Suunto compass/clinometers - and magnetic dip

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Suunto compass/clinometers - and magnetic dip

Postby NZcaver » Oct 2, 2005 3:12 pm

Last year I started getting more involved with cave surveying, and picked up a Suunto Twin (the combination unit with the prisms on top).

I like it, but sometimes I find it tough making sure my eye is square-on to the compass to avoid parallax errors. I have seen other cavers using Suunto's sister instrument, the Tandem. I've only had a brief play with one, but it's sighting system seemed slightly easier to use and possibly more precise.

I wonder if I should "upgrade" to a Tandem..? :?

On a side note, has anyone taken their compass and gone surveying in other parts of the world? If so, did you find it difficult to get accurate bearings because of magnetic dip and what (if anything) did you do about it? Considering most survey shots in caves aren't perfectly level anyway, what if you just manually compensate for the dip by tilting the compass slightly?

[FYI - magnetic dip is experienced when (for example) a compass disc manufactured for the northern hemisphere has a tendency to dip downwards toward the north pole when used in the southern hemisphere.]

Any comments? :caver:
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Postby George Dasher » Oct 3, 2005 12:49 pm

What do those prisms on top do?

The surest way to make sure you're not having parallax problems is to have someone else shoot backsights. In theory, if you're shooting your own backsights, a shift to the right with both shots could equal "an ok" azimuth.

Regarding the magnetic dip: I was told, long ago, that compasses are calibrated for their part of the world to correct for this. However, I have no idea if this is correct or not--and it seems strange that you could buy a compass that is "good anywhere in North America," since the dip is going to vary greatly between northern Canada and the southeastern US. Bruntons sometimes have a little wire on the compass needle to correct for magnetic dip, but I don't know what Suuntos have--only that they work. I don't think you could correct for magnetic dip by just holding the compass a certain way.
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Postby Cindy Heazlit » Oct 3, 2005 1:56 pm

We have rather nasty magnetic dip problems in Hawai`i. The dip is enough to cause the compass to catch on the roof of the case. In other words, it doesn't swing freely when you're trying to sight it, no matter what.

The solution we've used is to get the compasses rebalanced for Hawai`i.
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Postby NZcaver » Oct 6, 2005 12:50 am

Thanks for the replies. :D

George - the "prisms" on top of the Suunto Twin contain a lens and mirror to sight into - like a prismatic compass. (Unlike the Tandem, where you sight directly into the edge of the compass or clinometer disc.)

Accuracy hasn't been a problem with my Twin - we shoot foresights and backsights on most surveys and my bearings have been fine. I was just wondering if anyone has used both the Tandem and the Twin, and can comment on the finer points of shooting bearings with each model.

As for magnetic dip - I was also told if you buy a Suunto compass from a dealer in one country (or region), you get a model set for the magnetic dip of that location. This setting is non-adjustable, and is "invisible" to the user (the compass looks just the same as one purchased anywhere else in the world). I agree - this doesn't make sense considering the wide magnetic dip variation experienced throughout North America... and Hawai'i (hi Cindy!) :wink:
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Postby George Dasher » Oct 7, 2005 9:27 am

I think your dealer gave you the correct information about the magnetic dip. Maybe North America is divided up into more than one area.

I was concerned about the prism because prisms reflect the light at right angles to the prism. If the compass is held horizontal, then this reflected light will be directly above or below the station. You are thus "on station."

But if the compass is not held horizontal, then the reflected light will be off to one side of the station, and you are then "off station."

The bottom line is that you can hold the old-fashioned Suunto pretty horizontal by making sure the hairline is parallel to the increments on the dial and by making sure that the dial floats free. But it is my understanding that the compass cannot be held horizontal enough to make the use of a prism accurate. The reflected light is not vertical and the compass is "off station. Thus prisms on Suuntos are inaccurate.

But if you're getting good backsights, then maybe I am wrong...
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Postby NZcaver » Oct 7, 2005 2:33 pm

George - interesting comments, thanks. :D

I think I understand what you mean by the reflected light being at right angles to the prism. But as long as you keep the instrument horizontal so the disc floats (which you should be doing anyway), this won't be a problem - right? And I guess the same rule applies for keeping the instrument vertical when shooting inclinations.

On my brief look at a Tandem, I noticed the compass "sight picture" looks like a black circle with a vertical slot in the middle through which to read the numbers (almost like looking through a keyhole). This seems more precise to me that sighting with the Twin - as long as you can see the number and the disc is free-floating, you have your bearing. With the Twin, you need to take the additional step of ensuring that you are square-on to the lens. There is a red dot on the side of the housing to help with this, but if you're lighting up the top of the instrument to read your bearing it can be hard to see the dot on the side at the same time! :(

Maybe I should just shut up and buy a Tandem too - that way I can do a proper A-B comparison... :wink:
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