I'm glad Patrick mentioned the swing which can occur on a compass when you
bring it close to a building. I am in the process of making a vertical dial
for a library and depending on where I hold the compass along the wall of
the building I can get as much as a 40 degree change in the location of
North. In Sailing we refer to this as deviation (local attraction) which is
different from the geographic difference called Variation. We are taught a
simple formula to obtain true north (TVMDC). Meaning take Compass reading,
apply Deviation (which yields Magnetic), and then apply Variation to obtain
True North.
Another way to check the alignment of the building wall would be to ask for
a copy of the architects plans. The surveyor would have marked North and
often labeled local Variation. What I do to check my reading is to get out
my Nautical Almanac and H.O. publication 249 (called the air almanac and
great for celestial navigation) to obtain the Zn of the Sun for a particular
time (LHA).
Anthony
