Tony,

On Tue, 28 Feb 2006 20:50:39 -0600, Tony Harris wrote:

> Hi all,
> 
> I'm tinkering with an idea, but I think it is beyond me - I'm not sure tho. 
> Basically, it's a height at a given distance determination.
> 
> I was thinking of using 2 sensors one angled down and one angled up, using 
> the angle between them, and the distance calculated from the sensors to 
> determine the height.

Basic triangulation - measure the horizontal distance, and the angle from horizontal to the top.  I did it 
once to measure the height of a tree...  of course it may be that the thing you're measuring doesn't have 
anything directly below it, in which case you have to measure the slant distance.
 
> This worked out pretty good until I found that only short range sensors are 
> cheap :)  I'd like to figure out something that would read say a hundred 
> feet ahead.  The idea would be to measure the height from an overhang or 
> bridge or opening to ground from a distance.

I think Polaroid do ultrasonic sensors that go up to 30m, but laser would be much more reliable.  Not sure of 
the costs, though.
 
> I was then thinking of some sort of camera system to capture an image, say 2 
> per second, but then came the problem of determining automatically what is 
> above and where "ground level" is to that above area.

I'm not sure what your camera would tell you, or why you want more than one picture? 
 
> So, if you can imagine - in my rather lame ascii art....
> 
> Top of overhang
> --------------
>         \
>          |
>          | Heigth to calculate
>          |
>          |---------------------------------------------------------------------- 
>  > Point of origin, exact height (give or take) by combination
>          |                                     < --  Distance, 25 - 100 
> feet -->                       data entry or perhaps gps (got that idea from 
> looking at gps data
>          | 
> from a remote control helicopter video feed project on the net)
>         /
> ----------------
> Ground
> 
> I'm sure you can see why the triangle option is very tempting, the problem 
> is distance, doing it without lasers (would suck to have someone walk in 
> front of my new toy and get blinded and sue me because I was experimenting.)

Surveying lasers aren't dangerous to eyesight - they wouldn't be allowed otherwise!  Imagine roadside 
surveying blinding (even temporarily) a passing driver... I think they use Infra Red so you don't even see 
them.
 
> That's why I was thinking images, but I don't even know where I would start 
> research on something like this.

You need to measure a distance - there really is no other way unless you know this, otherwise a 10' height at 
100' looks the same as a 20' height at 200', so anything purely optical with no distance measurement won't 
work.

Cheers,


Howard Winter
St.Albans, England


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