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.

Landscape Features Visible From Space

Eye of the Sahara from Landsat 8 on 7th July 2015. Data courtesy of NASA/USGS.

Eye of the Sahara from Landsat 8 on 7th July 2015.
Data courtesy of NASA/USGS.

One of my favourite facts growing up was that the Great Wall of China was the only manmade feature visible from space. Of course, I now know that everything about that statement was wrong, and it is not a fact at all!

Firstly, it is actually very difficult to see the Great Wall of China from space due to the narrowness of the wall and the pixel size of satellites. For example, Landsat has a pixel size of 30 m meaning that it is impossible to distinguish anything smaller than 30 m and features need to be significantly larger to be visible. The astronauts astronauts Chris Hadfield from Canada and China’s Yang Liwei, both said they could not see the Great Wall with the naked eye when orbiting the Earth. US astronaut Leroy Chiao took a picture of the Great Wall from the window of the International Space Station in 2004; however, even this needed to be magnified to be able to see the Great Wall. Sadly, the other side of my childhood fact is also untrue; there are a number of manmade features that can be seen from space, including the Great Pyramids at Giza, the greenhouses of Almeria in Spain and Palm Tree Island in Dubai.

There are, of course, also many natural features visible from space. From the obvious Great Barrier Reef, Uluru (also known as Ayres Rock) and the Grand Canyon; to the more unusual and less well known features such as the Eye of the Sahara, which is the image at the top of the blog. I become aware of the Eye of Sahara, also known as the Richat, through a recent New Scientist article. It’s a 40 kilometre wide series of concentric rings of rocks of different ages, located in the Sahara Desert near Ouadene in Mauritania.

It’s not known precisely how this feature was created, nor why it is so circular; however, it is an interesting anomaly visible from space. The concept of exploring unusual Earth features seen from space is the basis of a new television series due to begin on Discovery UK at the end of this month. The series, Into The Unknown, will see presenter Ed Stafford travel to unusual and unexplained landscape features that have been spotted from satellites.

Who says there is nothing left to discover on Earth? Start scouring your satellite pictures; you never know what you might discover!