Today's cave survey programs can correct for the changing magnetic
declination. I had a 15 year series of surveys on a large Ozark cave
that looked terrible in closure, until I turned on the declination
correction in WALLS and made sure all the survey book entries had a
proper date.
It has been shown in the past with major survey projects that it is
always best to set the declination of each compass to "zero",
particularly when folks participate from different areas where survey
techniques may be different. (We found that some people set the
declination in the opposite direction.) Some projects have set up
50-100' long N-S and E-W lines between posts (using a transit with
large compass circle) near the expedition headquarters and require
people to sight in any compass (in both directions, 4 in all) they
intended to use in the cave prior to going into the cave. On Bruntons
this would detect any declination setting, hanging needles, bent
needles, and the sighting ability of the compass person. Of course it
was expected (but not always followed) that backsights would be made
in the cave. Of course, in Mammoth Cave where you sometimes had
current carrying cables and transformers emitting magnetic fields, you
had to very carefully choose on which end of a sighting line you
positioned the compass, and then just repeat that single-ended sight
for enhanced accuracy. With the switch to Suuntos and other similar
unadjustable units including some of the new electronic compass units,
this outdoor sighting check might also be a good idea to detect small
differences between compass units and the compass person's sighting
technique. At least on a Brunton if your needle is bent (I've seen a 2
degree error between FS and BS) or even reversed on North direction
(don't laugh, I've seen a NM Brunton that had this problem) you can
take it apart and fix it.
- Pete
On Wed, Oct 28, 2009 at 8:09 AM, Aaron Birenboim <[email protected]> wrote:
an anybody recommend a method (or place) where I can get a good
magnetic north reference?
Preferably one that does not have metal (or at least iron or steel)
in the markers.
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