Anyone have a nifty way of telling a hieght of an object without having to climb? I've no problem with doing a timed rock-drop, but sometimes I'm just passing by and would like a quick measurement for future knowledge. Thanks in advance!
Mike
Anyone have a nifty way of telling a hieght of an object without having to climb? I've no problem with doing a timed rock-drop, but sometimes I'm just passing by and would like a quick measurement for future knowledge. Thanks in advance!
Mike
You've probably already considered this, but laser rangefinders and a little math can go a long way. Keep in mind that the error is higher than if you were lasering the ground from on top of the object...
If D1 is your measured distance to the base (assuming you can get horizontal with the base; there are ways to do the calculation even if you can't) and D2 is your measured distance to the top, then:
H = sqrt(D2^2 - D1^2)
(that's 'square root' and 'squared' happening in there). That's pretty common knowledge. You can also estimate the error, though...
DH = (D1 + D2) * DD / H
where DH is your error in height and DD is the error in the rangefinder (usually about a yard). That works only if you're far enough away that the object isn't moving all over the place in your sight -- ideally you want to be about 1/3 of the height of the object away from the object.
Hello Mike!
Buy immediately a Bushnell Yardage Pro!!! They are a series of laser rangefinder, invisible and silent operation, capable of measuring the distance from an object even in the most total darkness!!!
Here through the BASE Board, under Store - Electronics, you can buy a Bushnell Yardage Pro 600 for 249$. The "600" stands for 600 yd of maximum measuring capability of a "tree" (object of average size and reflectivity), maximum distance for smaller objects is less than 600 yd, maximum distance for bigger (and more reflective) objects is more than 600 yd.
And it is not compulsory to get to the base of the object under measurement and measuring directly the object height (even if it is very adviseble to get as close as possible to the base): from the closest (and safe!) location, you can measure the diagonal distance (hypotenuse of a right triangle) to the top of the object and the horizontal distance to the base ("one of the two smaller sides" (cathet ?!?) of a right triangle): making a very simple calculation through a pocket calculator (Pythagorean theorem), you can easily get the object height ("the second of the two smaller sides of a right triangle"). If interested in this, drop an e-mail that I shall tell you how to do such a simple calculation.
Stay safe out there :7
Blue Skies
Andrea :D
#689
The error calculation mentioned by Jason is really helpful. For a rangefinder good to within a yard, expect 3 or 4 yards of error, sometimes more, if you're calculating height using Pythagoras.
Michael
Hello Michael,
I also agree with Jason's formula for error in height (given the error of rangefinder): it comes from Mc Laurin development of SQRT(1+x) being: SQRT(1+x) = 1+0.5*x if x<<1. So the formula is perfect. Also, in the particular case in which D1 = H, you can easily show that DH = (1+sqrt(2))*DD = 2.41*DD: if you are in the (beginning to be too bad) case in which your distance from the base of the object is equal to its height, the error in height itself is multiplied by 2.41 times the error of rangefinder (i.e., if DD is 2 yd, the error of height begins to be something like 5 yd).
Still.
From my practical experience (a building!) of measuring several couples of D2 and D1, approaching the base of the object and using Pythagora's theorem, I found that the calculated height was decreasing while I was getting closer. Roughly, 1st from 50m of distance, then 40m, then 30m and so on. I.e.: the farest from the base, the greater the error was. Approaching to the base, I was getting smaller values (yes, few meters indeed but much greater values than figures from Jason's formula, 10 or 15m when measured from 50m) down to an asymptotic value very, very close (2m?) to the value of height measured standing at the base and shooting exactly in vertical to a piece of the structure. Consider also that (unless you go straight to the base and measure directly the height!) you cannot go too close to the object base because rangefinders (at least mine!) cannot measure less than 14 yd-m (D1>=14 yd-m).
Have you got an explanation to this phenomenon?
Stay safe out there :7
Blue Skies
Andrea :D
#689
Jason's formula can also be derived by simple differentiation. But what this simple formula assumes is that all the error in measurement is due to instrument error (ie it's the rangefinder's fault). What it doesn't take into account is angular error.
That is to say, how sure are you that you're hitting the top of the object, or that you are shooting exactly horizontal?
The second case isn't so bad. For relatively small angular error (ie you're pointing in the wrong place), the error in distance is also very small.
The second case isn't so easy. If you're standing very far away from the object, it's the same as the first case and the error is very small. But if you're standing near the object, a very small angular error translates into a large error in distance. That is, if you're standing 10 feet from the building, and your hand shakes just a little bit, you'll go from shooting the 34th floor to shooting the 32nd floor very easily. There is a very good chance you won't hit the top of the building with the laser.
In addition, these lasers are most accurate when the beam encounters the wall at a right angle. As you shoot from closer and closer to the building, that angle gets smaller and the instrument error goes up.
It seems like your best bet is to shoot from very far away, except that also increases instrument error (see Jason's formula). So you have to find a happy medium. In my experience, this means being about as far away from the building as it is tall.
As you probably know, you can also decrease the error by taking several measurements and using the average. If you take five measurements, you'll get about one-fifth the error. The measurements should be taken from a variety of positions, not too close and not too far from the object.
My rangefinder (Bushnell YP Scout) has a scan mode, so I can hold the button down and get continuous measurements. I use this also to decrease the error. When measuring the distance to the base of the object, I move the rangefinder around a bit and take the shortest measurement. For the top of the object, I move it around and use the largest measurement I can get, usually it will flash between getting distances and showing no measurement at all because I'm shooting over the top of the building. That means I'm close.
I use ground measurements only as an estimate of the height, though. The final word comes when I'm at the top shooting down, which in most cases is about as accurate as you can get and will usually be within a yard or so. Again, I use the scan mode and take the shortest measurement I can find.
Michael
Arial obstruction maps as used by pilots. Mainly anntenas / smoke stacks and some buildings. Avalible at most airfields..........
Mostly I want to echo what crwper has said, with the (very minor) correction that error diminishes as the square root of the number of measurements, so that while halfing your error requires only four measurements, halfing it again requires sixteen. Life's a bitch. :P I find nine measurements is usually pretty easy, though clearly not all that appropriate for the "drive-by, just curious" type of measuring the original poster mentioned.
The error you've noticed is (as pointed out) probably due to angular errors. While the instrument error is plus or minus something, angular errors will always underestimate the height of a structure (it's easy to hit the 32nd floor of a 34-floor building, but not so easy to hit the 36th ;-) ) and will get worse as you get closer to the structure.
If that error is very small (which it isn't when you're close up), then the formulas above all apply. That should be the case (provided your hands are pretty steady) at as little as 1/3 of the object height from the base, and certainly at the full height away from the base.
Ditto on the "measure from the top", though, if you're looking to take full advantage of your rangefinder's accuracy.
If it's an A? Try www.berkana.com/:+
This doesn't open for me.
Do you have another?
Mike
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