How low is this airplane?
May 23, 2012 2:13 PM   Subscribe

Can someone explain this picture to me? It's a Google Earth shot of Russell Square in London. It aslo features an apparently low-flying airplane. It can't be as low as it looks, can it?
posted by Cobbler to Technology (21 answers total) 3 users marked this as a favorite
well, it's far enough away from the ground that its shadow isn't identifiably in the shot, at least.
posted by KathrynT at 2:21 PM on May 23, 2012 [4 favorites]

Depends on how low you think it is :)

It's not right over the treetops but yes it's likely that plane is pretty low. I'd guess under 10,000 ft and prob closer to half that.

Looking at the map, there is an airport only 8 miles away from this airplane. The plane is likely circling waiting for landing or descending to land.

I live near a Air National Guard base and not too far from a international airport, it's pretty common for airplanes to fly surprisingly low when on approach.
posted by rmathew1 at 2:22 PM on May 23, 2012

While this isn't conclusive, due to the composite nature of the images Google Maps serves up, on important clue to height is shadows. You have a rough idea of how tall the trees in the square are, as well as the buildings around it, and you see where the shadows from them are landing. On preview, what KathrynT says... the plane is high enough up we don't see its shadow.
posted by radwolf76 at 2:22 PM on May 23, 2012

The photos are taken from so high up that the difference between the plane and the ground are relatively trivial compared to the distance between the ground and the camera. Telefoto lenses make things look smashed together that are really quite distant. But, as rmathew1 notes, it's probably on approach or departure from a nearby airport. There are numerous examples of things like this on google earth/maps near other airports.
posted by LionIndex at 2:23 PM on May 23, 2012 [5 favorites]

Googling "Russel Square London low flying aircraft" brought up this.

"Geostationary satellites balance the Earth’s gravitational pull against the centrifugal force of orbit to stay at the same location, relative to the ground. But at that distance – around twenty-five thousand miles – they are much too far away to get high-resolution photographs of our planet.
Thus Low Earth Orbit satellites are used. Being less than a thousand miles up, these have to whiz around the Earth in order for the centrifugal force to balance the much GREATER gravitational pull of the planet.
But they are still way higher than aircraft. Also, when planes are flying over London, they are usually on their final approach – or being stacked – therefore their height is much less than the seven miles up they normally travel at. And they have generally slowed to far less than their cruising speed of 550 mph.
And so it was that while flying over Russell Square, a plane had just happened to be photographed by one of the satellites from which Google get their Google Earth pictures. Plus, the photo had been taken on high-speed film, making the aeroplane appear stationary.

posted by Cat Pie Hurts at 2:26 PM on May 23, 2012

At least there used to be numerous examples. I think Google tries to eliminate them.
posted by LionIndex at 2:26 PM on May 23, 2012

It looks strange because of foreshortening from the very long camera lens. However, one thing is puzzling me. Much of Google's "satellite imagery", particularly detailed imagery like this, is taken from airplanes. How far up was the photo plane vs. the subject plane/ I don't know how high the imagery planes fly, but air traffic control would provide at least 1000', maybe 2000' of separation.

I'm a little surprised to see a plane looking like it's flying an approach or departure right over the center of the city. Is that common? The only really close airport is London City Airport, but I haven't done the math to see if it makes sense for this to be Heathrow or Gtwick.

Bonus airplane in Google Maps image, showing what happens when a fast moving object is photographed with a different kind of camera than what we usually use.
posted by Nelson at 2:42 PM on May 23, 2012 [1 favorite]

Heathrow's about 20 miles from there. Whenever I've flown in there it's seemed like one long-ass queue of planes and the approach often seems to be from over the city. I've definitely seen the London Eye from my seat while stacked. You can see another few planes on approach closer in and heading back to the city - here, here and here.
posted by IanMorr at 2:52 PM on May 23, 2012

I'm a little surprised to see a plane looking like it's flying an approach or departure right over the center of the city. Is that common? The only really close airport is London City Airport, but I haven't done the math to see if it makes sense for this to be Heathrow or Gtwick.

You can see plenty of planes departing Heathrow and ascending over central London (at least when I looked now)
posted by vacapinta at 3:00 PM on May 23, 2012

If someone can find the shadow, figure out how far away from the plane it is, and figure out the height of a nearby landmark with a shadow we can measure, then we can roughly calculate the height of the plane.
posted by Blasdelb at 3:01 PM on May 23, 2012

The satellite that took this shot is thousands of times further away from the plane than the plane is from the ground. The plane of approx 60m length is almost "life size" compared to the dome of the British Library reading room just South (42m). It could look this way at any height between normal cruising altitude, and parked. We have no way to judge the height, and the appearance of low height is just because your mind imagines this was shot from another low flying plane.
posted by roofus at 3:13 PM on May 23, 2012

I went shadow hunting and either I missed it (easily possible) or the plane is fairly high and the impression is due to the foreshortening as others have noted. I also doubt this is on final at City airport. The profile of the plane looks most like an A320 (or variant). I'd even go so far to hazard a guess this is a Swiss airline plane, which would mean it's heading in or out of Heathrow.

Unless things have changed, City only handles planes certified for short field operations. The biggest jet I ever saw at City was a Jumbolina.
posted by michswiss at 3:48 PM on May 23, 2012

Basically, it's an artifact of how your brain interprets photos. We assume when looking at a photo that it represents about the same visual angle, the same angular field of view, as our eyes. Obviously we can interpret things like the zoom of a normal camera more-or-less correctly, but given that this is so SO zoomed in, orders of magnitude more in the zoom level you linked to, the visual angle is miniscule. Your normal 2d-to-3d filters can't handle that with any accuracy. Even if the plane were magnified only 5% over the ground, that would mean that the altitude is quite high in real terms, even if it's low in the relative camera-altitude-to-plane-altitude sense.

Someone could theoretically figure out what the plane-to-ground-object magnification factor (you'd have to have a good scale for both) and do the math; you should be able to figure out exactly how many times higher the camera is than the plane.
posted by supercres at 3:56 PM on May 23, 2012

(I think perceived visual angle is what I'm getting at. There are two S' lengths, you know what the actual lengths are, and \theta is the same, so you should be able to figure out the difference in D'.)
posted by supercres at 4:03 PM on May 23, 2012

"The satellite that took this shot is thousands of times further away from the plane than the plane is from the ground."

That's not a 'satellite' shot; that's aerial photography. Moving out, this is the last aerial photograph (notice that you can still see the same aircraft in the same location, if you squint), while this is the first satellite image.

Typically, aerial photography like that would be taken at ~30,000'. I'm no expert, but this one looks to be higher than that (although it's impossible to tell without knowing the camera details/settings).

Blasdelb &/or supercres have recommended the best / easiest options for estimating height, though I suspect the latter will be extremely sensitive to miniscule measurement errors (& depends on knowing the exact type of plane).
posted by Pinback at 4:16 PM on May 23, 2012

That airplane appears to be an Airbus A321. Using this site you can get an exact distance between points on Google maps, which lets you measure the length and wingspan of the image with some precision. Based on that, I calculate that the image of the plane appears about 1.65 times larger than it would if it was on the ground. If we had the height of the device that took this picture, we could work out it's altitude from that.
posted by Rhomboid at 6:17 PM on May 23, 2012

I calculate that the image of the plane appears about 1.65 times larger than it would if it was on the ground.

To be precise: if the ground projection of the plane is 1.65 times the plane's actual size, then similar triangles says that the altitude of the camera is 1.65 times the distance from camera to plane. So if the camera were indeed at 30,000 feet, the camera would be 18,000 feet above the plane, putting the plane 12,000 feet above the ground.
posted by flabdablet at 6:37 PM on May 23, 2012 [2 favorites]

OK, after doing some trig based on the size of various objects, I estimated that the flying height of the camera aircraft was ~5000' (that's a mean of 5 measurements with quite high error margins, so no promises). My original educated guesstimate of over 30,000' feet appears to be totally wrong, but 5000' does make considering everything else.

Using Rhomboid's calculation, that gives a flying height of the A320 of ~3000'.

Then, taking a different tack, I took my shiny barometer over to Google and found this post. They suggests it's about to make a tight turn to make its final approach to Heathrow, and given the distance and glide path etc. is at least 2500' up.
posted by Pinback at 8:44 PM on May 23, 2012 [1 favorite]

If 5000' is 1.65 times the height of the camera above the plane, then the camera is 3000' above the plane, which puts the plane's altitude at 2000'. If the plane is actually at 2500', the camera would be at 1.65 ÷ (1.65 - 1) × 2500' = 6300'. Is that still within your error margins?
posted by flabdablet at 10:02 PM on May 23, 2012

Ah, yeah, you're right.

I'm at home now & don't have the numbers handy, but they were all over the place - the lowest was 800-something meters, and I forget what the highest was. There were two based on buildings where I had decent distances to work with - building height estimated from Google Earth, length @ street level measured on GE, & length @ street vs roof level compared from the Google Maps view - so they were possibly the best (give or take my dodgy height measurement from Google Earth). IIRC they gave me camera heights of ~2000m & ~2500m. 2250m = ~7300', for an Airbus height of ~2900'. Close enough?

On one hand, the measurements - particularly of building height using Google Earth - are probably very wrong; I guesstimated them by flying down to roof level / ground level and comparing altitudes. On the other hand, I didn't bother correcting for any angular factors; it seemed pointless without knowing where the nadir or principle points were.

And on the third hand, if you look at Google Earth that Airbus is actually pancaked on the ground in the park. You probably would have heard it on the news if that'd happened ;-)
posted by Pinback at 12:23 AM on May 24, 2012

Yes, especially given that it's apparently suffered some kind of weird pressurization failure that's blown it up to 1.65× normal size!
posted by flabdablet at 4:54 AM on May 24, 2012

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