How many eyes with Suuntos?

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Do you use one or two eyes with a Suunto compass and inclinometer?

One eye with compass, one eye with inclinometer
16
59%
Two eyes with compass, one eye with inclinometer
1
4%
One eye with compass, two eyes with inclinometer
1
4%
Two eyes with compass, two eyes with inclinometer
9
33%
 
Total votes : 27

How many eyes with Suuntos?

Postby rlboyce » Mar 22, 2011 7:38 am

Searched all over the forum and I'm sure the discussion is here somewhere but I couldn't find it.

I'm curious, how many eyes do you use? Does it really make a difference with a normally functioning pair of eyes? I'm specifically looking for data where one and two eyes were used, and error analysis of each.
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Re: How many eyes with Suuntos?

Postby Scott McCrea » Mar 22, 2011 7:57 am

From the Suunto KB-14 users manual:
"USAGE
Keep both eyes open and aim with the compass so that the sighting line meets the target when viewing through the lens. In R-models the main scale indicates the reading to the target you are aiming at and the reversed scale indicates the reading from your target to the aimed location. This feature is of great help, especially at sea, when determining exact locations. Use your left or right eye for reading the compass depending on your personal preferences. When both eyes are open, an optical illusion makes the sighting line extend above the body of the compass over the target. This increases reading accuracy and speed.

Heterophoria (an eye disorder some persons suffer from) may deteriorate the reading accuracy at times. This can be controlled in the following fashion: Keep both eyes open and take a reading. Then close one of your eyes. If there is no significant change in the reading, there is no directional discrepancy in the vision axes of your eyes and both eyes may be kept open during orienteering. If a charge is noted, keep only one eye open and aim half way over the compass body. The sighting line now rises above the instrument body and is visible against the target."
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Re: How many eyes with Suuntos?

Postby rlboyce » Mar 22, 2011 8:24 am

Yes, I have read this from the user manual as well. Actually, I was looking for data. I believe in numbers! :big grin:

I've come across an older Compass & Tape article that claims you should use one eye for a compass, not two (page 16): http://www.caves.org/section/sacs/back/v06i2_21.pdf I was wondering if anyone else can back up this author's claims or disprove them with another error analysis. I am especially interested in inclinometer data since this author has not included it.
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Re: How many eyes with Suuntos?

Postby Chads93GT » Mar 22, 2011 8:36 am

Both eyes. had 2 1/2 to 3/4 mile long loops closed with .02% error with 2 eyes. i go by doing whatever works for you.
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Re: How many eyes with Suuntos?

Postby rlboyce » Mar 22, 2011 9:22 am

That makes sense Chad, seeing as the end result is what counts. If you can do it either way and still get acceptable results, I guess it doesn't really matter then.

Anyone have negative experiences from using one eye?
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Re: How many eyes with Suuntos?

Postby Extremeophile » Mar 22, 2011 10:16 am

I was taught from the beginning to use one eye. I don't have any data to share, but at the time it was demonstrated to me that by using 2 eyes and superimposing the line image over the target image you can get 1/2 to 1 degree error. This is due to parallax error, i.e. the angle difference between your right and left eye and the target.

So, as an example, if you use one eye (say your right) and line up to the target and you get a reading of 35 degrees, then with 2 eyes you might get a reading of 34.5 degrees. I would expect the error to be greater the shorter the survey distance (greater parallax angle). Also, if you have one surveyor always using the same technique then front / back sights will agree and loop closure errors should be low because you have the same error on all shots (i.e. a systematic error, much like an instrument out of calibration). I would expect that the line plot might just be rotated ~0.5-1 degrees from its true location (maybe not significant for most people). It's more likely to be a problem if you have multiple surveyors - some using one eye and others using two. I'm not sure you can correct for this error with something like a compass course due to the fact that the angle error is a function of distance to the target.

If you use two eyes and both eyes are in-line with the target (as with a normal clino shot), then there shouldn't be a parallax angle and therefore no error, but perhaps this would prove confusing to many new surveyors and should be avoided.

I believe it's much easier to read instruments with two eyes, but you need to be aware of the error that's being introduced.
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Re: How many eyes with Suuntos?

Postby Aaron Addison » Mar 22, 2011 10:29 am

I have used both methods with success. I lost most of the vision in my left eye in 2005 and have read instruments on several surveys using the single eye method since that time with no problems.

@Chad - I would agree that you should go with whatever works on smaller projects. This is more difficult on larger projects like Lechuguilla and Mammoth where surveys last decades and have people come and go. In this case I would recommend that all instrument readers use the same method.

Cheers,

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Re: How many eyes with Suuntos?

Postby Extremeophile » Mar 22, 2011 10:45 am

Maybe I should have read your linked Compass and Tape article before posting. I think for most surveyors the conventional wisdom has been that the error is due to parallax, but this article suggests otherwise. Regardless of the cause of the error the data here supports using one eye and not two. It's easy enough to prove this to yourself - take a reading both ways and see if there's a difference. I think you'll find there is, and then you can ask which of the two methods is more accurate.

I'm not a gun enthusiast, but I believe you'll be a more accurate marksman using one eye to sight rather than two.
Last edited by Extremeophile on Mar 22, 2011 2:03 pm, edited 1 time in total.
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Re: How many eyes with Suuntos?

Postby Scott McCrea » Mar 22, 2011 11:06 am

I believe, and I have no data to back it up, that two eyes is fine when you are standing normally on both feet. However, when standing on you ear, taking a challenging cave survey shot, one eye is better, because of the parallax thing and general wonkiness.
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Re: How many eyes with Suuntos?

Postby Tlaloc » Mar 22, 2011 11:27 am

rlboyce wrote:I'm curious, how many eyes do you use?


Did I miss something? Do some cavers have three or four eyes?

The correct way to do it is to look through the Sunto with one eye at the target with the other isn't it?

This is a joke, right?
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Re: How many eyes with Suuntos?

Postby cavedoc » Mar 22, 2011 12:17 pm

Tlaloc wrote:This is a joke, right?


Not at all. Surveyors have to fight about something :boxing: The directions that come with the instruments are assuming shooting at a mountain or something miles away. In that case the small distance between your eyes is irrelevant. When you're doing short distances that can turn into something that is outside of standard error. If you systematically changed eyes it would probably cancel out but most of us have dominant or preferred eyes for using the instrument so we keep using the same one over and over. That makes the error always happen in the same direction. For a few shots, no big deal. For long shots, no big deal. But there are no long shots where I survey so we tell everyone to use the one eye technique.

But I'm tempted to set up a course at the next Regional and have everyone do it one-eyed and then two-eyed and see how it turns out. Data is good.
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Re: How many eyes with Suuntos?

Postby Phil Winkler » Mar 22, 2011 12:32 pm

Interesting topic and taking me back to my days as a Fire Control Instrument Repair instructor at Aberdeen Proving Ground.

Parallax: the apparent movement of the reticle (of the instrument) over the field of view.

Dominant eye: Hold a finger in front of you and aline it on a distant object like a telephone pole. Now close each eye in turn and watch your finger. When your finger jumps to the left or right of the object when you close an eye then the other eye is your dominant one. It is termed being right-eyed or left-eyed. And don't get me started on crossed-eyes.

In general, aiming anything is usually done with both eyes open.
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Re: How many eyes with Suuntos?

Postby rlboyce » Mar 22, 2011 12:50 pm

Cavedoc, I'd very much like to hear the results if you decide to hold that course. I'm sure others would too, seeing as there doesn't seem to be a clear consensus or definitive proof as to which is the superior method, if either is superior at all.
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Re: How many eyes with Suuntos?

Postby Squirrel Girl » Mar 22, 2011 1:44 pm

So, I was reading this thread, not understanding how two eyes could possibly be better than one. I mean, how could anything be better than three points that form a straight line, eye, compass hairline, and target. If you add a second eye to the side, it could only make things worse [not counting empirical examples in which "real life" complicate theory, and there, all bets are off].

I tried reading the Compass and Tape article and it more or less agreed with me, but I zoned on it. So I downloaded a Suunto manual and it said this:

With both eyes open, aim the compass so that the hairline is superimposed on the
target, when viewed through the lens. The main scale (large numbers) gives the
bearing from your position to the target, the small numbers give a reverse bearing
from the target to your position. This feature is of great assistance when calculating a
precise position.
Use the left or the right eye as preferred. With both eyes open, an optical illusion
makes the hairline appear to continue above the instrument frame, superimposed on
the target. This improves reading accuracy and speed.


So now I think I understand. *If* you can make sure the hairline is directly above or below the target, you're golden. The problem comes in that the target and the hairline aren't super imposed (except in folding mirror style compi). So, it all depends on if the error you generate by trying to look into the compass and at the target is greater or less than the parallax issue of both eyes open. Suunto seems to think the parallax is the lesser of two evils. I would think that depends on the distance to the target and the skill of the instrument reader. And various other things like the awkwardness of the station.

So, an experiment is an ideal thing to do, to see which is better for any individual.
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Re: How many eyes with Suuntos?

Postby caverdan » Mar 22, 2011 2:28 pm

Squirrel Girl wrote:So, an experiment is an ideal thing to do, to see which is better for any individual.


Sounds like the perfect thing for you to do at this years convention!!! :bananabat:
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