Subscribe to Diving Deeper

Ocean Service Feeds

What is a Podcast?

A podcast is a an audio file published on the web. The files are usually downloaded onto computers or portable listening devices such as iPods or other players.

Read more about podcasting from

Find other podcasts from the US government

Diving Deeper: Accurate Positions: Know Your Location

Episode 41 (August 23, 2012)

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...How do we get accurate, consistent positions?

If you want to know where you are, you need a starting, or reference, point. Take a bunch of these points, and you have a spatial reference system. NOAA's National Geodetic Survey maintains a reference system to ensure that all U.S. positioning information, including every map made by any government agency, fits together. Accurate, consistent positioning data is fundamental to navigation, infrastructure, transportation, communication systems, and so much more.

Today we will talk with Joe Evjen on positioning. Joe is a geodesist with NOAA's National Geodetic Survey. Hi Joe, welcome to our show.

JOE EVJEN: Hi Kate, thanks for having me.

HOST: Can you tell us a little bit about how we get accurate positions?

JOE EVJEN: Sure. You get positions by measuring them from other positions of course. In this office building, we're on the fourth floor, we're 40 feet above ground and if you wanted to meet your friends for lunch you could tell them you were six and a half miles north of the Zero Milestone, a famous monument in downtown DC.

To make positions relevant to a lot of people across the country, we use more famous monuments, we use the Earth. We measure latitudes up from the equator and longitudes east from the Greenwich Meridian in England. Those are rather inconvenient to measure from of course so we seeded the country with about a million small concrete monuments, brass disks you may see on mountaintops, or in sidewalks, and we have determined the official latitude, longitude, and elevation of those points.

HOST: Great and so, we talked a little bit about this in the introduction, but can you expand on some of the applications of positioning data—maybe who uses this information and this data?

JOE EVJEN: Most famously, map makers use data to make their maps. And now map users, if you have a GPS instrument in your car, you are actually drawing yourself into a map in real time. More specifically, our customers tend to be surveyors and engineers. They need to figure out where things are, and how they might need to be moved or worked around in order to build bridges and roads through mountains and over rivers. And also for ship drivers if they're trying to fit a large ship under a bridge or over a shoal to get into a port, they need to know the size and location of things.

I like that reference, if you're using a GPS in your car, you're pulling yourself into a map—that's a really good way to think about that. So, this data's important for everybody all across the country and my next question for you Joe is, what does the National Ocean Service then have to do with positioning?

JOE EVJEN: We get that question asked all the time because we worked mostly in mountaintops for the last 100 years and we have National Ocean Service written on the side of our truck. In a word, we're here because of shipwrecks. Two hundred years ago when the nation started, we were a seaboard nation, clinging to the edge of a vast wilderness and we used the ocean as our roads. And we had lots of crashes. Lots of shipwrecks would cause turmoil in commerce—you never knew if you'd have to have a large profit in order to get a ship to move up the coast because you might lose everything along the way. Thomas Jefferson figured that out 200 years ago and he formed the nation's oldest scientific service, what became the Coast and Geodetic Survey.

We needed good maps or charts of the ocean so that ships could drive around shoals and find the deep water to get into ports. We must have done a good job because as the nation moved westward into the wilderness, we led the way with geodetic surveys to help build the roads and interstates.

HOST: So Joe, you've mentioned these brass disks a few times so far today during our episode. When someone finds a disk, what does that mean?

JOE EVJEN: When you see a disk, it's a placeholder for an earlier survey observation. If you find a brass disk on a mountaintop from 1957, you can touch the same location that the surveyor in 1957 touched when he determined the precise latitude, longitude, and height, and you can work from there to find your latitude, longitude, and height of a thing that you're interested in or track to see if the mountain has changed height.

Because we have these 200 years of historical descriptions of these marks, it's become a hobby for geo-cachers to go out and locate bench marks and read a description from 80 years ago and compare that with the site as they find it today. It's very refreshing to have a job that other people consider a hobby.

HOST: So what services then does your office, NOAA's National Geodetic Survey, what services do you provide to professionals requesting this survey information?

JOE EVJEN: Where is one of the five fundamental questions - who, what, when, where, and why. And we answer the where question about 50 million times per year through our various products, many of which are now maintained by state and local engineers. It took a large Coast and Geodetic Survey for the first 100 years to put in these million marks across the country. We use a lot of satellite techniques and a lot of borrowed data from around the world to produce our products now.

One of our main products is a datasheet which is just a text description of the official location of each of those one million brass disks that we've put in sidewalks and mountaintops around the country. We now give out raw GPS data files and we process GPS solutions for almost 2,000 active control stations which are just a GPS receiver glued on top of a bench mark.

We've now installed GPS receivers that 24 hours a day collect GPS data. A surveyor doesn't need to send a crew member out to the known location to start his survey that data can be downloaded from our CORS network automatically or we also have a product called OPUS, where you can submit your raw data and we'll do the processing for you.

HOST: Joe, how does the online positioning user service that you just mentioned, how does it work?

JOE EVJEN: Well, when you're using a GPS receiver, you're listening to GPS satellites and you can position yourself fairly accurately to within 10 or 15 meters. A lot of people need positions better than that if you're trying to position a house above a floodplain or make two ends of a bridge meet in the middle over a river, you want centimeters or millimeters instead. In order to get that you need to ground your GPS satellite data with some known positions. Helps us eliminate a lot of the errors that a GPS signal incorporates as it goes through the atmosphere and because it's based on a GPS satellite position which may or may not be perfect.

OPUS is a software tool which will combine the user's GPS data file with GPS data carefully recorded from our network of about 2,000 active control stations, we call CORS—Continuously Operating Reference Stations—where we record data 24 hours a day, 7 days a week.

HOST: So it's more of a computer program then that automatically generates this position for the user? How are people involved?

JOE EVJEN: The people mostly stand out of the way. We try to let the machines talk to the machines. A survey grade GPS receiver will record the actual signals coming out of the satellites. GPS satellites are essentially an atomic clock with a radio station attached to it. And they fly around the world, twice a day, at about 9,000 miles an hour. In fact, in the time it takes to say GPS satellite, a GPS satellite will move about 2.3 miles. But with computer techniques we can go super slow-mo with the satellites and we can pretend like they're standing still. We'll measure the time that it takes for a signal to go from the satellite to the GPS receiver and the time is actually different between the receiver at the known location and the receiver at the unknown location. We can tease out these tiny differences to determine the distance from the receivers to the satellite and then by basic geometry we'll figure out the difference in the locations between the known and unknown locations.

HOST: OK, so how accurate then is the data or the coordinate that the user gets back? Is the system still being refined?

JOE EVJEN: About ten years ago it was good to a decimeter and now we refine techniques where we can get positions for users usually within a centimeter or two. When we're able to collect data for 24 hours a day at our known locations, we're able to consistently produce positions good within about a millimeter.

HOST: That's pretty accurate.

JOE EVJEN: That's pretty accurate.

HOST: Can anyone use this online system or was it designed for a specific audience?

JOE EVJEN: It's open to anyone, but the price of entry is about $10,000 in GPS equipment. We're not talking about a dashboard GPS, we're talking about a dual frequency survey grade unit. They have come down in price very much from about $100,000, now they cost around $10,000. We do have techniques that are usable with cheaper receivers, but we haven't found a business case for them yet.

HOST: Joe, what do you think is the main benefit of this system?

JOE EVJEN: OPUS helps make the National Spatial Reference System real. Even though we put out a million bench marks, we never put them in exactly the same place you wanted. We put them out 70 years ahead of time and they have moved in the interceding years. With OPUS, you don't need the government to come out and set a bench mark in your area. If you have a clear view of the sky, you can establish your own bench mark and determine your own latitude, longitude, and height very precisely on the day that you need it.

HOST: So, does your office retain or archive the information in some way following upload of these approximately 30,000 monthly survey files?

JOE EVJEN: For the most part we forget. Due to privacy concerns, much of the data just get washed out at the end of the day. We do try to remember where people use it and keep usage statistics. We define our success by how often and well it's used. And it is used in just about every county and all around the country.

There are about one percent of users however, they think that the position they've got is useful for others, say it's on a tidal bench mark or engineering project for the Corps of Engineers, so they'll ask us to keep the data and they'll give the mark a name and they'll upload a description and some photographs so that other people can go out and visit the same mark later and make better use of the data that they've already collected.

HOST: And just to remind our listeners, we talked about this in an earlier podcast, but if we can touch on what is the National Spatial Reference System again?

JOE EVJEN: Sure. You can think of the National Spatial Reference System as standard time, but in four dimensions instead of one. We use standard time of course to make sure our VCRs start to record programs at the right time or to meet up with people at the same time. Standard time helps coordinate fiber optic communications and the Internet and cellular phone technologies, all based on very precise clocks. In the same way that time works in a single dimension, latitude, longitude, and height, and time now work in four dimensions to help people coordinate their activities.

HOST: And Joe, for those listeners who aren't in your immediate user community that we've talked about, what is the benefit to them that a product such as the online positioning user service even exists?

JOE EVJEN: If you need to get a survey done, it can be done more cheaply. If you need products delivered to stores from across the world, they can arrive at ports and airports more effectively, and then your dashboard GPS can get you from your house to the store.

HOST: Great! Do you have any final, closing words for our listeners today?

JOE EVJEN: I'd say commerce relies on good coordination of activities and good coordination begins with good coordinates.

HOST: Thanks Joe for joining us on Diving Deeper and talking about our accurate and consistent positions on the Earth's surface. To learn more, please visit

That's all for today's show. Remember, if you have questions on this episode or the National Ocean Service in general, you can contact us at And if you're on social media, don't forget you can find us, it's usoceangov, on Facebook, Twitter, Flickr, and YouTube. Please join us for our next episode in two weeks.