Foresights vs backsights

[volica]

http://www.volica.org/foreback.php

Always had problem with that....

[Scott McCrea]

Cheater.



That will make a nice addition to the back of the survey book. Just don't give it to the instrument guys--they will cheat.

[boogercaver71]

Especially when surveying in a water crawl

[Jeff Bartlett]

yeah, seriously keep it away from the instrument guys. when i'm reading backsights i specifically don't listen at all to what the other guy is reading, but it's tough.

by the way, Auriga does this for you =)

[Bob Thrun]

Or you can get a compass marked in grads, 400 to a circle. Then you either add or subtract 200 to get the backsight. The only problem with grads is sketching with a protractor. Grads protractors are not available in the US, although they can be found in Europe.

[NZcaver]

Or you can get a compass marked in grads, 400 to a circle. Then you either add or subtract 200 to get the backsight. The only problem with grads is sketching with a protractor. Grads protractors are not available in the US, although they can be found in Europe.

You can also find military compasses marked in mils (6400 to a circle), but like grads I can't say I've personally seen them used in cave surveying. Protractors in mils are also a little hard to find.

Volica - thanks for posting the table.

[Jeff Bartlett]

Or you can get a compass marked in grads, 400 to a circle. Then you either add or subtract 200 to get the backsight. The only problem with grads is sketching with a protractor. Grads protractors are not available in the US, although they can be found in Europe.

actually, i would go ahead and say that the major problem with using a grad compass is you either have to (A) buy two of them or (B) find someone else in your survey team who thinks it's a good idea too =)

[Martin Sluka]

The only problem with grads is sketching with a protractor. Grads protractors are not available in the US, although they can be found in Europe.

I may send you several of them, but much more easy is to make your own:

http://therion.speleo.sk/protractor/index.php

Print it on imagesetter and laminate it. If you need any modification it is not very difficult to change the code and generate it.

[Bob Thrun]

actually, i would go ahead and say that the major problem with using a grad compass is you either have to (A) buy two of them or (B) find someone else in your survey team who thinks it's a good idea too =)

Cave Research Foundation practice is to use only one set of instruments and have the same person reading both foresights and backsights. The makes the fore and back sights agree much better than having two sets of instruments and two readers. It eliminates compass eccentricity error and clinometer bias error. The differences between instruments and readers on different surveys are reduced by taking a shot between two permanent stations at the start of every survey.

I have done a survey where one set of instruments was in grads and the other was in degrees. In fact, we were leapfrogging. One shot would have the foresight in degrees and the backsight in grads. The next shot would have the foresight in grads and the backsight in degrees. It was about as easy/dificult comparing fore and back sights as if both were in degrees.

I find 6400 mils less convenient for doing backsight arithmetric. There are actually two kinds of mils. The American Army uses 6400 mils to a circle. The French and Russian armies use 6000 to a circle. The Suunto KB-14 can be gotten in any of these units.

[driggs]

Hint: you can make your chart half that size by cutting out everything from frontsight=180 and up (for frontsights greater than 180, lookup the second number instead of the first).

[NZcaver]

Cave Research Foundation practice is to use only one set of instruments and have the same person reading both foresights and backsights. The makes the fore and back sights agree much better than having two sets of instruments and two readers. It eliminates compass eccentricity error and clinometer bias error. The differences between instruments and readers on different surveys are reduced by taking a shot between two permanent stations at the start of every survey.

Hmmm, interesting. I must confess that's not how we did it when I've been an instrument reader/gopher on a few CRF surveys. We always used the standard method with each person having his/her own instrument. I can see how one instrument and one person may help improve the error between foresights and backsights, which if I recall should be less than 2 degrees to be "acceptable" anyway, but surely it must take at least twice the amount of time and effort to cover the same ground?

[cavedoc]

but surely it must take at least twice the amount of time and effort to cover the same ground?

Hmmmm. Methinks this is a way to slow down the instrument people to allow more time to sketch in pebbles on the floor. I'll have to try it!

[NZcaver]

but surely it must take at least twice the amount of time and effort to cover the same ground?

Hmmmm. Methinks this is a way to slow down the instrument people to allow more time to sketch in pebbles on the floor. I'll have to try it!

Ah of course it is. Good point, Roger! This sounds like similar reasoning to why you don't let the instrument guys operate the Disto.

[Spike]

In regards to the use of one instrument reader for CRF surveys, at times there is confusion among crews.
It is prefered to use one reader and one instrument, if the cave passage cooperates. Often the passage being surveyed is big enough for the people to move around. If your reader is good and makes few errors it is no slower than using two people even in a crawlway.
The lead tape would hold a light on station for the front site, and until recently we were required to have 4 people on every trip into the cave, this extra body often holds a light for the backsite and no switching is needed. If there is no need to reshoot the instument reader repositions and shoots the next shot. The crew inches along as normal. However I always take multiple sets of instruments(they can and do fail in cave) and often use 2 sets in nasty passage.

Gravel? it must be under all the breakdown in the cave:)

[rchrds]

Here is a quick rule I learned in flight school (of all places):

+2, -2 (or -2, +2)

Here is how it works-

take your azimuth, say 341 degrees.

if you can subtract 2 from the first digit, then add two to the second digit. (3-2=1, 4+2=6) so the reciprocal becomes 161.

another example, this time, the other way: 073 degrees. Since I cant subtract 2 from zero, I'll add 2 to the first digit, subtract 2 from the second digit. (0+2=2, 7-2=5) so the reciprocal becomes 253.

The only time this wont work is from 181-199 degrees but that is super easy (subtract 180), and 001-019 (add 180), so it's not perfect, but even the obtuse mathematician can figure it out in his or her head.

(Try doing it in the clouds on short final with turbulence when you forgot which runway your were landing to and approach is waiting for an answer... even I can figure it out! )

hope it helps.

J