calling all mathmaticians

[thubby]

i think...
is there any way to determine an objects distance from you based on its relative size? assuming you know how large it is to begin with.

[Dallas-Dakota]

Yep there is. I know there is.

But cannae remember the formula out of my head.

ZP we need you to save the day!:smalltongue:

[Phishfood]

Yes.

Basically, hold a ruler at arms length, measure the apparent height of the object at distance. Measure the distance from ruler to eye. Divide height of far object by apparent height of object. Multiply the answer from that by the distance from eye to ruler and you have the distance.

[Khanderas]

Normal trigonomitry ?

See it as two triangles, assume both items you are measuring are standing at a 90degree angle.

You have A (big a) as the item you want to know the distance to.
You have B (big b) as the lenght you want to find out.

You have a (small a) as your comparing item. You know how long it is.
You have b (small b) as the length between your eye to the item. You can judge this easily.

Since the angle opposite to A and a, are the same (since you see both items as equally "tall", you first calculate that angle, using tan (a/b) to get that angle. Lets call it angle X

Then calculate B with the forumla A / (arctan X)

I think it should be something like that.

[Zar Peter]

Yep there is. I know there is.

But cannae remember the formula out of my head.

ZP we need you to save the day!:smalltongue:

It seems I'm too late...

I approve all methods that are mentioned before, but I must warn that they are not very accurate (depends on the distance but I think it's about +/- 10%).

If you want to have an accurate method call your local surveyor and pay him/her a hugh amount of money :smallbiggrin::smallbiggrin:

[Totally Guy]

I always assumed it was some kind of inverse square law. But it must exist. 3D computer programmers must use it to calculate size relative to distance on the screen.

Then there would be a variable for perspective. Like in a scene in a horror movie where it looks like both the foreground is getting further away and the background is getting closer at the same time.

[Deathslayer7]

easiest way is probably using the two similar triangles and using trig as khanderas suggested, but maybe that is just me.

[golentan]

I always assumed it was some kind of inverse square law. But it must exist. 3D computer programmers must use it to calculate size relative to distance on the screen.

Then there would be a variable for perspective. Like in a scene in a horror movie where it looks like both the foreground is getting further away and the background is getting closer at the same time.

I guess you could do inverse square, with the visible cross section or something. I agree though that apparent height and trigonometry is the best method though.