Measuring building height with devices...

[Don B.]

EQUIPMENT:

Suunto Clinometer

http://www.cspoutdoors.com/clinometers.html

Leica Disto A3

http://www.leica-geosystems.com/cpd/.../lgs_29794.htm

INITIAL SETUP AND CALCULATIONS:

There are really two ways to measure building height with these devices. The method I use is simple and works best in a flat city like Phoenix (you have to make additional adjustments if you are trying to do this on a slope):

1. Position yourself near a tower (be sure to select a location far enough away that you can see the top and the base clearly--I also center myself on a flat side of the target) and measure your distance horizontally to the base of the tower using the Leica device. You must be level when you do this. This will give you a distance we will call y. For example, let's say y = 120 feet. The precise location of the building's entrance doesn't matter in Phoenix because all skyscrapers here are built on perfectly level ground, so the base all the way around the building is the same as the front entrance. In a hilly city like Seattle, you would have to make other adjustments.

2. Using the inclinometer, sight the top of the building (I do this process twice if there is a tip versus roof height) and read the angle percentage (I believe this is a tan figure) from the device. Convert this to a decimal (divide by 100). We will call this x. For example, let's say x = .42. This works best if the building's max height is at the perimeter - i.e. a flat-topped building, and the sides are at a 90 degree angle to the ground. If you are measuring the tip in a center of a high-rise (steeple or crown), you will need to determine the building's width and add half of that figure to your distance. If the building's sides are not at a right angle with the ground (i.e. pyramid-shaped structure), you will need to determine the slope and correct your distance figure - or you could lay on the ground to do this.

3. Multiply x and y, then add your height to this number. This yields the actual height of the building. So, for our example, .42 x 120 = 50.4. My eyes are about 5.7 feet above the ground, so the actual building height is 56.1 feet.

The other way is to figure out the ratio of your shadow to your height and then determine the length of the building's shadow, then multiply that last figure with the ratio you determined previously. In a bright, sunny city like Phoenix, determining the precise location of the highest point (longest shadow) is easy. The only drawback is if that point happens to be in the middle of a street, then you either risk life and limb to secure the measurements or come back at another time.

EFFICACY:

These methods are 99% accurate if done right, and I've measured several buildings with expected results. I calculated the height of Chase Tower (Phoenix's tallest) at 481.2 feet and the actual height is 483 or 486 feet, depending on what source you believe. Obviously, shorter buildings yield more accurate numbers.

This works the same with meters and feet.

--don

[Lecom]

Wow, I never knew how they measure building height before. Thanks.

[Upward]

Whoa, I had no idea you were into this kind of stuff...

[LMich]

Originally Posted by Lecom

Wow, I never knew how they measure building height before. Thanks.

Actually, this is but one way to measure a building, and probably one of the more rarely used ways, now.

[Kelvin]

True, but if you have good equipment it will be reasonably accurate. I would be interested to see what range of results (or scatter) Don gets with his instrument.

If one has access to some surveying equipment, then the process is the same and the results could be to within an inch!

As Don also noted, this technique is not without its difficulties. Sloping ground requires two shots. One to the top and one to the base. Also, one presumes that the face of the building is vertical. Those that slope or step back make for more elaborate measurements.

[Don B.]

^ I measured Chase Tower (Phoenix's tallest) at 481.2 feet. The actual (known height) is 483 or 486 feet, depending on the source you believe. I also measured from street level and Chase Tower is actually a bit recessed (very rare in Phoenix) as it sits in a plaza that is a few feet below the street.

This equipment I have IS professional surveying equipment. They aren't cheap, either.

Yes, sloping buildings with walls that are not at right angles with the ground would pose an additional challenge. Instead of trying to figure out the angle of the building's side, however, I just measure from ground level with the Leica Disto.

Another challenge is that posed by a building with a crown or other tallest point not on the edge of the building. In many cases, that point will be in the exact center of the building, but you either have to assume this or rely on some other source. In addition, you will now need to measure the width of the building and add half of that distance to your distance measure.

In any event, this method seems to work best with shorter skyscrapers, of which Phoenix has no shortage of. The taller the building, the greater the margin of error, probably because you are holding the clinometer in your hand which of course can move slightly. I wonder if using a tripod or even a monopod might solve that problem?

--don

[microlost]

We had to take a surveying course one summer in undergrad. In addition to getting drunk every night for 2 weeks, we also learned how to measure the height of objects with surveying equipment. Very fun experience.

[swimmer_or_sinker]

Why don't they just make tape measures of varying lengths and throw them off the roofs of buildings to someone at the base. Seems to me to be a hell of a lot faster, cheaper and easier than going through all that shit.

[Don B.]

^ And how do you get on the roof of a skyscraper? Most of them are locked down tighter than Fort Knox.



--don

[Kelvin]

Originally Posted by microlost

We had to take a surveying course one summer in undergrad. In addition to getting drunk every night for 2 weeks, we also learned how to measure the height of objects with surveying equipment. Very fun experience.

Survey camp is the best U/G course hands-down!

[KevinFromTexas]

Originally Posted by swimmer_or_sinker

Why don't they just make tape measures of varying lengths and throw them off the roofs of buildings to someone at the base. Seems to me to be a hell of a lot faster, cheaper and easier than going through all that shit.

Also, think of how much a 30 foot measuring tape weighs. Pretty light right? Say 1 pound. Imagine if it were a 1,000 foot measuring tape. It would weight about 34 pounds or so. Which isn't that bad. But imagine the size. A 30 foot tape is maybe 3 inches across, if it were a 1,000 foot tape the tape casing would be almost 8 1/2 feet across. If you used a cloth measuring tape then you'd have to worry with the tape stretching, or bowing/blowing in the wind, thus not giving you an accurate measurement. Even so the size would still be a problem. It wouldn't be anything portable that you could just walk into the building with.

[microlost]

Survey camp is the best U/G course hands-down!

Totally.

[statler]

On the same idea I just came across this:

R.L. Loeffelbein, a physics teacher at Washington University in St. Louis was about to give a student a zero for the student's answer to an examination problem. The student claimed he should receive a perfect score, if the system were not so set up against the student. Instructor and student agreed to submit to an impartial arbiter, Dr. Alexander Calandra, who tells the story.

The examination problem was: "Show how it is possible to determine the height of a tall building with the aid of a barometer."

The student's answer was, "Take the barometer to the top of the building, attach a long rope to it, and lower the barometer to the ground. Then, bring it back up, measuring the length of the rope and barometer. The lengths of the two together is the height of the building."

I, as arbiter, pointed out that the student really had a strong case for full credit since he had answered the problem completely and correctly. On the other hand, of course, full credit would contribute to a high grade for the student in his physics course, and a high grade is supposed to certify that the student knows some physics, a fact that his answer had not confirmed. So it was suggested that the student have another try at answering the problem.

He was given six minutes to answer it, with the warning this time that the answer should indicate some knowledge of physics. At the end of five minutes, he had not written anything. Asked if he wished to give up, he said no, that he had several answers and he was just trying to think which would be the best. In the next minute he dashed off this answer. "Take the barometer to the top of the building. Lean over the edge of the roof, drop the barometer, timing its fall with a stopwatch. Then, using the formula S=½at2, calculate the height of the building. At this point, I asked my colleague if he gave up and he conceded. The student got nearly full credit.

Recalling that the student had said he had other answers, I asked him what they were. "Well," he said, "you could take the barometer out on a sunny day and measure the height of the barometer, the length of its shadow, and length of the building's shadow, then use simple proportion to determine the height of the building. And there is a very basic measurement method you might like. You take the barometer and begin to walk up the stairs. As you climb, you mark off lengths of the barometer along the wall. You then count the number of marks to get the height of the building in barometer units.

"Of course, if you want a more sophisticated method, you can tie the barometer to the end of a string, swing it as a pendulum, and determine the value of 'g.' The height of the building can, in principle, be calculated from this.

"And," he concluded, "if you don't limit me to physics solutions, you can take the barometer to the basement and knock on the superintendent's door. When he answers, you say, 'Mr. Superintendent, I have here a fine barometer. If you will tell me the height of this building, I will give you this barometer.'"

Finally, he admitted that he even knew the correct textbook answer -- measuring the air pressure at the bottom and top of the building and applying the appropriate formula illustrating that pressure reduces as height increases -- but that he was so fed up with college instructors trying to teach him how to think instead of showing the structure of the subject matter, that he had decided to rebel.

For my part, I seriously considered changing my grade to unequivocal full credit.

R.L. Loeffelbein has been a teacher and writer for 20 years. He was an assistant professor aboard the first voyage of the University of the Seven Seas.

[Line_and_Grade]

I've used a chain 200' roadway in an elevator shaft. Then differencial leveling from there. Or set-up the job XZY coords and use the Nikon.

[Tom In Chicago]

What I would find interesting is what the actual blueprints state and what Don's measurements came up with. . . that way you could determine how much of the building has settled over the years. . . or the difference in height during the day vs. night based upon the temperature of the structural materials. . . should be measurable either way. . .

[Alpha]

Sources of inaccuracy occur from non-flat ground. If these values are very low, they may be not detected. If a structure is situated in mountaneous area, it may be difficult to find the best point of measurements. Then you must consider that you carry the instrument in the hand, perhaps 1.5 metres above the ground.

However, it may be an interesting "toy". Try to confirm with it the height of as much structures from which you know the height (would be a cool idea for a sunday with good weather) and then try to determine the height of structures, from which you have no height values.

This device should also not miss in your holiday luggage!

[LeeWilson]

I tried constructing my own device of this sort from odds and ends. The results it gave were vaguely correct, but not good enough.

[SkyWatcher]

A tape measure? Boy I sure got a good chuckle out of that one. I work with a tape daily as a deckbuilder. They are wildly inaccurate over distances greater than 10ft. without being perfectly aligned & taut. The result would be a most inaccurate measure.

And who's gonna climb to the top of the spires?