regarding other mountains, that is honestly not a National Geodetic Survey job to go out and survey the mountains. Typically, there's something actually USGS would do. Or, you know, a private surveyor can go up with an antenna two and measure the height of the mountain. We do expect, for example, you know, everything, New Mexico, Colorado, Wyoming, Montana, to drop by this two to three feet level. And as you go north and west, it'll be even more. So we do expect that, yes, everything will change, not just the Colorado mountains.

HOST:It is estimated that the update to the NSRS will lead to about $8.7 billion in social and economic benefits over the first 10 years to the U.S. through improved floodplain mapping, coastal resource management, construction, agriculture and emergency evacuation planning. This has been the NOAA Ocean Podcast. To learn more about this important work and the role of the National Geodetic Survey, check out our show notes. Be sure to subscribe to us in your favorite podcast player, so you never miss an episode.

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