• Saturday Science

I happened to be out on the back porch fiddling with my camera when an aircraft flew over, so I fired off a few frames.


Later, looking at the images in Apple's great iPhoto program, the n-number (registration) on the side of the aircraft tickled a few neurons, and I wondered why. A quick check of the FAA registry told me it was owned by someone in Coronado, and Google told me he was an air traffic controller. No help.

So I googled the n-number and out popped an NTSB report that the aircraft "was substantially damaged" in a landing accident at Hyde Field in 1976. Ah-hah!

I flew out of nearby Oxon Hill airport back in the early '70s when the Navy sent me to work at the Department of Defense Computer Institute in Washington DC. Sure 'nuff, a check of my log book revealed I have 5.7 hours in N3430T in late 1973.

So I asked a friend who works for the FAA to forward the picture and the 'small world' history to the owner via agency e-mail.

Then I started wondering if maybe I'd put whoever was flying it on report by mentioning the aircraft had flown low over the neighborhood and when.

How low, I wondered. Is there any way to tell from the picture? Turns out there is.

Virtually all digital cameras store EXIF data along with the image information. That's short for Exchangeable Image File. Information such as shutter speed, F-number, focal length, ISO number, date and time the image was taken, white balance, lens that was used, resolution, and other details are all saved with each image.

If I have the actual focal length (my darling gave me a fantastic Nikkor zoom lens so it might have been anything from 18 to 200mm), and if I know the size of the aircraft (google knows), I can compute it's distance using a simple ratio.

The image size (IS) of the object in the frame is to focal length (FL) as the object size (OS) is to object distance (OD). In other words

IS/FL = OS/OD

Solving for what we don't know, object distance (OD) we get

OD= OS (FL/IS)

So I loaded the image in Photoshop, used the ruler tool, and found the aircraft's image size (744 pixels in 3008 pixel image). The focal length was 200mm, and the size (length) of a Cessna 177 Cardinal is 8.22 meters (27 feet). Do the math, and the bird was 262 meters away.

That's a slant range of 860 feet. If he'd been directly overhead he'd have been 860 feet above the ground. If he'd been level with me he'd have been zero feet above the ground.

We could do some more measuring and more math to determine exactly what the angle was, but let's estimate it at 60 degrees. Now with Pythaogras' help, all we have to do is solve a simple geometry problem, with a little trig thrown in [sin (60 degrees) * h], and what you find is that the aircraft was at about 750 feet.

That's higher that I thought (I guessed about 500-600'), but nevertheless slightly lower than is legal over a populated area (1000'). To be charitable let's say that with measuring error on my part and instrument error on his part—he was close enough for government work.

So what does all this prove? Well, for one thing, don't believe everything you think. Humans are notoriously bad observers and even worse estimators. And if you're a pilot, keep in mind that even a backyard photographer can catch you busting the regs.

You might also conclude that I've effectively proven I have far too much time on my hands.