Great Wall from Space: The Answer

Original question posed here: Is it really true, as we've heard a million times, that the Great Wall of China is the only man-made structure you can see from space? Can you see the Great Wall from space at all? Is there an easy way to figure this out at home?

Summary answers:
1) Is it the only structure you can see from space? Far from it. By far the easiest structures to pick out are ones with perfectly regular -- and therefore "unnatural" contours. Airports. Malls or stadiums surrounded by huge parking lots. Bridges, or as we'll see, miles-long dike/ causeways. Plus the Pyramids of Egypt in the right light. Etc. Below is a NASA satellite look at the Afsluitdijk in Holland. (Click for larger.)


2) Can you even see the Great Wall from space? No. Or probably not, in any normal definition of "seeing." NASA answer on the subject here. Details after the jump.

3) Could you figure this out at home? Yes, and even without searching for that NASA answer. What do you think Google Earth, with its ability to view the globe from different perspectives, is for? (Yes, photo quality varies for images of northern China, but you get the idea of how hard it is to pick out the wall.)

Details, qualifications, more photos after the jump. Thanks to readers RZ, PA, JS, AR. 

Why can't you see the Great Wall from space? In the authoritative NASA post, Kamlesh Lulla gives this illustration of what it looks like -- red and yellow arrows pointing to wall segments -- and explains that, "In fact, it is very, very difficult to distinguish the Great Wall of China in astronaut photography, because the materials that were used in the wall are similar in color and texture to the materials of the land surrounding the wall -- the dirt."


Reader RZ, who sent the NASA link, adds:

I would suspect that part of the lack of visibility stems, as well, from the way the wall follows what appears to be watershed lines, an organic form instead of a man-made form composed of straight lines and square corners. The same posts shows astronaut photographs where the pyramids and the Toyota Center in Houston are clearly visible. The pyramids, at least, are built out of native-colored materials, but with the straight lines and equilateral triangles indicate man-made structures; hence my guess at form as wall as substance as the disguising factor.

From reader AR, a suggestion that we need to clarify the whole question:

My answer to whether the wall is visible is to look at Google Maps.[OK!] But I'm not sure I buy the rest of the arguments, i.e. should be clearly visible from space, should not be light/shadow assisted etc. Maybe the thing is to clearly define the parameters.

For instance: maybe you can see it clearly from space with binoculars but not with the naked eye. But even if you can only see the shadow, or only see a long ribbon, why wouldn't that count as seeing? From here on earth, there are a number of things you can see on the moon, and unless you'd read about them, you would have no idea what they are. They could be craters, they could be seas with water, they could be an old lady. But if someone said, you can see the craters on the moon from earth, would you disagree, because it is not clearly visible as such? How about if you saw a distant star, claimed it was a sun, would the claim be that since it was not clearly visible to be similar to our sun, it wasn't visible as such? Finally, you can see Saturn with your naked eye from Earth (on the right day, in appropriate conditions etc), but even when you could, you would not treat it as the breathtaking experience that it is when seen through a telescope. But you can still see it from here.

My guess is that when it comes to seeing things in/from space, our definitions of visible and not are looser than seeing things on earth.

And for those who want to approach the problem mathematically, reader JS explains how:

The distance the wall could be seen depends on the angular resolution of the imaging system (the eye) and the contrast of the wall vs. background.

For the eye: 20 / 20 vision = about 1 minute of arc = about 0.016 666 666 667 degree.

To measure resolution, consider a line pair -> i.e. the wall is a line but to differentiate it, there must be a contrasting equal width along side. So if the wall is 9m wide, use 18m as the width to consider resolution.

Distance for 1 minute of arc = 18m / Sin(0.016 666 666 667) = about 62 Km

If the wall is narrower, say 4m,then use 8. 8m / sin (0.016 666 666 667) = about 27 km.

This doesn't take into account atmospheric distortion, refraction through a window, or less than ideal contrast, all of which would lessen the real distance at which it could be discerned.

Which leaves only the question: If a frog were placed in a pot of tepid water on top of the Great Wall and slowly heated, what would you notice from space?