The Earth's magnetic field varies continuously in time and space, including by elevation. Thus, any computation you make has to consider location and time. The Geomag program is the world standard, it is used to calibrate aircraft compasses, although I don't imagine too many pilots rely on a compass any more. Geomag can be found at:
http://geomag.usgs.gov/models/models/ If you want to check a compass, it isn't difficult to set up a location to do so. I've done this before using survey-grade GPS equipment (I can borrow this from my job), setting markers (at least one non-metalic), computing geographic coordinates from the GPS, then using Geomag to compute the predicted compass bearings. Alternatively, highly accurate coordinates can sometimes be found for a location near you from the National Geodetic Survey web site for objects you could use to take bearings, such as water towers. Back in the days of optical geodetic surveying, water towers and smokestacks were sometimes used as reference marks and including in network adjustments. The NGS survey markers can be found at: http://www.ngs.noaa.gov/cgi-bin/datasheet.prl Theoretically you should use Geomag every time to compute the bearings but the bearings don't change very quickly. I could do this for Sandia Grotto if anyone thinks it would be useful to have a survey course. Another method would be to simply compare a bunch of compasses, but it is satisfying to have an exact answer. Scott Christenson On Wed, Oct 28, 2009 at 8:09 AM, Aaron Birenboim <ca...@boim.com> 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. > > I would think that most surveyed markers would try to align > to true north, which would not help my calibration problem much... > unless I have access to exactly what deviation they used. > > aaron > > > _______________________________________________ > NMCAVER mailing list > nmca...@caver.net > http://caver.net/mailman/listinfo/nmcaver_caver.net > _______________________________________________ NMCAVER mailing list nmca...@caver.net http://caver.net/mailman/listinfo/nmcaver_caver.net
