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Diving Deeper: Height Modernization

Episode 33 (Oct. 20, 2011)

HOST: Welcome to Diving Deeper where we interview National Ocean Service scientists on the ocean topics and information that are important to you! I'm your host Kate Nielsen.

Today's question is...What is height modernization?

Height Modernization is the establishment of accurate and reliable heights using GPS technology in conjunction with traditional leveling, gravity, and remote sensing information.

To help us dive a little deeper into this question, we will talk with Renee Shields on height modernization. Renee is a geodesist and the Height Modernization Manager with the National Geodetic Survey. Hi Renee, welcome to our show.

RENEE SHIELDS: Hi Kate, thanks for inviting me. How are you doing today?

HOST: Very good. So, Renee, can you elaborate on that initial definition that I just gave for height modernization?

RENEE SHIELDS: Sure, I'll give it a try. I really like the definition because it's nice and short and sweet. The ultimate goal of height modernization is to be able to use GPS technology to get heights that are accurate to, what we're shooting for is, one to two centimeters, or about a half an inch to an inch. Traditionally heights, or elevations, have been measured by surveyors through a process called leveling. Most of us have seen surveyors by the side of the road, they have an instrument on a tripod, they're looking through the instrument and they're sighting down the road to a rod that's got marks on it. What they're doing is they're measuring height differences from where they are to where the rod is.

Level instruments are optical instruments, so you need to have line of sight from where you are to where you're siting to. And, if you need a height a couple of miles down the road, you can imagine you have to keep setting this up over and over again and it can become very time consuming and very costly.

People are looking at GPS as perhaps a faster way of doing this because with GPS instruments you're sighting between the GPS and the satellite. You don't need line of sight. The difficulty is that the heights that you get with GPS and the heights that you get from leveling are not the same kind of heights.

HOST: OK Renee, so you talked about two different kinds of heights, what's the difference between these - the difference between the heights that you get from leveling and the heights that you get from GPS?

RENEE SHIELDS: To answer this question we really need to know what a height is and what we're using it for. If you think about it, a height is really a distance above a certain surface. Most people want heights because they want to know which way water is flowing. They want to know if they're in a flood zone, or they want to know, if they're building their house, they want to make sure the water's going in the right direction in their pipes.

As you can imagine, gravity plays a big role in this. So using gravity measurements, we can actually model the surface that we're measuring the heights from. This surface we call a geoid. A leveling instrument is designed to take gravity into account, and the height differences will indicate the direction the water will flow. The heights we get from leveling we call orthometric heights.

If you go back to GPS, what GPS is doing is it's measuring the distance relative to what we know about the GPS satellite orbits, and we describe that with a mathematical figure called an ellipsoid. So you're measuring a distance from the ellipsoid to where you are on the surface of the earth. The ellipsoid is terrific for doing mathematical computations, but it really doesn't have a clue where gravity is, or what the gravity is.

So, if we could figure out the difference between that geoid surface and the ellipsoid, and then take that difference and apply it to a GPS measurement, we're getting the measurement of the height from the geoid, the height that we're really after. The problem is that the geoid is pretty bumpy, it's not an even surface, so it's pretty hard to define that surface.

HOST: OK, so measuring heights and getting elevation data, is not quite so simple.

RENEE SHIELDS: Not at all.

HOST: Renee, what types of data do you need to determine these elevations?

RENEE SHIELDS: Well, first and foremost we need gravity data. At NGS, we have gravity data that we've measured on the land, terrestrial data. We have gravity data from ships offshore. And recently there are some satellite gravity missions, so we're getting satellite gravity, and we're also using an airplane to collect gravity. Where the terrestrial is point by point data, the satellite and the aerial gravity data is more of a constant stream of data, so that's filling in the gaps.

What we have to do with all that data is reconcile it to the ground, that's where we need that gravity measurement. Now at NGS, we have a particular surface that we're trying to relate everything to, this particular version of the geoid model, so in order to get that surface, we're taking additional data where we have leveling observations and GPS observations, so we have both kinds of heights, the orthometric height and the ellipsoid height, the GPS height, and we're using that data to kind of tweak the model that we've created from the gravity. So it's a specific version of the geoid model, specific surface that we're trying to define here.

HOST: Is the process set for how you collect data or are you constantly testing and evolving your methods for collecting this information?

RENEE SHIELDS: Well, we do have a process in place, but I'd have to say it's still evolving. We have a long way to go for our goal, our two centimeter/one inch heights objective here. The model needs improving, the more data you have, the better the model. We're finding better ways to develop the model from the data we have. And the technology is changing. GPS instrumentation is getting better. There are more satellites in the sky, so you get better data from that.

And the fact is, in the early years of height modernization we were so focused on getting that specific geoid model that we were doing a lot of the leveling and GPS heights data. We've changed our approach in recent years to look more at using the gravimetric geoid, the geoid that we build out of the gravity data by itself as a way of doing this, so we're sort of changing our approach. The other part of this is no matter how accurate you make it, users are going to want more accuracy or they're going to want it faster, or they're going to find a new application for it. People especially today want it in real time or near real time, that's going to save them money. Recently I heard about a new application the automobile companies are looking at for the trucking industry, where they will include topographic maps, so maps with heights on them, in the navigation system and somehow they're able to automate the speed of the vehicle in hilly or mountainous regions and save on fuel. So, of course to do this, they need these GPS observations in real time.

HOST: I like that example with the automobile companies. That paints a good picture for us. And that actually takes us to one of the next things that I wanted to ask you about now, which is why is height modernization important? What do we use this data for?

RENEE SHIELDS: There are a lot of products and services that people use that rely on having, accurate heights. When people are concerned about whether they're in a flood zone, they want to make sure those FEMA floodplain maps have accurate heights that they're based on. The U.S. Geological Survey and NOAA's own National Weather Service both use heights to create inundation models or storm surge models, so they need accurate heights. Aircraft and ships need accurate heights for navigation, and they're using GPS more and more. And since we can measure elevations faster and cheaper with GPS, we can go into areas where we know there's movement going on and we can do re-surveys of an area over time and try and model just what's going on there as far as vertical movement. If your house is not in a flood zone now, you want to know is it not going to be in a flood zone in ten years. So, how things are moving is one of the big things that we're looking at now.

HOST: Renee, what causes the elevation in an area to change?

RENEE SHIELDS: There are a few things that can cause heights to change in an area. Earthquakes, of course, especially if you're talking a few centimeters level change, the least little earthquake can make a difference. There's something called subsidence or land sinking, that will cause heights, obviously, to change. Some folks may be familiar with the Gulf Coast and Louisiana where they've been hearing in the news about subsidence. Up in the northern part of the country where glaciers are receding, where the glaciers are melting, they have something called post glacial rebound, it's sort of the opposite of subsidence, where the land is bouncing back.

And some of these things are man-made causes, those were all tectonic or natural causes, but there's man-made causes as well if you're extracting oil and gas or water out of an area, that can lead to subsidence. And there's relative changes we're looking at. If you know of an area, along the coast especially, where you've got some kind of activity like a subsidence going on but we also think that there's sea level rise going on, how do you measure the relative differences between those two. So, that's something NOAA particularly is interested in.

HOST: So Renee, what are some of the benefits of height modernization? We've talked a little bit about how the data can be used, but what are some of the benefits of this?

RENEE SHIELDS: Well, saving lives is a key part of this of course. I mentioned navigation. The FAA positions runways at airports using GPS and they have charts to show that and then the planes have GPS on them now, so in low-visibility conditions, the planes can land safely. Ships can also use accurate heights to navigate in and out of port. If they know the height that their ship is sitting at and they know the depth of the channel and they know the height of the bridge they're passing under, they can get through with minimal amount of clearance so that saves them money as well as lives because they can load more cargo onto the ship.

And there are farmers now, this is a new group of users that we have, the agricultural community. Farmers are using GPS on their farm equipment and they have equipment that lays down pesticides and fertilizer and they have their irrigation equipment, and they're able to more efficiently apply these things because they can minimize the amount of overlap they have, they have an idea what the slope of their land is, and this saves them not only money, but it's good for the environment because they're minimizing the amount of pesticides and fertilizer that they have to use. So that's a whole new community that we're able to reach out to.

HOST: Great, well, you've sold me. Height modernization has a lot of really good benefits. I mean, saving lives is huge, and then when you add in the saving money and good environmental benefits, it sounds wonderful. My next question is, if this is better as we've been talking about, height modernization, why isn't everyone using it?

RENEE SHIELDS: Yeah, that's a good question. Of course if we had our way, everybody would be using it. This would be the way of the future. We need to make sure that the models and the tools that are in place are the best that they can be. Some parts of the country have done a lot of work, they've got a good handle on this, other parts not so much. So, that needs to be improved.

Currently, you need good geodetic quality instruments, not all GPS are created equal so you need specific ones to get accurate vertical measurements and not everybody's willing to invest in that. There's the technology transfer issue, we're still educating people about the benefits, but also about how to do it, it takes a real specific process to be able to do this so we're building capacity in the community and we're teaching people about the importance and the value of this. And that there are still some applications where you need even better heights than an inch or half an inch, you need sub-centimeter level heights and currently leveling is still the best way to do that. So there are still people that want to use leveling. We've got a ways to go before everybody's embracing this technology.

HOST: Renee, at the beginning we mentioned that you are with NOAA's National Geodetic Survey. Why is your office involved in this effort?

RENEE SHIELDS: There are a number of government agencies who have responsibilities for different kinds of mapping products. I mentioned FEMA, they're responsible for the flood maps. U.S. Geological Survey makes topographic maps that are used for recreation, for engineering, for resource management. The Army Corps of Engineers uses maps for their survey projects. What NGS does is provides the most accurate positions, or position data, so all these agencies can make their maps consistent with each other, so that if you're using two mapping products together, you know that they're referenced accurately together. You can combine this data. Our agency started when the country was very young and the big focus then was on coastal waterways, on coastal charting because of course commerce depended on navigation along the coast so that's how we got started and now we do it across the land as well.

HOST: Is height modernization a project or an effort that will ever be completed or do you think there will always be work to do in this field?

RENEE SHIELDS: Good question. Of course we have a goal to get the two centimeter heights using GPS. But I think the technology's always changing so maybe we won't have to go to such rigorous procedures to do it in the future if the technology's better. We'll learn to build better models so again it'll be a little easier and I think there will always be new applications, I think there will always be people who want it more accurate, who want it faster, and so we'll always be striving to meet that need, to chase down that millimeter if that's possible down the road.

HOST: Renee, during your career working on height modernization and related efforts, what project are you most proud of?

RENEE SHIELDS: I can't say there's a particular project. I've been with NGS for 30 years, I'm proud of all the work that we do and all the projects I've been involved in. I do have to say I'm really proud and honored to have had the opportunity to work with all the partners that I'm working with that are helping us do this. We work very closely with other federal mapping agencies (FEMA, Corps of Engineers, USGS, Park Service). We have a lot of partnerships with local and state agencies like Departments of Transportation, where a lot of the surveying and mapping is done on the local level, sometimes Departments of Natural Resources is where that functionality is, they're all working to improve their mapping projects through height modernization. And we have a lot of university partners that we're working with. There's a lot of folks out there doing some great work and I think that's what I'm most proud of is those partnerships.

HOST: So Renee, my last question to you is, do you have any final words for our listeners today?

RENEE SHIELDS: I appreciate you having me on your show, on Diving Deeper, it's been a real pleasure. I think height modernization's really important and there's a lot of exciting things going on, so I'm just really pleased to be able to share that with your listeners. So, thanks a lot.

HOST: And thank you Renee for joining us on Diving Deeper and talking more about height modernization. For our listeners, to learn more, please visit or visit our site for more links and details.

That's all for today's show. Please join us for our next episode in two weeks.