Ben Peoples Speaks


United 747 did not almost graze the GG Bridge
October 19, 2010, 2:42 pm
Filed under: Uncategorized

Sorry, CNN, not even close.

This unattributed video was posted to break.com. CNN is now running a story about this plane almost striking the GG bridge.

Here’s a screen cap showing the plane passing well infront of the GG bridge:

Now, it looks like it’s really close, but this video is being shot from Fisherman’s Wharf (lining up the hills behind the bridge and striking a line). At this point the north tower of the bridge is 4 miles away.

So, now we know our distance to the bridge: 21120 feet, the tower above the deck is 500 feet tall and takes up 3/4 of the frame. At the bridge’s distance, we’re seeing about 675 feet of height. That means we have an equivalent 750mm lens on the camera. The vertical FOV (the only one we can calculate with this data is 1.8 degrees). The total image (the screen cap above) is 378 pixels tall, for about 17.1 arc seconds per pixel.

Now comes the fun part — how far away is the airplane? We can pretty accurately measure the height of the fuselage, in the bulge. This comes out to 28 pixels. That’s 25′-10″ tall in real life. 28 pixels = 0.133 degrees. That’s 13.5% of the height of the image. Were that 747 in the same plane as the bridge, it would be 91 feet tall.

I solved it geometrically, rather than breaking out the trig. Drafting the triangle with 0.133 degrees for the near side, and a height of 25′-10″, we get a 11,128′ range, or about 2 miles — halfway to the bridge.

So — in short: the plane was nowhere near the bridge, it was about halfway between Alcatraz and the Bridge, and was never in danger of striking it. This illusion wouldn’t even have looked close were you standing at the Marina Green, as the plane would have been cornering almost right infront of you.

 

UPDATE: Found the original Youtube Video here: http://www.youtube.com/watch?v=TSGT44hU4uc

User says they shot it with a Canon Powershot SX20IX, which has a maximum focal length equivalent of 560mm (100mm on a 1/2.3″ sensor).  So that’s 2.5 degrees vertical over 675 feet.  We get 15460 feet, which is only about 3 miles, meaning they were closer than Fisherman’s wharf.  13.5% of 2.5 degrees is 0.3375 degrees, meaning the airplane was  4384 feet away, or 0.8 miles.  Still, the plane was at least 2 miles from the bridge.

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