Many useful sites dealing with the divergence of compass north and true
north can be found by googling "magnetic declination".

Examples of those dealing with historical variation of the declination include:

gsc.nrcan.gc.ca/geomag/field/mdcalc_e.php
(1960+)  No 'www', but you may have to precede the URL with 'http://'

www.ngdc.noaa.gov/seg/geomag/jsp/Declination.jsp
(1900+)

Both the preceding allow you to calculate the declination at any point
(lat., long.) at any time within their respective spans.

www.phys.uu.nl/~vgent/magdec/magdec.htm
(16th c. on, generalized maps, not great for North America but better than
nothing)

Pete Toulmin

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