On Magnetic Declination

This is an excerpt of a wandering email discussion on magnetic declination with regard to cave survey software.

We have a West declination in West Virginia, which means that a magnetic compass points to the West of True North. The declination amount is subtracted from a magnetic compass reading in order to get the true bearing.

To specify declination in survex, you give the reading that the compass would read when pointing to True North. In our case, that means a positive value (because it is subtracted from the reading). So our "negative", or West, declination would be specified in survex with a positive number like this:

  *calibrate declination 8.983

Despite claiming to use a similar data format, therion calculates true bearing the "right way", the offset of Magnetic North from True North, which is opposite the way that survex does. The practical consequence of this is that if you use the declination value from .svx files in .th files, you must change the sign in therion. Specifying the same West declination in therion is:

  declination -8.983 deg

You can calculate declination for any point at any given date in time according to the IGRF geomagnetic model at this NOAA website.

Walls is smart enough that so long as you provide a location for the cave entrance and a date for every survey, it uses this geomagnetic model to automatically apply declination to every survey shot. Update: According to the therion book (PDF), therion (0.5.0 and higher) can do this also, provided that you specify a location, date, and don't manually specify any declination - though I have been unable to get this feature to work properly.

Interestingly, Bob Thrun analyzed a month's worth of actual measured magnetic declination taken at one-hour intervals and came to the following conclusions about short-term declination fluctuations (ie. not predicted by the geomagnetic model):

• Declination can change up to a quarter degree over the course of one day.
• There is some daily periodicity to declination change.
• Some days are more "noisy" or "quiet" than others, ditto for some locations.
• Declination is also affected by sunspot activity.

Since Bob's investigation in 1997, we now have declination data available in one-minute intervals, which could be studied in a similar way.

[trip] Evening Visit To Maiden Run

On Tuesday evening, the WVUSG took an enjoyable tourist trip to our own backyard cave, Maiden Run #1. Brian Masney, Rich Finley, John Harman, Dave Mason, myself (Dave Riggs), and Anastasia Heasley - on her first outing with the Grotto - met in Sabraton at 6PM, condensed vehicles, and drove out to Deckers Creek. The temperature was below freezing, and there was around 2 inches of snow on the ground - what better time is there to head underground where the weather is always great?


Rich Finley, Dave Mason, Anastasia Heasley, John Harman, Dave Riggs and Brian Masney in Maiden Run Cave #1. Photo by Brian Masney.

Rich headed in first and rigged the 15 foot drop with a cable ladder. He and I descended to the bottom of the first pit, which is about 8 feet in diameter, 25 feet tall, drippy-wet, and intersects the main passage in such a way as to prevent the casual visitor from seeing the majority of the cave. With the aid of a questionable webbing hand-line, I climbed up the far side of the pit and rigged the other side of our cable ladder to the even more-questionable rigging at the top. Rich climbed to the top, then wedged himself into a nook so that he could belay the climbers with a rope attached to his body. Note: One should NEVER rig to any questionable bolts for life support, we chose to do so only because we were able to back up and belay from a well-placed person, and only then because this was an easy "nuisance climb".

Everyone else climbed down and then back up the pit without incident, and we headed further into the cave. We were disappointed to find that most of the "biological speleothems" that we encountered on our last visit had either been carried off or weathered away. We soon popped out and chimneyed down into the second dome-pit, which appears to be about 35 feet tall and 15 feet in diameter. This room is covered in carbide-soot graffiti from the 1950's, 60's, and 70's. It also contains a register which was placed in the cave in 1995, and needs replaced as it has deteriorated extensively.


Dave Mason in the climb-up from Maiden Run's second dome-pit. Photo by Brian Masney.

Brian and Rich set up to take some photos, while Dave, John, Anastasia, and I headed up the dry upper route to push it to its bitter end. Unfortunately the end was quite bitter, as neither Dave nor myself were able to (willing to?) make a tight sideways-squeeze down into a slot with going passage heading off out of sight. Defeated, we headed back to the dome-pit to pose for a few of Brian's photos, taking care not to disturb the well-organized nest of an Allegheny woodrat.

The four of us then tackled the lower stream passage, accessible through a sloping hole near the floor of the dome-pit. Though I'd visited the cave several times, this was my first time down in the stream passage (the stream itself was mostly dry), and I was quite impressed with it. We followed it upstream as an incised canyon, up over a dry paleo bank where the stream meanders under a wall, until it connects back up with the stream. From here, the others followed the stream back downstream into a low canyon, while I continued upstream despite what looked like miserable caving ahead. I crawled for what seemed like approximately 300 feet in very consistent 2 foot high, wide streambed (not so dry), which had a perfectly flat, dipping ceiling, as if the passage were running exactly along the strike of the limestone. After a few moments of pushing, I headed back to the group, convinced that the crawl does in fact continue exactly like that, forever. On the way back to the dome-pit, I pushed an upper lead in the ceiling of the stream canyon, which was very low, very dry, and very disgusting (so this is where those bio-speleothems walked off to!), only to find myself at a dead-end with a dirt plug and a tiny hole. I shined my lamp into the hole and the hole shined back - Brian and Rich were on the other side, I was apparently on a small level in between the high dry passage and the low wet passage, which connects to the dome-pit right behind the rat's nest.

We arrived back at the dome, ate, drank, and planned our exit. John and Rich were to descend last, rigging a pull-down knot to de-rig the cable ladder behind them. Everyone descended the pit and climbed back up the opposite side without incident, and it was John's turn next. He tested the "bomb-proof" rusty rigging and pulled the hanger right off one of our three anchors. Yikes! Having expected issues with the rigging, we had hauled in tons of webbing, etriers, a bit of rope, etc., so within about one half-hour's time, Rich had rigged an etrier and handline to some rocks approximately 25 feet away from the lip. John and then Rich easily came down, back up the far side, de-rigged our cable ladder and headed out. Meanwhile, after they'd safely descended, the rest of us headed back to the vehicles - our wet cave suits frozen solid by the time we arrived. We headed back home at just about midnight, having spent around 4 hours underground on what was an excellent local caving trip.

The WVUSG will be heading back to Maiden Run #1 next week to place new bolts and hangers at the pit, and we plan on teaching new members how to survey by producing a modern map of the cave.

See also: More Maiden Run photos by Brian Masney (including past trips).