Mike: Have a look at: http://www.pacific.net/~brooke/Sensors.html#Earth's Magnetic
shamless plug. There is a web magnetic model that depends on lat,lon, date. Have Fun, Brooke Mike Cowham wrote: > Dear Expert Friends, > I have yet another question that I am sure that one of you will > be able to answer, or at least put me on the track to finding the > answer. > > We all know that Magnetic Declination marked on a compass is an > excellent guide to its date of manufacture. I have the figures for > London, Paris, Rome and New York for the last 400+ years. > > However, I have been studying a compass, actually a portable > dial, that was made for 22.5 degrees SOUTH! I measured this from its > gnomon angle, so there is some possibility of error, perhaps 1 degree. > This latitude passes through Australia, South Africa and South America > plus a few islands like Mauritius. I can virtually rule out the first > two as there are no major towns at this latitude. (Perhaps I should > consider Walfish Bay in S Africa, but the community there would have > been really small). However, Rio de Janeiro (or possible Sao Paulo) sit > almost on my line. It is an English made dial, and I know that Rio had > a large British population. The dial date is between 1800 and 1840, and > the magnetic declination indicated seems to be around zero. > > Therefore I am seeking for records of declination at around this > period to confirm that Rio is the intended location for the dial. If > not, where else had zero declination in 1820? > > One possibility would be to look for old shipping charts, but I > am hoping that someone will have a computer program that will calculate > such things for any point on the Earth for any date? This will be a > most important tool for anyone interested in Portable Dials or > Compasses. > > Any help from any source will be welcomed. > Regards, > Mike Cowham. > <[EMAIL PROTECTED]> > Cambridge, UK.
