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How does Google Earth work?


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How does Google Earth work?
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rasmits
rams it
 
Join Date: Oct 2004
Location: Seattle
 
2006-10-16, 20:16

How exactly can it take aerial photos of the Pacific Northwest, for example, without any clouds?
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billybobsky
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Join Date: May 2004
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2006-10-16, 20:30

It only uses photos from clear days...
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rasmits
rams it
 
Join Date: Oct 2004
Location: Seattle
 
2006-10-16, 20:41

Quote:
Originally Posted by billybobsky View Post
It only uses photos from clear days...
Sounds simple enough. Do they have a team of "cloud checkers" or does it do it all on it's own?
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chucker
 
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2006-10-16, 20:44

Quote:
Originally Posted by rasmits View Post
How exactly can it take aerial photos of the Pacific Northwest, for example, without any clouds?
It doesn't. Scroll to Montréal, Ville St. Laurent, and you'll see that there are indeed parts that sadly are covered in thick clouds.
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billybobsky
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2006-10-16, 20:50

Quote:
Originally Posted by rasmits View Post
Sounds simple enough. Do they have a team of "cloud checkers" or does it do it all on it's own?

They probably use the most recent set of data... which are collected by people who decide when to fly (most likely on clear days), but as chucker noted sometimes the weather is a bitch...
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drewprops
Magnificent Basturd™
sagacious-d
 
Join Date: May 2004
Location: Atlanta
 
2006-10-16, 21:14

Yeah... just in case you don't know (assuming you do), the photos aren't in real-time. They're not often even in the same five year span.
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Elysium
Environmental Bloodhound
 
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2006-10-16, 22:19

Simple answer: Magic.
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Brad
Selfish Heathen
 
Join Date: May 2004
Location: Zone of Pain
 
2006-10-16, 22:29

Better answer: Gnomes.

1. Take satellite photos around the world.
2. ???
3. Profit!
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rasmits
rams it
 
Join Date: Oct 2004
Location: Seattle
 
2006-10-16, 23:06

Quote:
Originally Posted by drewprops View Post
Yeah... just in case you don't know (assuming you do), the photos aren't in real-time. They're not often even in the same five year span.
I definitely knew that, I just didn't know how they could use satellite imagery without clouds showing up.

Someone mentioned flying though, so are these pictures from a plane?

You had me at asl
.......
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drewprops
Magnificent Basturd™
sagacious-d
 
Join Date: May 2004
Location: Atlanta
 
2006-10-16, 23:34

Yes, they're not from satellites.
THOSE are sweet, sweet machines that require at least a J25.
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Schnauzer
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Join Date: Feb 2006
Location: Arizona
 
2006-10-16, 23:42

there planes?
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BarracksSi
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2006-10-17, 00:10

They're awfully high up to be taken from aircraft, considering that you can see aircraft in some areas.
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*Joe*
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2006-10-17, 05:57

Definetly satelite images, you can see the source of the image in the google earth window. One of them is http://www.terrametrics.com/ and you can read where the source images come from.
The images of the Oceans are sourced directly from NASA.

They source mapping data for the roads and buildings from Europa technologies, and from Navteq.
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BarracksSi
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2006-10-17, 07:07

Users are now able to "build" their own buildings, too, and submit them to Google.
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AWR
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Join Date: Jun 2005
Location: State of Flux
 
2006-10-17, 07:18

Quote:
Originally Posted by Brad View Post
Better answer: Gnomes.

1. Take satellite photos around the world.
2. ???
3. Profit!



The recipe seems so simple!! Doh.
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autodata
hustlin
 
Join Date: May 2004
 
2006-10-17, 08:25

It's a combination both satellite imagery and super-high resolution aerial photography.
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sunrain
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Join Date: Jun 2004
Location: Portlandia
 
2006-10-17, 08:42

Quote:
Originally Posted by Brad View Post
Better answer: Gnomes.

1. Take satellite photos around the world.
2. ???
3. Profit!
Yeah, well...step two is apparently going after me. Those gnomes keep appearing out of nowhere in WoW and killing me. Twerpy little bastards.
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Dorian Gray
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Join Date: May 2005
Location: Paris, France
 
2006-10-17, 08:54

Quote:
Originally Posted by autodata
It's a combination both satellite imagery and super-high resolution aerial photography.
Exactly. The images with jet aircraft at altitude are from satellites (you can tell because the shadows of these aircraft are roughly the same size as the aircraft themselves), while the images showing incredible detail (even people) are from aerial photographs (satellites can't see people).

It's a fabulous resource. I never tire of "exploring" world cities and remote little islands.
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spotcatbug
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Join Date: May 2004
Location: Clayton, NC
 
2006-10-17, 09:59

Quote:
Originally Posted by Brad View Post
Better answer: Gnomes.

1. Take satellite photos around the world.
2. ???
3. Profit!
Number 2, obviously: Go back in time and create a best-of-class, easy to use, powerful, Internet search engine and use the photos to make a subsection which shows maps, etc. then simply Go Public and continue to step 3.

Ugh.
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drewprops
Magnificent Basturd™
sagacious-d
 
Join Date: May 2004
Location: Atlanta
 
2006-10-17, 10:08

Quote:
Originally Posted by Dorian Gray View Post
(satellites can't see people)
What year are you living in DG? You could play a Gameboy from a satellite these days.
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Kickaha
Likes his boobies blue.
 
Join Date: May 2004
Location: Hell
 
2006-10-17, 10:16

Almost, but not quite:

Regarding the KH (Keyhole) US surveillance satellites (from here):

"The black and white images are used by the military and civilian communities. Many of the details about this class of satellites remain classified, but it is known that there are several of these overhead at any given time. They have an imaging resolution of 5-6 inches, which means they can see something 5 inches or larger on the ground. These satellites probably can't read your house number, but they can tell whether there is a bike parked in your driveway."

And remember, that's just the public info. The actual specs are probably much better.

*lol* Check out the name for KH-10. Now we know why Dorian seemingly pulls information out of thin air...

http://en.wikipedia.org/wiki/Spy_satellite

My other brain is hung like a horse too.
#IRC isn't old school.
Old school is being able to say 'finger me' with a straight face.
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drewprops
Magnificent Basturd™
sagacious-d
 
Join Date: May 2004
Location: Atlanta
 
2006-10-17, 11:05

I can't believe it took me this long to post a link to the article on How Google Earth Works at HowStuffWorks.com.

Sheesh.
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Dorian Gray
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Join Date: May 2005
Location: Paris, France
 
2006-10-17, 13:40

True, I should have qualified that statement to exclude spy satellites, which obviously offer much higher resolution than the commercial satellites in operation. which to the best of my knowledge are incapable of resolving objects less that a few metres across.

Resolution is a much misunderstood concept anyway (ask ten Leica fans to define it and you'll get at least five different answers!). A much better method (and indeed the primary scientific method) for describing the quality of an optical system is the Modulation Transfer Function (MTF), which is simply a description of the spatial frequency response of the lens system. So we can say that a lens delivers a certain contrast ratio (in percent) at a given frequency (usually line pairs per mm at the image plane).

Quote:
They have an imaging resolution of 5-6 inches, which means they can see something 5 inches or larger on the ground.
This is a confusing statement. When one talks about resolution one normally refers to the maximum spatial frequency that is transferred with a contrast ratio of at least a defined value (often 20% but this figure is almost arbitrarily chosen). In other words, resolution is generally taken to be how small the line pairs can get before they blur into one grey mess, like so:



But the contrast ratio of any lens drops as spatial frequency increases, so by simply choosing an object with higher contrast in the first place, the resolution is increased (this is why resolution is so useless for describing lens performance). For this reason resolution is often tested at a particularly high object contrast ratio, usually 1000:1. But a person walking down the street, viewed from a satellite's position in space, is far less than 1000 times brighter or darker than the street.

So it's not accurate to say that a satellite that can just about detect an object 5 inches across against a uniform background (representing the object as a blur of just discernable contrast) actually has a resolution of 5 inches. When people think "wow, 5 inches!" they might imagine clear photos of their small dog sleeping in the garden. In reality such a dog would not be remotely identifiable as a dog. In fact, it's very possible that the dog would not even appear as a smudge on the photo, because for all we know they might mean a 5-inch object with a contrast ratio in excess of 1000000:1 (e.g. a spotlight shining upwards) rather than a dog on grass with a contrast ratio of perhaps 10:1.

… engrossed in such factional acts as dreaming different dreams.
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autodata
hustlin
 
Join Date: May 2004
 
2006-10-17, 14:40

Quote:
Originally Posted by Dorian Gray View Post
True, I should have qualified that statement to exclude spy satellites, which obviously offer much higher resolution than the commercial satellites in operation. which to the best of my knowledge are incapable of resolving objects less that a few metres across.
Actually, DigitalGlobe's QuickBird (one of the main sources of google images) has a b&w resolution of 60-70cm, although the color that google uses is 2.4m.
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billybobsky
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2006-10-17, 14:42

More to the point...


The spy sattelites can see paterns of objects placed five inches apart, but not say two inches apart...
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