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Old 2008-07-29, 06:26   #12
Uncwilly
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Quote:
Originally Posted by Brian-E View Post
The question about which bridge/tunnel combination it is does not quite seem to have total consensus here, but the proponents of the Monitor-Merrimac Memorial Bridge-Tunnel seem to have produced the best supporting evidence.
Here is a link to a satellite view of the MMBT. I checked maps I have of the area, looked at snopes, and checked wiki. I must disagree with Bob, it is not the CBBT
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Old 2008-07-29, 11:12   #13
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Quote:
Originally Posted by Brian-E View Post
Well, that was already clear to me (though is there any particular reason why you say "rectangular"? Does it need to be?). However I still consider it a feat of engineering. It would be easier, I guess, if the water at that point is particularly shallow. Now I think about it, probably that is indeed the case - there are likely to be shallow areas in wide rivers and sea inlets everywhere, and presumably one such area will have been selected on which to build the artificial island.
The rectangle's width is determined by the number of
carriageways, and its length by the maximum sensible gradient
required. In this case our definition of "island" can be generalized
to any sensible means of excuding water from the tunnel and
supporting it until it is out of the sea/river. Ideally you would
use an existing island, but then it would be described as a bridge
and a tunnel rather than a "bridge-tunnel".

As for shallowness, I presume the purpose of the tunnel is to
allow ships to pass over it.

Since river estuaries fluctuate geologically on a relatively
short time scale, one could be forgiven for thinking that they
are building castles on sand

Last fiddled with by davieddy on 2008-07-29 at 11:15
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Old 2008-07-29, 16:54   #14
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Quote:
Originally Posted by Uncwilly View Post
Here is a link to a satellite view of the MMBT. I checked maps I have of the area, looked at snopes, and checked wiki. I must disagree with Bob, it is not the CBBT
Actually, I don't think that's satellite, it's probably an aerial photograph instead. In fact, a lot of the closer-zoomed photos on Google Earth and Maps are actually aerial photographs--it's the only way they'd be able to obtain the resolution shown. I guess they just call it Satellite view for simplicity (though it probably would be more clear if they called it "Satellite/Aerial view").
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Old 2008-07-29, 18:37   #15
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Originally Posted by Anonymous View Post
Actually, I don't think that's satellite, it's probably an aerial photograph instead. In fact, a lot of the closer-zoomed photos on Google Earth and Maps are actually aerial photographs
Actually it is possible to get a resoultion of 4" (10cm) from space.
http://en.wikipedia.org/wiki/Spy_satellite The google material is not even that good. Based upon my looking at things that I know and some reasonable understanding of image compression, etc., I am guessing that most of the closest zooms of are in the 25-30cm range. It appears to be a B&W base with color poured on top of it.
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Old 2008-07-29, 21:14   #16
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Originally Posted by Anonymous View Post
Actually, I don't think that's satellite, it's probably an aerial photograph instead.
Remember when the Ikonos satellite photos were posted on the Web Sept. 12, 2001, showing overhead views of World Trade Center and Pentagon after attacks? One-meter resolution.

http://www.space.com/news/ikonos_wtc_010912.html

http://www.space.com/images/pentagon_damage_02.jpg

http://www.space.com/images/manhattan_010912_02.jpg

Each of the Pentagon's outer walls is 921.6 feet long. Looks to me like the width of that posted photo (taken by the Ikonos satellite, remember) is about twice the length of one of those, certainly no more than 2000 feet.

Now look back at the Google overhead view at http://maps.google.com/maps?ll=36.95...5458&t=k&hl=en and notice in the lower left corner how small 2000 feet is in that MMMBT photo. It could be aerial, but a satellite is certainly capable of at least that resolution.

Try centering the view on the island, then pushing up the scale indicator at left by three steps, so that the lower left corner bar is 200 feet and the width of surface shown in the MMMBT photo is roughly the same as the Ikonos Pentagon shot. Compare vehicle sizes in the two photos - I think they're roughly similar, though the Google photo does look sharper at this 10x magnification than the Ikonos Pentagon shot. OTOH, the repeated references to 1-meter Ikonos resolution may be a hint that 1-meter is not the Ikonos's limit.

- -

Another comparison, to the Hubble Space Telescope:

The optical systems of satellites looking down at Earth are basically the same type as Hubble's -- reflector telescopes. Hubble's CCDs are too sensitive to point at sunlit Earth, but Ikonos could have the same optics with less sensitive, but equally-sized CCDs. Hubble's resolution is about 0.085 arc second. At a height of 423 miles (Ikonos's orbital altitude, according to article), 0.085 arc second resolution in a ground photo would be about ... ( (423 miles * 5280 feet/mile * 12 inches/foot) * (2*pi) / (360*60*60) ) * 0.085 = about 11 inches, or about 28 cm. If Ikonos has a smaller mirror (or larger CCDs) than Hubble, its resolution would be worse by the ratio of mirror diameters (or CCD sizes).

Last fiddled with by cheesehead on 2008-07-29 at 21:41 Reason: math
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Old 2008-07-29, 22:11   #17
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Quote:
Originally Posted by cheesehead View Post
Another comparison, to the Hubble Space Telescope:

The optical systems of satellites looking down at Earth are basically the same type as Hubble's -- reflector telescopes. Hubble's CCDs are too sensitive to point at sunlit Earth, but Ikonos could have the same optics with less sensitive, but equally-sized CCDs. Hubble's resolution is about 0.085 arc second. At a height of 423 miles (Ikonos's orbital altitude, according to article), 0.085 arc second resolution in a ground photo would be about ... ( (423 miles * 5280 feet/mile * 12 inches/foot) * (2*pi) / (360*60*60) ) * 0.085 = about 11 inches, or about 28 cm. If Ikonos has a smaller mirror (or larger CCDs) than Hubble, its resolution would be worse by the ratio of mirror diameters (or CCD sizes).
One would do well to read the wiki article referenced above. I have seen the 3-4" numbers from several sources. HST is an upward pointing KeyHole sat, basically. The SpaceShuttle cargo bay was designed to be big enough to haul them to orbit. There was a nice program that I saw on MOL that gave similar size and resolution numbers.
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Old 2008-07-29, 22:53   #18
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Quote:
Originally Posted by Uncwilly View Post
I have seen the 3-4" numbers from several sources.
There's no contradiction.

At 200 miles altitude, 0.085 arc second resolution would be 11 * 200/423 = ~ 5 inches. At 100 miles altitude, 0.085 arc second resolution would be 11 * 100/423 = ~ 3 inches. A dippy-doo to 75 miles perigee would allow ~ 2 inches. They do have the spy satellites in lower orbits than Hubble's or Ikonos's, and mirrors that were not incorrectly ground, as Hubble's was, could have somewhat better resolution.

Note: resolution is also proportional to the wavelength of light being used. The 0.085 arc second that I'm using is for mid-visual spectrum. Infrared will be fuzzier.

One of several ways that the Hubble mirror mistake could have been caught before launch would have been for the DoD to allow the Hubble mirror to have been tested by the same facility that tests the spy satellites' mirrors. But, according to some, DoD didn't want to allow folks to deduce that spy satellites have about the same accuracy as Hubble -- which was just silly, because elementary optical considerations and Shuttle bay dimensions lead to that conclusion anyway. So the Hubble mirrors were ground and tested by a company that did sloppy work -- low-bidder and all that. Shame on NASA for not requiring enough double-checking tests.

Last fiddled with by cheesehead on 2008-07-29 at 23:26
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Old 2008-07-30, 03:36   #19
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Ah, I see--actually, though, what originally prompted me to assume that the high-resolution shots are aerial instead of satellite is that more than once, when I was going over the outskirts of a more urban-ish area either in Google Earth or Maps (at a zoom level high enough to make out the colors and the general shape of the cars), following a given path along the streets, when the area would get more rural, I would quite often suddenly see the image get a lot, lot less clear--it would suddenly look all moosh-y and muddled (staying at the same zoom level as before). Thus I assumed that to be the boundary of the region that's been aerial photographed by Google. (After all, if it was all satellite, then why would there be such a clear boundary when you get out to the rural areas?)
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Old 2008-07-30, 05:00   #20
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Quote:
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Thus I assumed that to be the boundary of the region that's been aerial photographed by Google. (After all, if it was all satellite, then why would there be such a clear boundary when you get out to the rural areas?)
Thus under that assumption, you would want to save hi-res data on ~70% of the earth that is ocean? Or would lower quality do? A rice paddy, thick jungle, expansive sand dunes, or wheat field has less 'important' detail than Manhattan, Moscow, Johannesberg, or Rio. If it was an areial photo, the detail could be much higher. The U2's camera (from high altitude), was powerful enough to show a golf ball, from a contact print (no enlargement). This impressed DDE considerably (and helped him understand the potential of spy sat's).

Last fiddled with by Uncwilly on 2008-07-30 at 05:05
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Old 2008-07-30, 14:37   #21
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Quote:
Originally Posted by Uncwilly View Post
Thus under that assumption, you would want to save hi-res data on ~70% of the earth that is ocean? Or would lower quality do? A rice paddy, thick jungle, expansive sand dunes, or wheat field has less 'important' detail than Manhattan, Moscow, Johannesberg, or Rio. If it was an areial photo, the detail could be much higher. The U2's camera (from high altitude), was powerful enough to show a golf ball, from a contact print (no enlargement). This impressed DDE considerably (and helped him understand the potential of spy sat's).
Oh, I see. I hadn't thought of the storage costs--I mean, of course, I didn't think that anyone would want to get hi-res data on the oceans, but I would think that Google would at least have a hi-res photo of my house (somewhat rural area, but no more than half an hour's drive to the nearest suburb/town area, and an hour to a full-size city). (Nope, sorry, not giving out the coordinates. )
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Old 2008-07-30, 23:56   #22
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Google just needs to reverse engineer and build a couple SR-71 Blackbirds.

We cannot find the article but we remember somebody used a SR-71 camera to take a "normal" picture of the Grand Canyon, or something like that. The results were stunning.
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