The Great Wall of China visible from space?

[dersk]

In the 20 November edition of CBC's Quirks and Quarks, they interviewed Canadian astronaut Chris Hadfield, who's been up on the ISS. He said that while it's true that many human artifacts are visible from space, the Great Wall actually isn't. This is mainly because it's long and narrow and made from local materials, so it blends in. As opposed to a four-lane highway cutting through a forest, say.

Link: http://www.cbc.ca/quirks/episode/201...-trees-for-th/

Column: http://www.straightdope.com/columns/...see-from-space

[Carptracker]

Well, if you've used Google Earth recently, you'd know that even objects as small as a person are visible with the right magnification.

I've been to the Great Wall, and hiked a lot of the restored bits. It is (well, mostly "was") not a single wall, but many walls, almost a network like a road system with many branches, usually running along the mountain ridge tops. Some parts of this (in fact, many, many miles of it) have been restored so they supposedly look like they used to when they were made. Those restored parts are difficult to pick out, even on Google Earth, partly because they are narrow and partly because the magnification is not as high in most parts of China as it is in the USA.

The parts that have not been restored, which is most of the system, are so far gone that they don't look like much at all even if you are standing on them. Just a bit of rough terrain with a bit different rock than the surrounding area Some parts are so grown up you have trouble telling when you are actually on the wall if you are standing on it.

No, you can't see it from space with the naked eye. Not even close.

Search for "Badaling, China" if you want to see the wall from Google Earth. If you search for "Great Wall" or "Great Wall of China" you'll get a bunch of restaurants. Badaling has the most extensive restored section.

[John W. Kennedy]

Originally Posted by Carptracker

Well, if you've used Google Earth recently, you'd know that even objects as small as a person are visible with the right magnification.

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.)

[Carptracker]

Hunh - I did not know that. But nevertheless, I stand by the remainder of my post. I'd almost say that the unrestored portion of the Great Wall of China is not that impressive if you are standing on it, much less from outer space. Except that it is, in its own way. Its impressive as to its length. and in its history and in the extreme amount of person power that must have been required to build such a thing. But now, most of it would be far less visible from outer space than an interstate in the USA.

[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.

[Floater]

My experience is that if you want good aerial pictures you shouldn't use Google Earth. Google maps is much better (I thought both programs used the same photos, but apparently they don't).

[Ed Zotti]

Originally Posted by dersk

In the 20 November edition of CBC's Quirks and Quarks, they interviewed Canadian astronaut Chris Hadfield, who's been up on the ISS. He said that while it's true that many human artifacts are visible from space, the Great Wall actually isn't. This is mainly because it's long and narrow and made from local materials, so it blends in. As opposed to a four-lane highway cutting through a forest, say.

Link: http://www.cbc.ca/quirks/episode/201...-trees-for-th/

Column: http://www.straightdope.com/columns/...see-from-space

Thanks for the tip. The Master has appended an update to the column.

[John W. Kennedy]

Originally Posted by 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.

You can usually work it out from the copyright notice. This is the reason that some places have better resolution than others (different air-photography vendors), and why some places have really bad resolution (only satellite), and why the different zones typically have sharp lines between them.

[b2220128]

According to an article on space.com

Adding a new chapter to a longstanding mythical story about what on Earth is visible from space, several media reported this week that Chinese astronaut Yang Liwei did not see the Great Wall of China during his historic mission.

"I did not see our Great Wall from space," Yang said in an interview with China Central Television.

The discussion also depends on just what you mean by "see from space." Bright electric lights are man made objects and can be seen from space. I suspect that a parabolic reflector carbon arc light shining at the space station would be easily noticed from orbit.

[dhkendall]

Originally Posted by b2220128

The discussion also depends on just what you mean by "see from space." Bright electric lights are man made objects and can be seen from space. I suspect that a parabolic reflector carbon arc light shining at the space station would be easily noticed from orbit.

A bit of a hijack, but tangentially related.

This brings to mind a question I've always wanted the Straight Dope on, namely those pictures of "the earth at night" where you see all the cities lit (here is but one of many examples). Now, while I know the picture is fake to a degree because it's cloudless (regular cloudless satellite pictures of earth are largely just pieced together of cloudless bits taken over the period of many months), I've always wondered about those lights, my gut tells me that each and every one of those lights are fake, that they are all just Photoshopped in using population density (which also makes me at similar pictures which show a darkened North Korea). Surely, you can't see lights like that from space on a cloudless night, can you? Would it look exactly like the pictures?

[Chronos]

If it were faked, APOD would say so.

Why is it so hard to believe? We produce a lot of light.

[dhkendall]

I agree we produce a lot of light, but I didn't think it would look like that on a map, nor did I believe that our light can be seen that way from space.

[John W. Kennedy]

Astronauts have reported being able to see earth lights from the beginning.

Although the intensity of light falls back with the square of the distance, it is canceled out by the fact that the amount of lit-up surface you can see over a given angle increases with the square of the distance. If that weren't so, we could hardly see the moon, which is a lot darker than, say, Tokyo.

[Irishman]

dhkendall said:

Now, while I know the picture is fake to a degree because it's cloudless (regular cloudless satellite pictures of earth are largely just pieced together of cloudless bits taken over the period of many months),

Fake is a harsh word for a digital composite. It accurately shows the ground without ground cover. Sure, digital composites can be used to make fake images, but it's all in how it is done, what source images are used.

I've always wondered about those lights, my gut tells me that each and every one of those lights are fake, that they are all just Photoshopped in using population density (which also makes me at similar pictures which show a darkened North Korea).

And your gut is accurate because...? Why do you think they photoshopped in fake lights vs the technique they describe of making a composite of real photographs?

Surely, you can't see lights like that from space on a cloudless night, can you? Would it look exactly like the pictures?

Why not? Light has this distinct feature of standing out from darkness. Many city lights are not blocked from shining directly into the sky. Even ones aimed at the ground reflect off that ground and shine back into the sky. That's what skyglow it - the brightness in the air around cities that keep you from being able to see stars when the sky is clear.

How far away can you see a car's headlights if not obscured by hills, trees, bushes, etc? Car headlights are ludicrously dim compared to the accumulated amount of light any city is dumping into the sky.

I didn't think it would look like that on a map, nor did I believe that our light can be seen that way from space.

Why not? What would keep it from looking that way? We've already eliminated the clouds by the use of digital composites.

[Floater]

Originally Posted by Irishman

Fake is a harsh word for a digital composite.

True. Going from the very large to the very small, these photos (insects for the squeamish), are all composites, but I doubt anybody would call them fake.

[Irishman]

Furthering the discussion on pictures of Earth at night, and how they look, I give you

http://eol.jsc.nasa.gov/EarthObserva...from_Space.htm

This article shows several pictures, and discusses the techniques the astronauts use to take detailed night pictures. There is a linked video where Astronaut Don Pettit discusses the technique, demonstrates using the "barn-door" tracker he built, and shows a world tour of cities at night and discusses observations about each image.

One thing that stood out to me was comparing the pictures shown with your original sample. In the video Pettit briefly mentions the issue. The first pictures are from higher distances in black and white, at lower resolution - primarily because of the difficulty tracking because of orbital motion. The shutter has to be open long enough to expose the image, but the motion makes it tricky to keep steady.

The more detailed images taken from lower altitudes by astronauts using their steadying apparatus look a lot more distinct. The link I provide includes a side by side day/night comparison of Chicago with the same resolution.

[Elendil's Heir]

Incidentally, NYU "garbologist" Robin Nagle was on PRI's Bob Edwards Weekend radio program this morning, and mentioned in passing that the giant NYC garbage dump/landfill Fresh Kills could be seen from space - said she'd even confirmed it with NASA. I have my doubts.

[Chronos]

"Can be seen from space" is a fairly low hurdle to pass. I have no difficulty at all in accepting that a garbage dump large enough to service New York City could be distinguishable from low Earth orbit. From the Moon? Probably not.

[Elendil's Heir]

Whether or not you can see it from space, it's longer than we thought!: http://news.blogs.cnn.com/2012/06/06...sly-estimated/

[gnoitall]

Originally Posted by dhkendall

I agree we produce a lot of light, but I didn't think it would look like that on a map, nor did I believe that our light can be seen that way from space.

A) The (multiple) spacecraft that produced the imagery orbit at 830 miles above the surface of the Earth. At night, their Operational Line Scan (OLS) instrument has a night mode which is extremely sensitive to light radiating from the Earth's surface and atmosphere, for operational reasons: it can detect and image aurora borealis and australis, which is useful for military space weather observation of auroral extent (an indicator of how much solar particle energy is flowing into the atmosphere), and also pick up on large-area fire phenomena (wildfires).

So, the instrument is plenty sensitive, and 830 miles isn't that far. 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.

Long-winded, sorry. But it's a pretty technical subject, and the truth really is more amazing than a mere "I think it's 'shopped" blowoff.

BTW, the earliest "Earth at Night" composites were made before the early '80s. I can't figure out exactly when, but before when I arrived at Air Force Global Weather Central for active duty in 1983, because they already had an Earth at Night framed poster in one of the hallways. That predates Photoshop by half a decade at least, so if it was faked, it wasn't done using Photoshop or any other PC photo manipulation software.

[Irishman]

Since the thread is renewed...

Originally Posted by Floater

True. Going from the very large to the very small, these photos (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.

Originally Posted by 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.

[Veneriable Slacks]

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

[Irishman]

Only problem is that "space" starts much lower than geosych orbit.

[Derleth]

Originally Posted by 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, 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.

[ftg]

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/ba...und-the-earth/

One of the first historical tales of city lights being visible at night was during John Glenn's Friendship 7 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.

[njtt]

Originally Posted by Chronos

If it were faked, APOD would say so.

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, population 2,115. I am skeptical.

[Irishman]

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, 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.

[tullsterx]

Originally Posted by 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.

[Morgenstern]

Originally Posted by 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.

[C K Dexter Haven]

[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 :::

[JKellyMap]

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.

[JKellyMap]

Originally Posted by 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.)