Can You See The Great Wall of China From Space?

Area north of Beijing, China, showing the Great Wall of China running through the centre. Image acquired by Sentinel-2 on 27th June 2017. Data courtesy of ESA/Copernicus.

Dating back over two thousand three hundred years, the Great Wall of China winds its way from east to west across the northern part of the country. The current remains were built during Ming Dynasty and have a length of 8 851.8 km according to 2009 work by the Chinese State Administration of Cultural Heritage and National Bureau of Surveying and Mapping Agency. However, if you take into account the different parts of the wall built by other dynasties, its length is almost twenty two thousand kilometres.

The average height of the wall is between six and seven metres, and its width is between four to five metres. This width would allow five horses, or ten men, to walk side by side. The sheer size of the structure has led people to believe that it could be seen from space. This was first described by William Stukeley in 1754, when he wrote in reference to Hadrian’s Wall that ‘This mighty wall of four score miles in length is only exceeded by the Chinese Wall, which makes a considerable figure upon the terrestrial globe, and may be discerned at the Moon.’

Despite Stukeley’s personal opinion not having any scientific basis, it has been repeated many times since. By the time humans began to go into space, it was considered a fact. Unfortunately, astronauts such as Buzz Aldrin, Chris Hatfield and even China’s first astronaut, Yang Liwei, have all confirmed that the Great Wall is not visible from space by the naked eye. Even Pixalytics has got a little involved in this debate. Two years ago we wrote a blog saying that we couldn’t see the wall on Landsat imagery as the spatial resolution was not small enough to be able to distinguish it from its surroundings.

Anyone who is familiar with the QI television series on the BBC will know that they occasionally ask the same question in different shows and give different answers when new information comes to light. This time it’s our turn!

Last week Sam was a speaker at the TEDx One Step Beyond event at the National Space Centre in Leicester – you’ll hear more of that in a week or two. However, in exploring some imagery for the event we looked for the Great Wall of China within Sentinel-2 imagery. And guess what? We found it! In the image at the top, the Great Wall can be seen cutting down the centre from the top left.

Screenshot of SNAP showing area north of Beijing, China. Data acquired by Sentinel-2 on 27th June 2017. Data courtesy of ESA/Copernicus.

It was difficult to spot. The first challenge was getting a cloud free image of northern China, and we only found one covering our area of interest north of Beijing! Despite Sentinel-2 having 10 m spatial resolution for its visible wavelengths, as noted above, the wall is generally narrower. This means it is difficult to see the actual wall itself, but it is possible to see its path on the image. This ability to see very small things from space by their influence on their surroundings is similar to how we are able to spot microscopic phytoplankton blooms. The image on the right is a screenshot from Sentinel Application Platform tool (SNAP) which shows the original Sentinel-2 image of China on the top left and the zoomed section identifying the wall.

So whilst the Great Wall of China might not be visible from space with the naked eye, it is visible from our artificial eyes in the skies, like Sentinel-2.

China’s Geo-Information Survey

Yuqiao Reservoir, east of Beijing, China from Landsat 8 acquired March 2017. Data courtesy of NASA/USGS,.

The first national geo-information study of China was released last week at a State Council Information Office press briefing.

The study, also referred to as the national census of geographic conditions, was originally announced in March 2013. Over the last three years 50,000 professionals have been involved in collecting a variety of data about China and it’s reported that they have achieved a 92% coverage of the country, generating around 770 terabytes of data in the process.

Data has been collected on natural resources, such as land features, vegetation, water and deserts; together with urban resources such as transport infrastructure, towns and neighbourhoods. This information was gathered, and verified, through remote sensing satellites, drones, aerial photography, 3D laser scanning and in-situ data. It’s reported that the accuracy is 99.7% with a 1 m resolution.

China is one of the largest countries in the world by land mass, at approximately 9.6 m square kilometres. Therefore, simply completing such a study with the accuracy and resolution reported is highly impressive.

It may take years to fully appreciate the variety, size and usefulness of this new dataset. However, a number of interesting high level statistics have already been released by the Chinese Ministry of Land and Resources including:

  • 23.2% of China’s land is above 3,500 m altitude, and 43.4% is below 1,000 m altitude.
  • 7.57 million sq km of the country has vegetation cover, with 21.1% being cultivated lands and the remainder grasslands and forests.
  • 1.3 million sq km of land is desert and bare lands, whilst rivers cover 6.55 million sq km.
  • 153,000 sq km of land has buildings on it.
  • 116,500 sq km of railway track and there is 2 million sq km of roads.

According to Kurex Mexsut, deputy head of the National Administration of Surveying, Mapping and Geoinformation, the Chinese Government will be looking to establish a data sharing mechanism and information services platform for this dataset, together with a variety of data products. It is hoped that public departments and companies will be able to use this to help improve the delivery of public services.

Although not from the survey, the image at the top is of the Yuqiao Reservoir, situated just to the east of Beijing. It has a surface area of 119 sq km, with an average depth of 14 metres.

Not only is this a comprehensive geo-information dataset for a single country, but there is also huge potential for further information to be derived from this dataset. We’ll be watching with interest to see how the data is used and the impact it has.

Four reasons why 2016 will be big for Earth observation

Artist's rendition of a satellite - 3dsculptor/123RF Stock Photo

Artist’s rendition of a satellite – 3dsculptor/123RF Stock Photo

2016 has caught its first few rays of sunlight, but is already shaping up to be an exciting year for Earth observation (EO). Here are four reasons why:

Reason One
China launched the world’s most sophisticated geostationary satellite, Gaofen 4, on the 28th December – okay I know that was technically 2015, but it will begin operating in 2016! Gaofen 4 is part of the China High-Resolution Earth Observation System (CHEOS) that aims have a suite of seven high definition satellites, with varied specifications, providing real-time all day global coverage in all weathers by 2020. Unusually for EO, the Gaofen 4 high-resolution satellite is in a fixed-point 36,000 km geosynchronous orbit focusing on China and the surrounding area.

It has two optical instruments: a visible light imager with 50 m resolution, and an infrared imager with 400 m resolution. The main applications are disaster prevention, disaster relief, agricultural planning and climate change monitoring.

Reason Two
NASA awarded the contract to build Landsat-9’s Operational Land Imager-2 (OLI) instrument to Ball Aerospace & Technologies Corporation on 31st December – yes, I know that was 2015 too!

OLI will have eight spectral wavebands with a resolution of 30 m, and one panchromatic waveband with a resolution of 15 m. This will help extend the Landsat archive that has over 40 years of continuous satellite images. Interestingly despite having a similar number of optical bands as ESA’s comparable Sentinel 2 satellites; the spatial resolution is poorer as Sentinel 2 has 10 m resolution for its visible wavebands.

Reason Three
There are a number of significant EO satellite launches planned for the coming twelve months. Highlights include for:

  • Jason 3 ocean altimetry mission on January 17th
  • ESA’s Sentinel 3A on 4th February
  • Astro-H X-ray observatory on February 12th
  • ESA’s Sentinel-1B on 14th April
  • Ten SkySat Earth observation satellites for Google/Skybox Imaging over the summer
  • Worldview 4 in September
  • Geostationary Operational Environmental Satellite R-Series Program (GOES-R), a NASA/NOAA next-generation geostationary weather satellite, in October
  • Planet Labs are expected to deploy a significant number of small satellites from the International Space Station during the year, starting with Flock 2e’s twelve satellites, to enable them to provide terrestrial images for the entire Earth.

Reason Four
EO is a growing industry that had sales of $1.6 billion in 2014, up 60% from five years earlier. With the investment and development currently happening within the industry, it is anticipated that this growth will continue. Pixalytics is one example!

The focus in this, and future years, will be getting a broader user base for satellite imagery including providing more operational services using near real time imagery. This should offer potential new applications, services and markets to support the ongoing growth.

You can be part of it! Satellite imagery is no longer just for governments, space agencies or research bodies. Satellites still provide the large scale climate change, ocean and land monitoring; but there can also provide small scale support on everything from crop/field management, building and smart city planning, traffic/parking monitoring and even counting animals from space.

If you want to see how Earth observation might benefit your company, get in touch. We’d be happy to talk through what might be possible – you’ll never know unless you ask!

Sentinel-2A Data Released Into The Wild

False Colour Image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

False Colour Image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

Sentinel -2A is already producing some fantastic images, and last week ESA announced the availability of Sentinel-2A orthorectified products in the Sentinel Data Hub. This will enable Sentinel-2 data to be accessed more widely, although as we found out this week there are still a few teething problems to sort out.

At the top of the blog is a stunning image of the Chinese city of Qingdao, in the eastern Shangdong province. The false colour image shows the city of Qingdao and the surrounding area with the centre dominated by Jiaozhou Bay, which is natural inlet to the Yellow Sea. The bay is 32 km long and 27 km wide, and generally has a depth of around ten to fifteen metres; although there are deeper dredged channels to allow larger ships to enter the local ports. The bay itself has decreased by around 35% since 1928, due to urban and industrial growth in the area.

Jiaozhou Bay Bridge a sub-set of a false colour image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

Jiaozhou Bay Bridge a sub-set of a false colour image of Qingdao, China, acquired by Sentinel-2A on the 21st August 2015. Data courtesy of ESA.

There is a tenuous linguistic link between Plymouth, where Pixalytics is based, and Qingdao. Plymouth is branded as Britain’s Ocean City and Qingdao is home to the Ocean University of China. Qingdao does however, have a much greater claim to fame. It is home to the World’s Longest Bridge. The Jiaozhou Bay Bridge is 42 km long and transects the bay. It is clearly visible on the satellite image, although you might not be able to see it on the thumbnail image at the top of the blog. Therefore, if you look at the subset to the right, you should be able to see bridge clearly and boats on the bay.

Now Sentinel-2A data has been released into the Sentinel Data Hub, images like this are waiting for everyone in the world to discover. We’ve been testing Sentinel-2A data for a few months already, as were part of the community who gave feedback to ESA on the quality of the data. Sentinel-2A carries a Multispectral Imager (MSI) that has 13 spectral bands with 4 visible and near infra-red spectral bands with a spatial resolution of 10 m, 6 short wave infrared spectral bands with a spatial resolution of 20 m and 3 atmospheric correction bands with a spatial resolution of 60 m. When the identical Sentinel-2B is launched in late 2016, the pair will offer a revisit time of only 5 days.

The data from Sentinel-2A forms part of the Copernicus program and is freely available to use, as such it is bound to be very popular. So popular in fact, we found it difficult to get on the Data Hub this week, with slow data speeds and a few elements of the functionality not working efficiently. Although, we’re sure that these will be resolved quickly. Also, there are user guides and tutorials available on the website to help people use the data hub.

The Sentinel-2A data release, following on from the microwave data from Sentinel-1, is a watershed moment for Earth Observation companies, given their spatial resolution, revisit time and free availability, they offer a unique opportunity to develop satellite data services. We’re intending to use this data, are you?

Ocean Colour Cubes

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

August 2009 Monthly Chlorophyll-a Composite; data courtesy of the ESA Ocean Colour Climate Change Initiative project

It’s an exciting time to be in ocean colour! A couple of weeks ago we highlighted the new US partnership using ocean colour as an early warning system for harmful freshwater algae blooms, and last week a new ocean colour CubeSat development was announced.

Ocean colour is something very close to our heart; it was the basis of Sam’s PhD and a field of research she is highly active in today. When Sam began studying her PhD, Coastal Zone Color Scanner (CZCS) was the main source of satellite ocean colour data, until it was superseded by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) that became the focus of her role at Plymouth Marine Laboratory.

Currently, there are a number ocean colour instruments in orbit:

  • NASA’s twin MODIS instruments on the Terra and Aqua satellites
  • NOAA’s Visible Infrared Imager Radiometer Suite (VIIRS)
  • China’s Medium Resolution Spectral Imager (MERSI), Chinese Ocean Colour and Temperature Scanner (COCTS) and Coastal Zone Imager (CZI) onboard several satellites
  • South Korea’s Geostationary Ocean Color Imager (GOCI)
  • India’s Ocean Colour Monitor on-board Oceansat-2

Despite having these instruments in orbit, there is very limited global ocean colour data available for research applications. This is because the Chinese data is not easily accessible outside China, Oceansat-2 data isn’t of sufficient quality for climate research and GOCI is a geostationary satellite so the data is only for a limited geographical area focussed on South Korea. With MODIS, the Terra satellite has limited ocean colour applications due to issues with its mirror and hence calibration; and recently the calibration on Aqua has also become unstable due to its age. Therefore, the ocean colour community is just left with VIIRS; and the data from this instrument has only been recently proved.

With limited good quality ocean colour data, there is significant concern over the potential loss of continuity in this valuable dataset. The next planned instrument to provide a global dataset will be OLCI onboard ESA’s Sentinel 3A, due to be launched in November 2015; with everyone having their fingers crossed that MODIS will hang on until then.

Launching a satellite takes time and money, and satellites carrying ocean colour sensors have generally been big, for example, Sentinel 3A weighs 1250 kg and MODIS 228.7 kg. This is why the project was announced last week to build two Ocean Colour CubeSats is so exciting; they are planned to weigh only 4 kg which reduces both the expense and the launch lead time.

The project, called SOCON (Sustained Ocean Observation from Nanosatellites), will see Clyde Space, from Glasgow in the UK, will build an initial two prototype SeaHawk CubeSats with HawkEye Ocean Colour Sensors, with a ground resolution of between 75 m and 150 m per pixel to be launched in early 2017. The project consortium includes the University of North Carolina, NASA’s Goddard Space Flight Centre, Hawk Institute for Space Sciences and Cloudland Instruments. The eventual aim is to have constellations of CubeSats providing a global view of both ocean and inland waters.

There are a number of other planned ocean colour satellite launches in the next ten years including following on missions such as Oceansat-3, two missions from China, GOCI 2, and a second VIIRS mission.

With new missions, new data applications and miniaturised technology, we could be entering a purple patch for ocean colour data – although purple in ocean colour usually represents a Chlorophyll-a concentration of around 0.01 mg/m3 on the standard SeaWiFS colour palette as shown on the image at the top of the page.

We’re truly excited and looking forward to research, products and services this golden age may offer.

Controlling the Space Industry Narrative

The narrative of the satellite industry over the last week had all the components of a blockbuster novel or film: with new adventures beginning, dramatic challenges to overcome, redemption and an emotional end.

Artist's rendition of a satellite - paulfleet/123RF Stock Photo

Artist’s rendition of a satellite – paulfleet/123RF Stock Photo

Like lots of good stories, we start with those characters setting off on new adventures. Firstly, China launched its most powerful imaging satellite, Gaofen-2. It carries a High Resolution Optical Imager capable of providing images with a spatial resolution of 80cm in panchromatic mode and 3.2m in multispectral mode, and has a swath width of 48km. It is the second in series of seven Earth observation (EO) satellites, following Gaofen-1 launched in April 2013, which will provide environmental monitoring, disaster management support, urban planning and geographical mapping. The Long March 4B rocket launched Gaofen-2, redeeming itself following a failure last December causing the loss of the CBERS-3 EO satellite. The second significant launch was from the International Space Station on the 19th August, when the first pair from the twenty-eight constellation satellites of Flock 1B were launched; with further pairs sent on the 20th, 21st and 23rd. Flock 1B is part of three earth imaging nanosat constellations from Plant Labs, providing images with a spatial resolution of between 3 – 5m.

ESA’s Galileo satellites, Doresa and Milena, provided the drama by failing to reach their planned altitude of 29.9km, reaching an orbit of 26.9km; in addition, their inclination angle is 49.8 degrees to the equator, rather than 55 degrees. They were the fifth and sixth satellites in Europe’s version of the American GPS satellite navigation system, launched on the Soyuz rocket. Getting the satellites to the correct position is likely to require more fuel than they carry. Like Long March 4B, Soyuz will get its chance of redemption in December with the launch of the next two Galileo satellites.

The Tropical Rainfall Measuring Mission (TRMM), a joint mission between NASA and Japan Aerospace, provides the emotional end to our story with the announcement last week that it had run out of fuel. Launched in 1997, TRMM had a three year life expectancy, but will now provide an incredible nineteen years worth of data. It will continue collection until early 2016, when its instruments will be turned off in preparation for re-entry.

It’s interesting to see how this news has been reported in the mainstream media, little mention of China’s progress, or the second Flock constellation or the amazing longevity of TRMM; instead, the focus was the failure of the Galileo satellites. There is rarely widespread coverage of the successful launches of satellites, but there is a push within the UK for the community to celebrate our successes more so the full range of space activities can be seen.

Earth observation is all about data and images, and whilst these may interest people, it’s only through the power of storytelling that we can describe the positives of the industry motivating and inspiring people. Remember to create stories for your industry, and your company, or someone else will dictate the narrative.