Mapping Earth's surface in 3D

Iceland (DLR) TanDEM's view of Iceland: The country was beyond the sight of the shuttle topography mission in 2000

The German satellite radar twins - TanDEM-X and TerraSAR-X - are a year through their quest to make the most precise, seamless map of varying height on Earth.

They've now acquired data across the entire globe at least once. However, some tricky sampling areas, such as tall mountains and thick forests, will require several passes and so we don't expect to see a fully finished product before 2014.

The Digital Elevation Model, or DEM, has become one of those must-have technical tools.

Planners use them to work out where best to put new buildings, roads and railways; airline pilots need the information to fly planes safely; telecos look at DEMs before siting their transmitters; and generals feed the data into autonomous weapons. Archaeologists, volcanologists, glaciologists, hydrologists, ecologists - they all want to know the shape of the Earth's surface. Even a games developer wanting to produce ever more realistic landscapes will have reason to resort to a DEM.

Space shuttle Endeavour famously made a world DEM in 2000. It hung a radar system out of its payload bay and mapped about 80% of the Earth's land surface.

I've heard it said that this is the most used Earth observation dataset in history. Someone can put us right on that if it's wrong, but the popularity of the shuttle's 3D map of the Earth doesn't surprise me.

And it explains why the German space agency (DLR) and Europe's biggest space company, Astrium, are now trying to produce a more detailed and a more extensive version a decade on.

The Shuttle Radar Topography Mission (SRTM) product has a best spatial resolution of 30m by 30m, and a vertical resolution that varies from 16m to 10m.

Artist's impression of TanDEM-X and TerraSAR-X (DLR) The satellites trace a helix across the sky as they move just 150m apart

The intention of the TanDEM mission is to go down to a spatial resolution of 12m by 12m and a vertical resolution of two metres.

"The two metres is what we call the relative accuracy," explained Dr Manfred Zink, the project manager for the TanDEM-X ground segment at DLR. "It means if you take an area one degree by one degree, latitude by longitude, which at the equator is roughly 100km by 100km, you can take any two points in the DEM and the error between the two points is less than two metres," he told me. "In absolute terms, each and every single point has to be better than 10m."

Airbone lidars can achieve much better precision, but these maps are necessarily regional in extent - they will cover only relatively small areas. The purpose of TanDEM is to build a world DEM that is single-source and has "no joins".

Of course, what I find fascinating about the TanDEM mission is the way that data is being acquired.

The pair's radars work by constantly bouncing microwave pulses off the ground and sea surface. By timing how long the signal takes to make the return trip, the instruments can determine differences in height. As they circle the Earth, so they build up their DEM. That much is obvious.

But TanDEM-X and TerraSAR-X do something a little more interesting.

Their slightly offset orbits bring them extremely close to each other - to a separation of just 150m, as of last week.

Think about that for a moment - 514km up, moving at 7km/s with a gap of 150m. It leaves simply no room for error.

This compact orbital dance gives the pair "stereo vision", by enabling them to operate an interferometric mode in which one spacecraft acts as a transmitter/receiver and the other as a second receiver. Varying the geometry allows the pair to discern better the signal they see from different types of surface.

Error map in data acquistion (DLR) Earth is being mapped in strips. Some hard to see areas will need perhaps three or four more acquisitions

If you look at the map of acquisition, the colours tell you how well the TanDEM mission is progressing.

The green areas are where the two-metre requirement has already been met.

Yellow denotes those areas that will need at least a second acquisition. If you know your geography, you can see these include forested and desert locations like the Sahara which are not easy surfaces to read for radar.

Red areas are in most need of further acquisitions. You'll see these include the big mountain ranges.

"We always image a swath at an off-nadir angle of 30-40 degrees. In other words, we don't look straight down," said Dr Zink.

"So, if you see a tall mountain and you image it only from one side, there is going to be a slope on the farside that you cannot see. This applies to places such as the Alps, the Andes and the Himalayas where after one acquisition, we still do not yet have enough data. These are the gaps we have at the moment."

If you are wondering, the grey describes those areas where the data has been acquired but not yet processed.

Salar de Uyuni Salar de Uyuni: The largest salt flats (blue) in the world covering 10,000 sq km, located in southwest Bolivia

This is a monumental job. SRTM collected 8.6 terrabytes of data. TanDEM will collect 350 terrabytes of data in mapping the Earth's roughly 150 million square kilometres.

All the radar information has to be downlinked to ground stations, and in the case of the remote O'Higgins facility in Antarctica, the data is even put on tapes and physically carried back to Germany.

"The first coverage of the Earth is done and was very successful," says Dr Vark Helfritz from Astrium Geoinformation Services.

"The test data is really very promising and we believe we could potentially outperform the original specification.

"We need double coverage for the easier parts of the Earth's surface, and triple - and maybe even quadruple - for the more difficult parts.

"The mountains in northwest America and Norway - its water fjords - have been particularly challenging. But the hardware is working really well.

"I'm really looking forward to the first dual coverage because I think when you see that, you'll really notice a big difference."

The TanDEM-X Digital Elevation Model of Mount Etna, Sicily, Italy The TanDEM Digital Elevation Model of Mount Etna, Sicily, Italy
Jonathan Amos Article written by Jonathan Amos Jonathan Amos Science correspondent

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  • rate this

    Comment number 16.

    #15 sporpo

    I think the real difference is that you can get a good radar reflection off the surface of water- whereas the land is going to be largely radar invisible.
    Even in rough weather every wave will have surfaces facing in the right direction plus water is a single material with relatively predictability including high radio reflectivity. Land is a mixture but is mainly non-radio reflective.

  • rate this

    Comment number 15.

    I think I understand a little better now. There is a difference between measuring a surface like the ocean where you can assume that it is broadly level and the average water height peaks and troughs is equal to the sea level, but the solid ground is more uneven so you cannot average it out except over the 12m squares described in the article.

  • rate this

    Comment number 14.

    12. That's to do with the significant wave height and not Sea Level. These are two different things, the actual sea level varies very slowly both spatially and temporally. I know it doesn't seem like it when seasick but when talking about sea level even large waves have no appreciable effect.
    Back on DEMs by the way the key error here is the absolute error of 10m not the relative one.

  • rate this

    Comment number 13.

    "If you think that's poor I would like to see you do better."
    Well yes, of course I can't do better, but then I haven't got a multi-million euro spacecraft. I stand by my comment. We can measure the distance to the moon to centimetre accuracy and it is about 1,000 times further away than the distance at which these satellites are orbiting.

  • rate this

    Comment number 12.

    10. The sea is fairly lively, Several members of my family suffer seasickness when on ferries due to being bounced about by it and I have surfed waves the size of houses that do change their position in time and space a fair bit, the topography of the land seems relatively stable in comparison.

  • rate this

    Comment number 11.

    "Think about that for a moment - 514km up, moving at 7km/s with a gap of 150m. It leaves simply no room for error."

    I thought about it. Their relative speed is about zero, and the height has nothing to do with it, so if it's the danger of a crash you're worrying about 150 m is fairly safe, I'd say.

    I often sit less than a metre from my wife and we've hardly ever crashed.

  • rate this

    Comment number 10.

    The reason Sea level accuracy is mm is due to Satellite Radar Altimetery, the high reflectivity of the water and because the sea level changes very slowly in height over a very large area.
    Land topography changes very quickly and is therefore harder to get accurately however (shameless plug) the ACE2 GDEM has used the accuracy of the altimeter to improve the accuracy of the SRTM and is also free!

  • rate this

    Comment number 9.

    My brother works in a company that actually uses this kind of data. They are building detailed verifiable models of the territory around airports, that are designed to improve the efficiency and safety of GPS landing and take-off systems. I am sure they will be using these new data sets.

  • rate this

    Comment number 8.

    IF illegal systems like HAARP create artificial but catastrophic disturbances, this new system would be able to question what has transpired. I see its development as well worth the effort; I can foresee feedback to such nations as China, Pakistan, Haiti - areas that, in accordance with the system, should (or should not) have been struck with flooding, earthquakes, etc.

  • rate this

    Comment number 7.

    Sporpo, there are radar and laser altimeters that bounce beams off the surface to measure spot heights. These are much more accurate for height measurements but lack the horizontal coverage of interferometry. A third alternative is stereoscopy which uses optical images from different angles to provide stereo vision (i.e. ASTER GDEM). Though this has poor vertical accuracy relative to TanDEM-X.

  • rate this

    Comment number 6.

    For better understanding Earth as system, to employ data for climate & traffic research. The TanDEM-X capabilities are to be used to address questions of land usage & vegetation, hydrology, geology & glaciology. System can also generate information about height of snowline or change in ice masses, as well as provide geological maps of regions subject to volcanic and/or earthquake activity.

  • rate this

    Comment number 5.

    Satellite mission is first of its kind; it remains unique & highly complex, even for experienced engineers. Radar system views ground from 2 different points, achieving 'depth perception' - like binocular vision in humans. But what's its purpose? Intention is to create exceptionally accurate, global & homogeneous 3D elevation model of equal interest for commercial & scientific purposes.

  • rate this

    Comment number 4.

    The SRTM data set is free to use. This will make it quite popular for some time to come with students and hobbyists. I hope TanDEM (cute) will in future make portions of their data available for all.

    The longer duration mission should give us fantastic insights into how dynamic the Earth's surface really is.

  • rate this

    Comment number 3.

    2m accuracy mapping an approximate sphere 12756000m across you think? That's 0.00000015%! Sounds amazingly accurate to me. The sav-nav in your car which uses minute differences in the time taken a beam of light to travel to it from an average of four satelites are accurate to within about 10m and they allow you to forget where you are. If you think that's poor I would like to see you do better.

  • rate this

    Comment number 2.

    Terrabytes. Earth mapping. Hohoho, very drôle.

  • rate this

    Comment number 1.

    Somehow the claimed accuracy of 2 metres seems very poor. Have we not had missions measuring things like sea level to millimetre accuracy already?


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