1500x30 vision anyone?

March 13, 2007 5:00 PM

Balloonfilter: How big should a sign be to be visible from a hot air balloon?

I intend to propose in a month or two by means of a giant sign on the ground during a hot air balloon ride. I've got a balloon pilot for who is confident he can pass over/near enough a given field by adjusting the starting location of the balloon and height during flight. I believe the plan is to be between 500 and 1500 feet around the time we'd pass over my giant sign, but I can't figure out how big I'd need to make the sign.

I plan on using white butcher paper on a grass field. Butcher paper comes in rolls 3'x1100'. I'm figuring if I have, for example, a lower-case L 'l' be 30' tall by 6' wide, it would be easily visible, but I can't find any way to confirm this. I got the 30' estimate as approximately equal to the letters in the HOLLYWOOD sign.

So, hot air ballooners, what do you think?
Alternately, any ideas for some way to simulate the view from a given height? Google Earth isn't a whole lot of help from such low heights, but all my searching has turned up nothing. Help!

posted by krakedhalo to science & nature (12 comments total)

There's got to be an easy way to do this with trigonometry. Sadly, I can't remember my trigonometry.
posted by Dasein at 5:07 PM on March 13, 2007

You can napkin-math this:

Write your message in block letters on, say, a napkin, and hold it one foot from your face. Clearly legible? That's a usable size, then.

Measure the line-height of your handwritten message. Multiply by 1500—from fifteen hundred feet, that should be roughly equivilant. (Severe myopia, heavy fog, etc notwithstanding.)

I got a good reading just now with quarter-inch-tall letters. Scaled by a factor of 1500, that's 375 inches or just a hair over 30 feet tall.
posted by cortex at 5:19 PM on March 13, 2007

The Carew Tower at approx 574 feet might give you a good idea of what stuff looks like from up there. (I assume you near the coords in your profile)

When you place the proposal, make sure to stake out string lines so all your letters line up.
posted by Megafly at 5:34 PM on March 13, 2007

With all that's riding on this, I'm not sure how far to trust my approximations. I can read a 1 inch letter from 10 feet away pretty easily, which by a straight proportion gives 150 inches - 12.5 feet. I don't trust that, because from 200ft up my office building, a 10 foot car looks pretty small. Thinking back to my time atop the Empire State building, 1 car sized letters would be pretty illegible, 3 car length (30ish feet) maybe okay.

Also, 1500 ft only works if we get directly over the sign. If we're two fields over, where it's easily visible, I'd also like it to be legible. Pythagorean thm gives approx 1500*sqrt2 = 2100 ft distance...

I could second guess myself at this until it's time to do it (where 'this' = the optics, not the proposal) but thought someone out there has to be a hot-air balloon person.
posted by krakedhalo at 5:35 PM on March 13, 2007

I have read in several survival guides that SOS letters should be at least 18' high and 3' wide in order to be "visible from the air." I'm not sure if this refers only to visibility by rescue planes, which probably fly at a lower altitude, or by most aircraft. I imagine your balloon flight will be at an altitude closer to a rescue plane than commercial airliner.
posted by cocoagirl at 5:47 PM on March 13, 2007

You could always bring some binoculars, just in case -- they seem like a pretty unsurprising thing to have on a hot air balloon ride.
posted by advil at 5:54 PM on March 13, 2007

Can't the balloon guy give you some tips? Like, how easily can he read the STOP painted at intersections?
posted by DU at 5:59 PM on March 13, 2007

Only went hot air ballooning once, but we mostly traveled just above tree line, if that helps.
posted by clarkstonian at 6:07 PM on March 13, 2007

Off topic but relevant. You can contact your local newspaper and ask if they have any newsprint "tails." These are the ends of the paper rolls used to print newspaper, and you can usually get this stuff for free. I have some 4' wide in the office I use for flip chart paper. Might save you some time and money by using free wider paper.

Great Optimism,
posted by choragus at 9:14 PM on March 13, 2007

Check out this page. It has a chart of letter-height visibility.
posted by PEAK OIL at 9:31 PM on March 13, 2007

And on a related note, I'd ask an outdoor advertising company what they'd charge to print up a really nice sign for it. It might be less than you'd expect. (and it might make it far more readable.)
posted by PEAK OIL at 9:33 PM on March 13, 2007

There is a lot that goes into making a sign readily "readable" from the air. Interstate highway signs for exit services often specify lettering as small as 6" in height, and while a 6" white stripe for highway speed measurement marks can be readily seen by aerial observers from an altitude of 2000' above average terrain, the chances of this size lettering being readable for an untrained observer, perhaps with haze and parallax issues resulting from not being directly over the sign, can not be overestimated.

Many, if not most people, who are flight novices are somewhat disoriented by an altitude perspective; if you really want to be sure the message can be read by anyone, bigger is better, and maximum contrast is your friend, as is very, very simple wording. Anything more than "Marry me, Jen?" is liable to be mis-read, if it is made out at all. From an altitude of 500 feet, 6 foot tall letters are likely to be so big, as to be difficult to recognize as letterforms, while from 2000' feet and a 45° parallax angle caused by mistrack over the target, with ground haze due to 30% relative humidity, the same letters are likely not to be noticed, unless pointed out, and they still might not be easily read by a novice.

The next thing I'd mention, is that if you want your message to be read and understood, for sure, on a given date, you'd be much smarter to book a charter with a fixed wing aircraft service, than with a hot air balloon. The only flight variable a balloonist controls is altitude. They use their best judgement regarding prevailing winds, and can change altitude to use different wind layers in the atmosphere, but the safety of the ship and passengers is always their first priority, and balloon pilots promising you a position over a given field on a given date are generally hoping you are particularly guillible. If you want to be sure your message is read, book a powered flight in a reliable aircraft. You can be sure you get a pass, at an exact altitude, in the most favorable orientation, over a given position, and if you like, you can repeat the pass for photographs, as many times as your pilot has fuel aboard. And there are FBO operators in many locales who specialize in such events. But even with an engine, such scenarios don't always go smoothly.
posted by paulsc at 10:52 PM on March 13, 2007