Hi Silas, Nice to see you on here.
Re your interest in looking up magnetic variation; sadly, I doubt that even if you could it would be very reliable. The magnetic variation not only changes within a comparatively short time but it has local geological variations too. On top of that you don't know if the building in question has any steel framing that might also affect things. So, I fear you wouldn't really be able to use it except possibly as a back stop check that the client did make any other of your requested measurements correctly. The variations are short term and non-periodic as well as diurnal, 27 day (that's the period of rotation of the sun), seasonal and 11 year (the sun spot cycle) periodic depending on the position of the Sun and the degree of solar activity. In Britain there are 3 magnetic observatories (out of about 200 world wide). At the one at Hartland (north Devon) the variation (or declination as it is more properly called) is as follows - all declinations are West of true North. 1955 10deg 30mins 1960 9 59 1965 9 30 1970 9 06 1975 8 32 1980 7 44 1985 6 56 1990 6 15 1995 5 33 As a guide to the way in which distance affects things, in London (some 200 miles away?) the 1975 figure was 6 degs 39mins. The guide in Britain at say 50 degs latitude at the Greenwich meridian is to use the 1997 figure of 3 deg 06mins West and allow an increase of 15 mins for each degree of latittude northwards and one of 28 mins for each degree of longitude Westwards. The average annual change at the moment is about 9 mins decrease. However local anomalies have magnitudes which approach those of the normal field (!) - usually as a consequence of mineral deposits. It is this particularly which makes it hard to correct a compass with any accuracy in the way that you would like. As if all this is ot enough magnetic storms can arise suddenly - the record in Britain (I think!) is a declination change of 8 degrees in less than an hour! - and we don't think of ourselves as being in a weird part of the world magnetically speaking! Still, the above might be of help. Patrick
