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.

Optical Imagery is Eclipsed!

Solar eclipse across the USA captured by Suomi NPP VIIRS satellite on 21st August. Image courtesy of NASA/ NASA’s Earth Observatory.

Last week’s eclipse gave an excellent demonstration of the sun’s role in optical remote sensing. The image to the left was acquired on the 21st August by the Visible Infrared Imaging Radiometer Suite (VIIRS) aboard the NOAA/NASA Suomi NPP satellite, and the moon’s shadow can be clearly seen in the centre of the image.

Optical remote sensing images are the type most familiar to people as they use the visible spectrum and essentially show the world in a similar way to how the human eye sees it. The system works by a sensor aboard the satellite detecting sunlight reflected off the land or water – this process of light being scattered back towards the sensor by an object is known as reflectance.

Optical instruments collect data across a variety of spectral wavebands including those beyond human vision. However, the most common form of optical image is what is known as a pseudo true-colour composite which combines the red, green and blue wavelengths to produce an image which effectively matches human vision; i.e., in these images vegetation tends to be green, water blue and buildings grey. These are also referred to as RGB images.

These images are often enhanced by adjustments to the colour pallets of each of the individual wavelengths that allow the colours to stand out more, so the vegetation is greener and the ocean bluer than in the original data captured by the satellite. The VIIRS image above is an enhanced pseudo true-colour composite and the difference between the land and the ocean is clearly visible as are the white clouds.

As we noted above, optical remote sensing works by taking the sunlight reflected from the land and water. Therefore during the eclipse the moon’s shadow means no sunlight reaches the Earth beneath, causing the circle of no reflectance (black) in the centre of the USA. This is also the reason why no optical imagery is produced at night.

This also explains why the nemesis of optical imagery is clouds! In cloudy conditions, the sunlight is reflected back to the sensor by the clouds and does not reach the land or water. In this case the satellite images simply show swirls of white!

Mosaic composite image of solar eclipse over the USA on the 21st August 2017 acquired by MODIS. .Image courtesy of NASA Earth Observatory images by Joshua Stevens and Jesse Allen, using MODIS data from the Land Atmosphere Near real-time Capability for EOS (LANCE) and EOSDIS/Rapid Response

A second eclipse image was produced from the Moderate Resolution Imaging Spectroradiometer (MODIS) sensor aboard the Terra satellite. Shown on the left this is a mosaic image from the 21st August, where:

  • The right third of the image shows the eastern United States at about 12:10 p.m. Eastern Time, before the eclipse had begun.
  • The middle part was captured at about 12:50 p.m. Central Time during the eclipse.
  • The left third of the image was collected at about 12:30 p.m. Pacific Time, after the eclipse had ended.

Again, the moon’s shadow is obvious from the black area on the image.

Hopefully, this gives you a bit of an insight into how optical imagery works and why you can’t get optical images at night, under cloudy conditions or during an eclipse!

First Small Steps in Remote Sensing

The International Space Station is seen in silhouette as it transits the moon at roughly five miles per second, Sunday, Aug. 2, 2015, Woodford, VA.  Photo Credit: (NASA/Bill Ingalls)

The International Space Station is seen in silhouette as it transits the moon at roughly five miles per second, Sunday, Aug. 2, 2015, Woodford, VA. Photo Credit: (NASA/Bill Ingalls)

It’s not often you get given the opportunity to travel, live in an exciting new city and get an incredible internship all in one. So when I heard about the Erasmus+ Programme I applied right away! I wanted to gain more experience in remote sensing.

When I was little I had a very big poster of the moon surface hung on my wall, it had so much detail and I would stare at it every night before I went to bed. After my parents bought my first computer, I started to search for more images of the moon and other planets and I was impressed by the complexity of what I found. This was the beginning of my fascination with remote sensing. When it came to choosing my career path, it was not hard. I knew what I wanted to become and now it sounds, and feels, right to call myself a Geomatics Engineer.

I’m currently studying two undergraduate degrees in Surveying, and Civil Engineering; but it was still hard to find an Erasmus work placement for remote sensing. I managed to find the Pixalytics Ltd with my teacher’s help, as he had previously met Dr Samantha Lavender.

After finding a place to do your internship the rest is should be easy, but not for United Kingdom. Getting my work permit from British Council was a really challenging process, and took me exactly three months. Despite doing everything right, getting responses to my emails for sponsorship was hard. It was the most awful part of the process for me, because there was nothing I could do except wait. Finally, after a lot of patience my visa arrived and I was on my way to Plymouth!

The last issue, and some people’s main concern, is getting accommodation. I did not find it hard to find a place to stay because most of the students were out of town. With a basic search on the internet I found a flat in four days, it is based a few hundred metres from the centre of Plymouth and close to the bus route to Pixalytics.

I thought I had read and traveled enough to be prepared when I stepped off the plane in London, but it was still a shock standing alone with my suitcase and hearing all the British accents around me. At first, it was difficult to adapt to the language as the accents are sometimes hard to understand. But once I’d grasped the pronunciation, I believe I’m improving every week.

Working at Pixalytics will be my first internship experience, and I am so grateful to Samantha Lavender for giving me this opportunity. Working abroad will be a memory and lesson in itself but I hope to also I hope to enhance my discipline and knowledge as well as applying my existing engineering and personal skills.

Getting my internship was a long, difficult and exhausting process, but I realized that it’s totally worth it as soon as I got to Plymouth, If anyone is thinking of applying to the Erasmus+ programme, I would totally recommend it!

Blog by Selin Cakaloglu, Erasmus+ Intern at Pixalytics