Laser Targets: Important?

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Laser Targets: Important?

Postby GroundquestMSA » Oct 16, 2013 9:31 pm

I just re-read this old thread and wondered if some of the issues discussed therein have been settled or refined. Obviously the cost of laser rangefinders has dropped since then, or at least lower-cost models have emerged. And the pros and cons of laser surveying are of course well established. My main questions are about the targets used for laser shots. In my first exposure to surveying with a disto, the WUSS crew used the back of the Suunto as the target. Since then, I've been one of the "dummies" referenced by NZ who uses his hand as a target, or my survey book, or a rock, or if there's a need for a larger target, my belly. Since I've never gotten any obviously bad measurements, I hadn't considered the possibility that my targets were hurting my accuracy. I often have multiple measurements taken to verify a proper reading, with no noticable issues.

Is this a problem? Do you folks have any experiences that suggest that the proper target is vital?

Also, I have had a few issues taking longer shots in misty passage. Often the laser will give an error reading, and I'll try several times before getting a measurement. Should I trust these misty measurements? This has been a problem when shooting ceilings, so I have no way to verify the real distance.
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Re: Laser Targets: Important?

Postby NZcaver » Oct 17, 2013 5:37 am

Since you were kind enough to quote my words from long ago, I must now confess I occasionally used some of those "dummy" techniques myself. Especially for shorter shots in awkward positions - which, judging by your fantastic literary works, most of your surveying seems to be.

Back when I was last on a major survey project (sadly a while ago), we found using reflective targets (or at least white paper) made it substantially quicker to get a measurement - especially at around 40-50 feet or more . A faster fix also seemed to indicate a more consistent result when confirmed by a second measurement each time. If you're not doing so already I would suggest shooting 2-3 distance measurements for each shot, and if they are consistent then your question has been answered and you should concern yourself no longer. Accuracy is important in all cave surveys of course, but just a little more critical when you're dealing with extensive, complex systems and frustrating loop closures.

For what it's worth, I also found reflective targets especially useful surveying the circumference of large pits and entrances in daylight. Shooting through water or hanging tree roots - difficult at the best of times - also seemed a tad less frustrating when aiming at a target. All this was with older model Distos though, and technology has likely improved. Our biggest concerns were holding the laser steady and not shining it in any eyeballs.

P.S. I moved your topic into the survey and cartography forum where it seems to fit better.
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Re: Laser Targets: Important?

Postby GroundquestMSA » Oct 17, 2013 7:58 am

NZcaver wrote: If you're not doing so already I would suggest shooting 2-3 distance measurements for each shot, and if they are consistent then your question has been answered and you should concern yourself no longer.


I've been doing this for many shots, not all of them. If the target affects the accuracy though, then would shooting my hand twice give me two identical incorrect readings?

NZcaver wrote:Since you were kind enough to quote my words from long ago, I must now confess I occasionally used some of those "dummy" techniques myself. Especially for shorter shots in awkward positions - which, judging by your fantastic literary works, most of your surveying seems to be.


I'm wasn't complaining, I am a dummy. I wouldn't be too surprised to learn that I've been doing something tragically wrong for the past few years and that all of my surveys are crap. However, if you keep poking fun of my trip reports like this I just might run off and cry. Probably not. Should have a new one tonight.
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Re: Laser Targets: Important?

Postby NZcaver » Oct 17, 2013 3:56 pm

GroundquestMSA wrote:If the target affects the accuracy though, then would shooting my hand twice give me two identical incorrect readings?

I doubt it. But to really be sure, you could always double check with a tape. Rather defeats the convenience of a laser but it's an option.

I'm wasn't complaining, I am a dummy. I wouldn't be too surprised to learn that I've been doing something tragically wrong for the past few years and that all of my surveys are crap. However, if you keep poking fun of my trip reports like this I just might run off and cry. Probably not. Should have a new one tonight.

Here's a tissue. But seriously, I was being honest about your literary works. You really do have a talent for writing and I enjoy reading your regular installments.
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Re: Laser Targets: Important?

Postby JSDunham » Oct 31, 2013 8:51 am

We've been cutting out circles from the bottom of white plastic quart yogurt containers and bringing them in with the survey equipment. Durable, cheap, waterproof, washable, and the most reliable (according to my testing in the dark in fog, which is the best analog for our caves here in the northeast; tested compared to white paper, shiny white plastic, foil, mirror, and shiny black plastic).
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Re: Laser Targets: Important?

Postby trogman » Oct 31, 2013 9:14 am

JSDunham wrote:We've been cutting out circles from the bottom of white plastic quart yogurt containers and bringing them in with the survey equipment. Durable, cheap, waterproof, washable, and the most reliable (according to my testing in the dark in fog, which is the best analog for our caves here in the northeast; tested compared to white paper, shiny white plastic, foil, mirror, and shiny black plastic).


Mirror? Seems to me using a mirror for a laser target could have bad results- as in, the laser reflecting directly back into your eyes! :yikes: That would not be good!

The yogurt lids sound good, although I imagine there a number of other similar items that could be used.

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Re: Laser Targets: Important?

Postby Bob Thrun » Oct 31, 2013 7:46 pm

JSDunham wrote:We've been cutting out circles from the bottom of white plastic quart yogurt containers and bringing them in with the survey equipment. Durable, cheap, waterproof, washable, and the most reliable

Better yet, put some retroreflective tape (like Scotchlite) on your target. You can get it at any auto parts store. It greatly increases the range. Having > 50 meter shots would make sketching hard, though.
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Re: Laser Targets: Important?

Postby Scott McCrea » Oct 31, 2013 8:24 pm

Yup, if you look at land surveyors targets, they are reflective. And, they shoot in the daylight over long distances.
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Re: Laser Targets: Important?

Postby GroundquestMSA » Oct 31, 2013 8:55 pm

Bob Thrun wrote:Better yet, put some retroreflective tape (like Scotchlite) on your target. You can get it at any auto parts store. It greatly increases the range. Having > 50 meter shots would make sketching hard, though.


If my laser had that range, I might consider a "proper" target. I'm using a Bosch with a range of only 120'.

Recent testing in the yard showed no variance when different targets were used. I used the following targets: tree, skin, white paper, black t-shirt, and the back of a Suunto. I shot from 20' and 46' and the results varied no more that 3/8", and that possibly due to operator movements. I also tested the consistency near the range limit. 119' measurements were repeatedly consistent to within 1/8", though I used only one target (sheet metal barn siding) for this test.

It appears that for practical cave surveying purposes, any target will do. At long range though, a small target is easy to miss, and there have been cases when the laser "slipped" off target and measured to some point behind the intended station.
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Re: Laser Targets: Important?

Postby JSDunham » Nov 4, 2013 8:42 am

GroundquestMSA wrote:It appears that for practical cave surveying purposes, any target will do.


I have to disagree--I think it very much depends on the cave. Using a Disto, I have found dramatic differences in high-humidity conditions. Outside testing in good weather was always fine (hence my testing in fog rather than clear air). The humidity in our caves means swirling clouds of fog, dust, and particulates that diffuse the laser and make long readings difficult. A good target makes all the difference in these circumstances. Likewise, irregular, angled, wet, or muddy surfaces (i.e., most stations) are not always reliable and the target again makes the difference.
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Re: Laser Targets: Important?

Postby Chads93GT » Nov 4, 2013 5:23 pm

river caves make laser surveys very tricky. As if trying to keep the disto dry isnt enough, long shots are almost impossible ( in my experience) due to heavy humidity while cavers are breathing and overheating in wetsuits while in and out of the water.

Also, in my experience if you drop your disto a giant catfish may eat it, but im sure that isnt the norm in most cave environments ;)
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Re: Laser Targets: Important?

Postby GroundquestMSA » Nov 4, 2013 5:55 pm

JSDunham wrote:
GroundquestMSA wrote:It appears that for practical cave surveying purposes, any target will do.

I have to disagree--I think it very much depends on the cave.


I suppose so. I guess it's time for some more complete testing. I'll have smoke, fog, and time at my disposal in a bit. I've always assumed that the reason dust and fog are problematic is that the laser reflects off of a myriad particles, confusing the sensor whose job it is to measure the time it takes for a pulse to travel to and from the target. You're shooting at thousands of targets at once. If that is the case, how does the target make any difference? I understand that range could be enhanced with reflective targets... but accuracy?

Did you get it back Chad?
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Re: Laser Targets: Important?

Postby Extremeophile » Nov 4, 2013 6:31 pm

JSDunham wrote:
GroundquestMSA wrote:It appears that for practical cave surveying purposes, any target will do.


I have to disagree--I think it very much depends on the cave. Using a Disto, I have found dramatic differences in high-humidity conditions. Outside testing in good weather was always fine (hence my testing in fog rather than clear air). The humidity in our caves means swirling clouds of fog, dust, and particulates that diffuse the laser and make long readings difficult. A good target makes all the difference in these circumstances. Likewise, irregular, angled, wet, or muddy surfaces (i.e., most stations) are not always reliable and the target again makes the difference.

I've had difficulty getting readings when the shot is long and there's a lot of mist, but I haven't had issues with false readings. With the Leica distos you generally get the "255" error when the laser reflects off multiple surfaces (e.g. partly glancing off an intermediate wall, or a dome full of water droplets). When I got mine I also tested it on many different substrates and got identical measurements. I also verified the accuracy with a fiberglass tape. It's safe to say I survey a lot, and almost always use a disto for distance measurements, and the number one target is a gloved hand. There is a small sacrifice in accuracy when using a hand, and I generally record distances to the nearest 0.1 ft (i.e. less than the precision of the device), but I have not found this to cause loop closure problems. In one cave I've been working in we've done disto shots up to 275 ft. The point person was using a white trash bag as a target, and shots this long do create some additional work for the sketcher.
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Re: Laser Targets: Important?

Postby GroundquestMSA » Nov 4, 2013 7:44 pm

After 35 shots, here are my results. I lifted and reset the laser between each shot. the first 4 sets were measured between the same points. The fifth was seperate.

Target 1: white reflective plastic, clear atmosphere

Bosch DLR130
shot 1 - 33' 7" 1/2
shot 2 - 33' 7" 1/2
shot 3 - 33' 7" 9/16
shot 4 - 33' 7" 7/16
shot 5 - 33' 7" 7/16

Kobalt LDM-35
shot 1 - 33' 7" 9/16
shot 2 - 33' 7" 9/16
shot 3 - 33' 7" 9/16
shot 4 - 33' 7" 7/16
shot 5 - 33' 7" 7/16

Target 2: olive cordura survey book, clear atmosphere

Bosch
shot 1 - 33' 7" 5/8
shot 2 - 33' 7" 5/8
shot 3 - 33' 7" 5/8
shot 4 - 33' 7" 5/8
shot 5 - 33' 7" 9/16

Kobalt
shot 1 - 33' 7" 9/16
shot 2 - 33' 7" 7/16
shot 3 - 33' 7" 7/16
shot 4 - 33' 7" 7/16
shot 5 - 33' 7" 5/8

Target 3: dirty concrete block, through very thick smoke from 3 variably placed sources. To further obscure things, I also shined a headlamp into the smoke, which was heavy enough to make the target hard to see clearly. (by this time I had tired of redundancy, and stopped using the Kobalt)

shot 1 - 33' 7" 5/8
shot 2 - 33' 7" 9/16
shot 3 - 33' 7" 9/16
shot 4 - Err
shot 5 - 33' 7" 7/16

Target 4: white reflective plastic set at an extreme angle, through smoke (not as thick this time, but enough to pick up the laser beam) In order to hit the same point of the target each time, I did not move the laser between shots.

shot 1 - 34' 0" 1/8
shot 2 - 33' 11" 7/8
shot 3 - 34'
shot 4 - 34'
shot 5 - 34' 0" 3/16

Target 5: brown rock stuck in the mud bank. The spring beside my house isn't throwing as much fog as I had hoped, it being a bit too warm outside. These shots were fired directly over the stream through enough fog to show intermittent sections of laser beam.

shot 1 - 80' 2" 13/16
shot 2 - 80' 2" 7/8
shot 3 - 80' 2" 13/16
shot 4 - 80' 2" 11/16
shot 5 - 80' 2" 1/2

This was enough for me to decide that if the instrument gives you a measurement, it's probably right, no matter the target. This doesn't mean that you're always going to hit your target. Shooting to a small rock on the floor is obviously risky. If you jiggle and miss you aren't going to know it. My original concern was that I may be hurting my accuracy by using whatever I had at hand (including my hand) for a target. Laser targets: important? To maintain accuracy... nope.

I finished my testing by measuring with a fiberglass tape. My wife and I took turns with the tape and wrote down the measurements.
She - 33' 7" 15/16
I - 33' 7" 7/8
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Re: Laser Targets: Important?

Postby driggs » Nov 4, 2013 11:00 pm

GroundquestMSA wrote:shot 1 - 33' 7" 1/2


Everyone involved in your cave survey -- including yourself -- will thank you when you configure that laser to read decimal feet rather than fractional inches. Same goes for your fiberglass tape.

:big grin:

(Cue obligatory metric system quip)
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