Compass measurement locations even 30 feet apart are plenty good
enough to spot problems with your particular compass. You should do
foresights & back-sights both N-S and E-W and use 3 posts, which could
be set to magnetic (easier) or true N (more difficult). What matters
is to record in every dated survey book the difference between compass
& clino instruments used on the project. We used such a test station
at Flint Ridge for years to monitor bent needles in Bruntons, and just
plain "off" alignments for Suuntos. Most modern cave survey programs
simply use the survey date and Lat-Long. to apply a built-in Mag. N.
correction to the data. This is particularly useful if the cave is
large (like Mammoth) and the surveys are made over a long period of
time (ditto).
At one 15+ year survey project in NW Arkansas we carefully installed
three 3" PVC pipes in the ground, being careful to place them out of
the way from the expedition headquarters, and away from some metal
fencing. Alas, within the first year the owner (NPS) started mowing
once a year around the cabin and later around a potential heliport.
You guessed it! They "mowed" two of the three white posts :-(
- Pete
On Dec 30, 2009, at 11:39 AM, Gill Edigar wrote:
If the North Pole moves, where will Santa Claus live? In a travel
trailer?
Regarding survey instruments. I have long held--though never acted
upon--the idea that we should set up True North calibration posts at
popular caving project sites such as CBSP & Punkin/Deep & PEP, etc.
That would require 2 posts set up by high accuracy survey to True
North (via Polaris shot) and at a set distance apart, dead level--say
100 feet or 30 meters--& clearly marked on a brass or aluminum plate
attached thereon. Survey instruments could be calibrated to True North
(their magnetic variation and local declination then could be known),
clinometers could be calibrated to dead level, and survey tapes could
be compared to a known distance and an adjustment made for their
dimensional changes as they age. Most, but not all, good survey
instruments have a serial number stamped into them and that number
should be recorded in the heading information for each survey in the
survey book, along with the instrument's inherent error.
Whether such an effort to know an instrument's error is warranted is
worth questioning. Most surveys seem to go perfectly well without that
information since closure errors seem to be handled by the computer--
however far from their true location the passages may appear on the
map--to satisfy most people's needs. It is accepted by most cavers
that such a survey is not accurate enough to use for putting a well
down into a cave passage. Better methods are available for that.
--Ediger
On Wed, Dec 30, 2009 at 10:13 AM, Geary Schindel <gschin...@edwardsaquifer.org
> wrote:
Folks,
I’ve also seen data that indicates the magnetic field over the
southern ocean near Australia has greatly decreased or disappeared.
There is certainly evidence that magnetic pole reversals occur in the
geologic record and that we’re overdue for the next one (if I remember
correctly).
Placing the date of the survey, along with the magnetic north and true
north arrows on your cave map should allow folks in the future to
orient you cave maps in relation to topography easier. Depending upon
the use of the map, and the extent of the cave, it may or may not be a
significant issue. Does raise an interesting issue for long term cave
projects and whether they do compass corrections to true north based
on survey date.
On a more serious note, a magnetic reversal may throw off my sense of
direction. I’ve been able to use the weak magnetic field set up by
the planet, along with the steel plate in my head, to help with my
ability to tell north in the field (assuming I’m not near any large
metal bodies, electrical lines, or the internal voices aren’t too
loud). This is particularly acute when caving as my sense of
direction always tells me the way out of the cave is usually parallel
to the passage I’m in rather than needing to move through one of the
walls. Anyway, I know I should have spent the extra money for
stainless steel rather than pig iron. LOL
G
