Objet: RE: Cave-Surveying Digest, Vol 12, Issue 1

[stuart]

Hi, Interesting stuff....
But it seems the issue is around the type of laser used, the method, the 
objective and the survey grade....
basically laser are all the same, the brand name may change unless you guys 
somehow end up with real under powered unit...

There is no question that a laser is far more accurate... +-2mm per 100m and 
about +- 5mm up to 200m.
In a cave environment they are not really effected by atmosphere 
conditions...
However on the surface this effect and error can be +-25mm over 200m
best to use at night or dark places...

The issue of time to take a laser shot is less than a second....in fact they 
do about 100 samples to calculate a distance in that time....
It more the issue how still you can hold a dot, the size of the dot, the 
unit sampling time which will effect the sampling result...
hand held is only good for distances less than 50m. the shorter the 
distance, the less movement, the smaller the dot, the more accurate.
tri-pod or a stable surface 50 to 100m...
100m plus requires a Tri-pod and a target plate... (regardless how good the 
laser is)

Water, water on cave walls,  marble, dolomite caves, smooth angular surfaces 
create an issue for the reflecting laser dot, ineffective returns....
The reverse, a surface can be too light absorbing or too rough fracturing 
the dot for a good return...
So it take practice to shoot straight for a straight return, and to know 
what a good target is.

Bottom line, an off the self laser rated for 200m is only effective for 
around 70metres in a cave... (less than a 200m rating is almost a waste of 
time)
the shorter the distance naturally increases the number of hits...or 
measurements., less trouble some...
Compass is a compass, the error is what it is...the Laser however adds an 
advantage given you can take a plethora of measurements per 1 compass...
so while you may stay in short compass legs, you can over shoot each station 
to the next...Hence a stack of  triangulation measurements
ceiling heights, cross sections, details etc....
The trick is always shoot at either level or 90 degrees down...even if you 
carry a stick around or use 2 lasers is best...
Your caving lights will impact on the laser dot, giving it a very poor 
return, avoid direct lighting of the station or dot...
A very good way to over come a poor surface...is lights out or go near dark, 
it will return off all most any thing and increase your distance...
if the dot is being bounced, you will see it...

The thing is, with a Laser it can massively increase your collected data to 
a point of overkill for any survey...and 3D gets very wild...

I have been on the list for about 3 years and i find your topics when your 
in a chatty mood, which is rare, interesting...
i am not at your level of surveying or being so pedantic..
I am Australian cave diver...So underwater we don't have the luxury of time 
or can we achieve such a high survey standard as you guys...
hence it is carried over to our dry cave standards which is poor...
A cave like Cocklebiddy which is 50m across wall to wall, 10 to 25m floor to 
ceiling, 100, 300m leg lengths.. 6km long, 90% water filled...
Needs, we use every trick in the book, Radio location, Pingers,GPS, Lasers, 
dive computers, Tapes...per tested and tension survey lines to a Kg rating 
etc
and always our weakness is compass...There are no compasses we can use for 
accuracy, so have to make our own which about the size of a CD.
and rely heavily on triangulation and other tools...
On the subject of Lasers is something  i use allot...even to designing an 
underwater laser which is very tricky to do...
Wouldn't leave home without 1 or 2 for a dry section cave survey. , but they 
are just a tool like a tape is...don't leave home without them either...

The argument is not quite right...you cant match a lasers for accuracy over 
distance...at 100m you going to be +-2mm...
it is a light wave it doesn't change... it has a percentage calibration 
error which is known.... .+-.025 per metre...
Even a perfectly calibrated 100m Tape Measure can not be pulled straight, 
and if by chance the ground surface was perfectly flat..
heat /cold would shrink or expend the tape more than 2mm...you would need a 
perfect temperature to go with the rest...

You can argue the method of how a laser is used...the shorter the distance 
the more likely human error will effect the true distance..
if using a laser i feel you should always take the longest shot you can as 
part of the overall survey...
laser are used for everything from mapping the moon to mapping earth to less 
than a 1 metre contour.

Cheers  Stuart


----- Original Message ----- 
From: "Luc Le Blanc" <lleblanc at cam.org>
To: <cave-surveying at survex.com>
Sent: Wednesday, July 16, 2008 3:20 AM
Subject: Objet: RE: Cave-Surveying Digest, Vol 12, Issue 1


> Paul De Bie wrote:
>
>> I agree that the nominal accuracy of laser tape is better, however at 
>> several occasions we have seen
>> that, when taking long shots (over 20 m) there is a higher risk of errors 
>> than when using a tape.
>> The longer the distance, the bigger the possibility to "miss the target".
>> And many laser tapes need some time (sometimes over 1 second) to actually 
>> measure the distance. Even
>> when you are on target when pressing the button, you might easily 
>> "tremble off" the target while the
>> laser is measuring.  The use of a tripod is much recommended in such 
>> cases!
>
> That's why I like the timer option on my Disto A6. Press the button to
> start a 5-second countdown after which the measurement is actually
> taken. It leaves you plenty of time to lock in on the target - too
> much time in fact, but that delay is not adjustable. I always use this
> option on long shots. Alas, this option is not available on lower end
> Distos.
>
>
> Luc Le Blanc
>
> Take your PDA where the Sun never shines...
> Go map a cave!
> http://www.speleo.qc.ca/Auriga
>
> -- 
> Cave-Surveying http://lists.survex.com/mailman/listinfo/cave-surveying