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Diving Deeper: Episode 9 (May 20, 2009) —
What is geodesy?

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 geodesy?

Geodesy is the science of measuring and monitoring the size and shape of the Earth including its gravity field and determining the location of points on the Earth’s surface. Many organizations use geodesy to map the U.S. shoreline, determine land boundaries, and improve transportation and navigation safety.

To help us dive a little deeper into this question, we will talk with Dr. Dru Smith on geodesy – what it is and why it is important. Dru is the chief geodesist at the National Geodetic Survey. Hi Dru, welcome to our show.

DRU SMITH: Thanks Kate, it’s great to be here.

HOST: Dru, before we begin, I have to tell you that geodesy wasn’t a word I was very familiar with before preparing for this interview. Will you tell us what inspired you to study geodesy in the first place?

DRU SMITH:  Sure, Kate. I was actually studying Land Surveying in college and during my senior year, I took a course called Elements of Geodesy. That was also the first time I’d heard that word. That course basically took the flat Earth world of surveying that I’d been studying and put it on a curved Earth. It opened my eyes to a whole new way of measuring the world. I absolutely loved the course and I decided to stay on and study geodesy through my Ph.D. After I finished, I joined the National Geodetic Survey and I’ve been here ever since.

HOST: That’s great. Dru, how do we measure points on the Earth’s surface?

DRU SMITH: Well, geodesists, that’s what scientists are called who study geodesy, basically assign coordinates, which is kind of like a unique address, to points all over the Earth. If you were to stick pins in a model of the Earth and then give each of those pins a coordinate, you’d be doing what geodesists do. Before the space age, geodesists used to determine the coordinates of points by using Earth-based surveying tools, measuring angles, distances between points. Now most of our work is done using space-based tools like the Global Positioning System or more commonly known as GPS. But no matter what the tool is, geodesists still do the same work, which is very accurately defining the coordinates of points on the surface of the Earth in a consistent manner. This set of accurately measured points then becomes what we call a spatial reference system, which is basically a system that allows different kinds of maps to be consistent with one another.

HOST: Do geodesists need to re-measure or continuously monitor sites after they collect these spatial points?

DRU SMITH: That’s a great question, Kate, and yes we definitely do, but it’s not always been done in a very efficient manner. In the past, most of geodesy was done using what we call passive control. This is a mark in the ground that needs to be surveyed or re-surveyed if we’re going to actually monitor how it’s moving. But with space-based techniques, especially say GPS, we’re able to actually monitor a site’s movement 24/7.

HOST: Well, why is that so important? Are there different kinds of motion on the Earth?

DRU SMITH: Absolutely. There’s a few different types of motion that we’re especially concerned with. First of all, the Earth’s surface changes for a lot of different reasons. For instance, you may not notice it, but the surface of the Earth itself rises and falls about 30 centimeters, that’s about a foot, every day just due to the gravitational pull of the sun and the moon on the Earth’s crust. Because this repeats itself day in and day out on a well-known schedule, this motion is called periodic and we can usually compute or model this effect and remove its effects from the coordinates themselves.

A second kind of motion is secular, which is where things are moving at a certain pace such as the slow grind of tectonic plates, the uplift of Hudson Bay caused by the withdrawal of the last ice sheet, or the slow sinking or subsidence of land in areas such as the Louisiana and Texas Gulf Coasts. This kind of motion can be the most annoying to surveyors and mapmakers because it means that coordinates are constantly moving away from their previously computed values.

So, finally there’s also something we call episodic motion, and this is sort of one-time shifts and they’re usually caused by Earthquakes or say volcanic eruption.

Basically, geodesists have come to realize that when you’re trying to determine the coordinate of a point to a centimeter or so – which is really the sort of level of accuracy we’re working with today – that there’s a lot of motion that we have to take into account out there.

HOST: So Dru, how do we actually measure the Earth?

DRU SMITH: To measure the Earth, we, geodesists, look at it as a whole. Obviously the surface is very irregular with many mountains and valleys it makes it impossible to measure every single thing because you would need an infinite amount of data to actually model what the true Earth looks like. Knowing this, geodesists prefer to build simple mathematical models of the Earth which capture the largest, most obvious features, and then build residual models from that, to sort of describe variations from the simple model.

I’ll give you an example, as a first, sort of crude approximation, the Earth’s shape could be considered an ellipsoid. An ellipsoid is just, it’s sort of a flattened sphere. If you take a basketball and push it from the top to the bottom, you’d get an ellipsoidal shape.

Anyway, geodesists have adopted the ellipsoid as the most basic model of the Earth with which we work. Because the ellipsoid is a very simple, mathematical model, it can be completely smooth and doesn’t include any mountains or valleys.

Now, that’s useful in a number of cases, but we sometimes need a more accurate model of how the Earth actually looks. So, to account for additional detail of the Earth’s shape, geodesists have adopted the geoid, which is a shape very similar to global mean sea level, but this exists over the whole globe, not just over the oceans.

This is where we get into the gravity field, because the geoid is a surface of equal gravity potential energy. Now that’s a mouth full, but basically the geoid is not a smooth, regular shape like the ellipsoid, it rises and falls due to minor variations in the pull of gravity that’s caused by mass variations of the Earth itself. Basically the Earth’s mass is unevenly distributed, you have mountains, you have valleys, you have the ocean, you have heavy rocks in one location, lighter rocks in another – and it means that certain areas of the Earth will experience more gravitational pull than others, and as such, the shape of the geoid in different parts will vary by as much as plus or minus one hundred meters when we look at it relative to this really simple surface which we call the ellipsoid.

If you want to picture what the geoid looks like, you could imagine what the ocean would look like if there were no tides, no currents, no waves – if the ocean were to just settle down and become smooth as glass, then all that would remain to give the ocean any shape would be these minor fluctuations in gravity, causing these slight bulges and dips in the ocean and so the ocean at rest would then be the geoid. 

HOST: OK, so there is a shape to resemble the Earth with water, which is the geoid that you just described. How does this all fit into the spatial reference system that you mentioned a little earlier?

DRU SMITH: That’s a great question. It all ties in through one other topic that we really haven’t brought up, and that is datums. Historically, geodesists worked in two primary datums, which have come to be known as the horizontal datum and the vertical datum. These separations were made because the horizontal datum (which is the thing that helps us know where latitude and longitude are) and the vertical datum (which helps us know where heights are) these were each measured in very different ways. However, that isn’t as true today, now that we have GPS.

HOST: Dru, can you explain these datums to us a little bit more?

DRU SMITH:  Sure. First, datums are the basis for all geodetic survey work. They sort of act as reference points in the same way that starting points do when you give someone directions. Geodesists and surveyors use datums as starting points when they create maps, mark off property boundaries, when they build roads, bridges, other structures.

While there’s a lot more to it than this, basically when people think of the datums they think of geodetic marks in the ground. These markers, which we call geodetic control points served as known starting points for surveyors. In the case of the horizontal datum, we would provide a latitude and longitude on a mark. For the vertical datum, we would provide a height. Today, with GPS a lot of that has changed. You can get latitude, longitude, and heights out of GPS and you don’t need to start at a passive mark in the ground.

So, today the horizontal and vertical datums make up the greater part of what we call the National Spatial Reference System, which is the spatial reference system that for the United States itself. Geodesists, surveyors, anybody interested in precise positioning – uses the National Spatial Reference System as their foundation of reference in the United States.

HOST: Dru, a little earlier you mentioned gravity also as one of the elements needed to measure the Earth.

DRU SMITH: Gravity plays such a major role in geodesy, but it’s really so hidden from the public eye that it seems almost obscure. For example, knowing the gravity field of the Earth is required to model the orbits of all satellites which includes GPS. That means we need to know the gravity field to know where the satellites are. And you need to know where the satellites are to use them to position yourself. So, in that application, the gravity field’s really important.

Closer to home, knowing the gravity field is required to know the true height differences between any two points. Gravity’s pull changes from point to point. We don’t feel it, but water definitely feels it. And so, if you take an area that’s extremely flat such as New Orleans and you have very minor fluctuations in how gravity is pulling at different locations, those fluctuations are going to determine where areas will flood relative to one another.

HOST: It sounds like there are four main elements of geodesy based on what we have talked about so far today. And these include the size and shape of the Earth, the gravity field of the Earth, how all of these things change, and then how we position everything on a changing Earth. And these elements make up the National Spatial Reference System. Why is this system so important?

DRU SMITH: Kate, you’re absolutely right. All of the elements of geodesy are joined together in the National Spatial Reference System. A lot of maps are now digital and we build them on our computers. So to do this, we take data from a number of different places. Let’s say I want to start with the contours from a map from the U.S. Geological Survey and then I tell my computer to add a few more pieces of information such as floodplains from a map from the Federal Emergency Management Agency, and I say add the levee heights from the U.S. Army Corps of Engineers maps, and say the population statistics from a U.S. Census map. If there isn’t one consistent coordinate system tying all these maps together, then the result would be useless – each of these would not lay on top of each other properly and I couldn’t compare the data. The National Spatial Reference System is that system which gets all of these maps together on the same grid.

HOST: Dru, what are some of the other benefits of geodesy?

DRU SMITH: That’s a good question Kate. Some of the other benefits of geodesy are, for example, to the transportation industry. So here’s a great example, I really love this one. By providing charts that are accurate to centimeters and using GPS technology on a ship which also can position the ship to centimeters, if those two locations are done consistently, say in the National Spatial Reference System., then you have the ability to know where the bottom of that ship is relative to the bottom of the ocean to a couple of centimeters and this means that a ship could put on extra cargo, sink deeper into the water, and still know that they’re safely navigating through a channel. And the beauty of that is more cargo directly relates to an economic benefit to the shipping industries. So, extra accuracy due to geodetic tools allows the shipping industry to have a pretty significant economic benefit.

HOST: So, it sounds like there could be many benefits from geodesy that impact our everyday lives. Dru, what is the role of the National Ocean Service in geodesy?

DRU SMITH: Well, the National Geodetic Survey is just one office in the National Ocean Service. And so, while we’re responsible for the development and maintenance of the National Spatial Reference System at the National Geodetic Survey, there are related components within the National Ocean Service. For example, the National Ocean Service has offices responsible for tide gages and navigational charts. Both of these functions are directly tied to the National Spatial Reference System.

HOST: What are the tools that NOAA geodesists use to measure the Earth?

DRU SMITH: Well, historically geodesists from the National Geodetic Survey used manual instruments such as theodolites or distance-measuring tools to determine vertical and horizontal positions. If you’re not sure what a theodolite is, I’ll bet you’ve seen one without realizing it. Most of us have, for example, driven down the highway and seen someone standing there with a reflective jersey on looking through something that looked like a small telescope sitting on top of a yellow tripod. That instrument was likely a theodolite, a level, or a total station – these are all the manual instruments that I mentioned before that were used by geodesists for so long. Starting around the 1960s, the National Geodetic Survey started using satellites for global positioning. Around 1984, the first launches of GPS itself occurred and so GPS continues to this day to really be the primary operational tool of geodetic surveying.

HOST: What are some of the applications or how does NOAA use spatial reference data?

DRU SMITH: Global positioning data supports a lot of different applications – safe development, efficient transportation and navigation, emergency response efforts – for both coastal areas as well as the middle of the country. What’s especially critical are our abilities to monitor the changes to coordinates. For example, this gives the U.S. Army Corps of Engineers the ability to know changing heights of their levees, which is really important. It allows surveyors in southern California to track the movements of points or state departments of transportation to know if their evacuation routes are prone to flooding. More importantly it really allows us to predict when a currently safe evacuation route might become unsafe due to subsidence.

One of the programs that the National Geodetic Survey is focused on is called the National Height Modernization Program. This is where we combine GPS technology with existing surveying techniques to determine elevations for positioning and navigation.

HOST: Dru, can you highlight a little more on height modernization? Maybe a little bit about why this is important or what is the need for this kind of information?

DRU SMITH: Absolutely, those are great questions Kate. Like many of us, geodesists are also looking for ways to do things quickly and accurately. So, height modernization is about getting heights quickly and accurately, but the only way to get accurate heights used to be through a really tedious surveying process called leveling. But when GPS came along, surveying efficiency shot through the roof. The only problem was that GPS couldn’t tell you the right information about heights above sea level. But it turns out that if you have a GPS receiver and a good map of the gravity field, or the geoid more specifically, then you can get good accurate heights quickly from GPS.

So this is the basis of the National Height Modernization Program, which we’ve come to call Height Mod over the years. Height Mod has been a partnership program since its very beginning, where NOAA works with state partners to improve their ability to get these heights from GPS. More recently, as part of Height Mod, NOAA has implemented a project which we call GRAV-D that stands for Gravity for the Redefinition of the American Vertical Datum. It’s a nationwide effort to make sure everyone, no matter what state or territory they live in, can get accurate heights from GPS.

HOST: Thanks Dru. Do you have any final closing words for our listeners today?

DRU SMITH: Well first, thanks for the invitation. We don’t often get to talk about geodesy and, so, it’s nice to be able to get the word out. It’s especially nice for me to not just talk about the science which I love, but about some of the applications that geodesy has. I really am glad for the chance to tell your listeners why geodesy is important whether you live at the coast, whether you live inland, you rely on geodesy in ways that you don’t realize. It’s especially useful for us to make sure that people are aware of the benefits of geodesy – that you rely on it for transportation, for safety of your bridges and roads, to keep your office building, your school, or your home out of floodplains, to be able to tie all that together through the science that I enjoy so much, this really has been a nice opportunity to talk about those benefits as well to your listeners.

HOST: Thanks Dru for joining us on today’s episode of Diving Deeper and talking more about geodesy and how we rely on this every day in more ways than probably most of us realized. To learn more about geodesy, please visit

That’s all for this week’s show. Please tune in on June 3rd for our next episode on resilience.