# The Great Wall of China visible from space? **URL:** \ **Category:** Cecil's Columns/Staff Reports\ **Created:** [December 2, 2010, 5:39pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686 "2010-12-02T17:39:25Z")\ **Posts on this page:** 12\ **Page:** 2
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [June 8, 2012, 6:03pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/21 "2012-06-08T18:03:49Z")
Since the thread is renewed… > [@Floater](#): > > True. Going from the very large to the very small, [these photos](http://photoity.com/40-brilliantly-captured-macro-photographs-by-john-hallmen/) (insects for the squeamish), are all composites, but I doubt anybody would call them fake. I’m not sure how those are “composites”. They are extremely magnified and thus close focus, but each image appears to be a single image, not a composite of multiple shots of each bug. > [@gnoitall](#): > > The thing is that the spacecraft fly in an orbit which allows them only to see a swath of the Earth about 1800 miles across, so a full globe view like the famous “Earth at Night” is a composite, made by picking the best images (cloud-free, etc.) and stitching them together (like with photoshop). Since the ground processing of those image swaths calculates accurate geolocation for each point (the magic of orbital mechanics and trigonometry), it’s easy to place any given chunk of OLS swath on the correct part of the map projection, so the glow of city lights really is being placed where the city is, and not by eyeball or paintbrush. The thing is, since the data is collected in swaths anyway, any image showing multiple swaths at once is a digital composite. But the very process that allows them to make any picture this way is what allows them to correctly make the clear sky composites. Just substitute the swath that doesn’t have clouds. But as you say, this is a far thing from “fake”. All image content is from that location and is correctly aligned with all other image content. ---
**Author:** ![Veneriable\_Slacks](https://avatars.discourse-cdn.com/v4/letter/v/838e76/32.png) [@Veneriable\_Slacks](https://boards.straightdope.com/u/Veneriable_Slacks)\ **Post date:** [June 16, 2012, 3:48pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/22 "2012-06-16T15:48:19Z")
The reason you can’t see the Great Wall of China is because of simple physics. The word in physics to describe the reason why is called “resolution”. To see the Great Wall of China from a geo-orbitting satellite would require a big lens, really big. And I’m too drunk and tired to calculate now. Maybe 10 meters in diameter? Any sober physics professors out there? Veneriable Slacks ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [June 17, 2012, 4:17am UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/23 "2012-06-17T04:17:26Z")
Only problem is that “space” starts much lower than geosych orbit. ---
**Author:** ![Derleth](https://avatars.discourse-cdn.com/v4/letter/d/b9e5f3/32.png) [@Derleth](https://boards.straightdope.com/u/Derleth)\ **Post date:** [June 17, 2012, 5:18am UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/24 "2012-06-17T05:18:29Z")
> [@Irishman](#): > > Only problem is that “space” starts much lower than geosych orbit. Right. Apparently, what we call the ‘edge of space’ right now is the [Kármán line](http://en.wikipedia.org/wiki/K%C3%A1rm%C3%A1n_line), a paltry 62 miles or so (100 km) above sea level. That’s roughly the point where the atmosphere has become so thin a craft at that height might as well not have wings at all: It would need to go fast enough to orbit the Earth to maintain its altitude. (To fly in an atmosphere you need to go fast enough the wind beneath your wings supports you, which is a function of how dense the air is, with thinner air requiring higher velocity; to orbit, you need to go fast enough your momentum counters the force of gravity such that you’re constatly falling and always miss the ground, which is a function of the acceleration due to gravity, which decreases as the inverse square of your distance from the ground (don’t make me go into barycenters or I swear to Gődel I’ll make you compute the gravitational attraction between Uranus and mine). The Kármán line is roughly where those velocities are equal, rounded to a nice number of kilometers.) (There are other definitions, such as the one we use to determine who gets to be called an astronaut.) By comparison, a geosynchronous orbit is about 22,236 miles (or about 35,786 Canadian) above mean sea level; this is an orbit such that the craft goes around the Earth at the same speed the Earth itself rotates, maintaining a fixed position in the sky. ---
**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\ **Post date:** [June 17, 2012, 1:58pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/25 "2012-06-17T13:58:32Z")
Our friend, **the Bad Astronomer** , has regularly posted videos taken from the space shuttles of the night side of Earth with city lights clearly visible. E.g., [http://blogs.discovermagazine.com/badastronomy/2011/09/17/flying-around-the-earth/](http://blogs.discovermagazine.com/badastronomy/2011/09/17/flying-around-the-earth/) One of the first historical tales of city lights being visible at night was during John Glenn’s [Friendship 7](http://en.wikipedia.org/wiki/Mercury-Atlas_6) flight when he commented on seeing lights, turned out that some folks in Perth turned a bunch of lights on just to make it easier for him to spot them. While the better “Earth at night” photos are composites of longish exposures from higher orbits, seeing cities at night from low orbit is basically unavoidable. ---
**Author:** ![njtt](https://avatars.discourse-cdn.com/v4/letter/n/ecd19e/32.png) [@njtt](https://boards.straightdope.com/u/njtt)\ **Post date:** [July 16, 2012, 12:22am UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/26 "2012-07-16T00:22:16Z")
> [@Chronos](#): > > [If it were faked, APOD would say so](http://antwrp.gsfc.nasa.gov/apod/ap020810.html). > > Why is it so hard to believe? We produce a lot of light. That picture shows a clear light at what must be the Falkland Islands, only city, [Port Stanley](http://en.wikipedia.org/wiki/Port_Stanley), population 2,115. I am skeptical. ---
**Author:** ![Irishman](https://avatars.discourse-cdn.com/v4/letter/i/b487fb/32.png) [@Irishman](https://boards.straightdope.com/u/Irishman)\ **Post date:** [July 16, 2012, 6:30pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/27 "2012-07-16T18:30:03Z")
Why? Light has the distinct feature of standing out from dark. Port Stanley is a modern city with electricity and lighting surrounded by darkness. Looking at the map, you can see the outline of the islands and undersea terrain features by shading. Comparing to a different region, look at Alaska. There are lights showing in the Aleutian islands. One of them is clearly Unalaska, the largest city in the Aleutians, which has a population of 4,376 as of 2010, according to Wikipedia. Another light shows west of there. The best I can tell, that appears to be [Adak](http://www.citytowninfo.com/places/alaska/map), population of **326**. It’s a little hard to tell because (a) that image is stretched differently than the wiki map of Alaska, so island geometry is a little trickier to pick out; (b) I’m relying on Wiki for identifying the population centers of Alaska, which I realize is not the best source. (Pulled a different map search, found Adak.) Which does make the darkness of North Korea curious, but I don’t know enough about the geopolitics to make sense of it. I do see lights associated with most of the major cities on [this map](http://www.lonelyplanet.com/maps/asia/north-korea/). ---
**Author:** ![tullsterx](https://avatars.discourse-cdn.com/v4/letter/t/e9bcb4/32.png) [@tullsterx](https://boards.straightdope.com/u/tullsterx)\ **Post date:** [July 16, 2012, 7:27pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/28 "2012-07-16T19:27:46Z")
> [@Carptracker](#): > > Hijack - Kennedy - is there any way to tell which parts were taken from space and which from an airplane? I live in the middle of nowhere, and last time I looked you could see my full-size van in my driveway (long since gone to the junkyard for scrap, but it lives on in Google Earth, or did last time I looked). I don’t think you could pick out a person if they were standing in my back yard, probably not even if they were laying down, but you can sure see the van. But if you look at Chicago, you can see people walking around on Navy Pier. Makes sense that those would be taken from planes, but I wonder if my house in Missouri also was pictured from the plane. A vast majority of the Google Maps “satellite” images are orthophotos, taken from a series of flights and then stereo-graphically corrected for relief displacement and whatnot. A “single” shot taken from a satellite, or an airplane, would have a lot of “distortion” and would not overlay with other layers, like road networks, political boundaries, etc. Rectified color orthophotos of all of the US were flown and compiled in 2010, that’s mostly what you’re seeing in the US. ---
**Author:** ![Morgenstern](https://avatars.discourse-cdn.com/v4/letter/m/f9ae1b/32.png) [@Morgenstern](https://boards.straightdope.com/u/Morgenstern)\ **Post date:** [July 16, 2012, 9:53pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/29 "2012-07-16T21:53:41Z")
> [@John\_W.Kennedy](#): > > Hate to burst your bubble, but the high-magnification parts of Google Earth were taken by low-flying planes, not by satellite. (And, of course, the _very_ high-magnification parts were taken by a truck.) I’m not sure that’s totally correct. All one has to do is open Google Earth, and proceed to Area 51 for example. There, you can see runways, aircraft on the runways, etc. The magnification is pretty high. Since Area 51 is clearly marked on aeronautical charts as a prohibited (flight) area, regardless of altitude. No pilot would fly in that airspace without significant consequences, unless they had a clearance to do so. ---
**Author:** ![C\_K\_Dexter\_Haven](https://avatars.discourse-cdn.com/v4/letter/c/b2d939/32.png) [@C\_K\_Dexter\_Haven](https://boards.straightdope.com/u/C_K_Dexter_Haven)\ **Post date:** [July 18, 2012, 11:33am UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/30 "2012-07-18T11:33:00Z")
[Aside]In Cecil’s mail last week, someone asked why the Great Wall of China is called that, when it’s made out of bricks.[/aside] ::: duck ::: ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [July 18, 2012, 11:56am UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/31 "2012-07-18T11:56:00Z")
In the mid-to-late 1990s, Geographer Denis Wood wrote a very interesting chapter, in his book _The Power of Maps_, about the digital processing that went into the “Earth as viewed from Space” illustrations, including the “Night” one with the famously dark North Korea. The discussion is in the context of how few of us realize how much human massaging even the most “natural-looking” image requires, let alone any “authoritative-looking” map where all kinds of decisions had to be made about what to include and how to show it. ---
**Author:** ![JKellyMap](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/jkellymap/32/15880_2.png) [@JKellyMap](https://boards.straightdope.com/u/JKellyMap)\ **Post date:** [July 18, 2012, 12:09pm UTC](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686/32 "2012-07-18T12:09:02Z")
> [@Morgenstern](#): > > I’m not sure that’s totally correct. All one has to do is open Google Earth, and proceed to Area 51 for example. There, you can see runways, aircraft on the runways, etc. The magnification is pretty high. Since Area 51 is clearly marked on aeronautical charts as a prohibited (flight) area, regardless of altitude. No pilot would fly in that airspace without significant consequences, unless they had a clearance to do so. Sorry, but it is correct. As another poster mentioned, it’s obvious what parts of GE or GM are air photos vs. what parts are sat. images – even though there is variation in resolution within those two categories. I can’t speak to the specific example you mention, but I’m sure that, generally speaking, the same national governments which fund full-coverage aerial photography projects (among other things, from which to create public-domain topographic maps) will allow any temporary variances from their own aeronautical laws and regulations necessary to complete those projects. (This isn’t to say that some sensitive details don’t occasionally get altered or obscured before the photos or maps are made available to the public. But nothing as big as a runway. These days, especially, that kind of thing wouldn’t work for very long..even India has had a great worldwide high-res sat imagery available for sale since 15 years ago.) [Previous page](https://boards.straightdope.com/t/the-great-wall-of-china-visible-from-space/562686.md?page=1)