|How much does Declination change? #2639|
Back To Discussion List Written: 2009.09.08 by: Denis Rogers
I intend going backpacking in the area of the Tuchodi river, and got some 1.50,000 maps of the area. The declination for 1965 is listed as 30 degrees 45 minutes East with an annual decrease of 4 minutes. This would make the present declination 27 degrees 45 minutes. There is a resource on the net under "Geological Survey of Canada" whereby one can type in the Lat. and Lon. of any area and get the current magnetic declination. This webpage gives the current declination for the same area as 21 degrees 24 minutes, -a difference of over 6 degrees. A significant difference that would result in an error of about one mile in every ten. I assume that the declination calculation on the net is the more accurate. Is this assumption correct? If so the rate of change for that part of Canada has increased quite a bit since 1965, and would be fairly unreliable on older maps.
#1350 - 2009.11.09 Don Serl - expect big declination changes vs old maps Bottom line: don't prorate from old mapsheets, go to the calculator if it's important to you. Scott also suggested another site which would be useful in the USA: http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp Cheers,
I just dealt with exactly this issue in helping Fred Beckey establish declinations for the Waddington and Monarch areas. The declination shown on the 1974 Monarch sheet was just under 27 degrees, decreasing 3 seconds per year. That would make you expect a current declination of about 23-24 degrees. Similarly, the 1978 Waddington sheet gives 26 degrees decreasing 3.7' annually, so you might expect maybe 23-24 degrees there too now. However, the NRC website calculator [ http://geomag.nrcan.gc.ca/apps/mdcal-eng.php ] gives about 19 degrees for Monarch and 18 1/2 degrees for Waddington. Apparently (as I'm informed by Scott Bogue, an expert in the subject, whose information Glenn Woodsworth kindly passed along) the rate of change increased quite dramatically beginning about the mid-70s. It is currently in the neighbourhood of 13' per year! (i.e. one degree every 4 years!)
Bottom line: don't prorate from old mapsheets, go to the calculator if it's important to you.
Scott also suggested another site which would be useful in the USA: http://www.ngdc.noaa.gov/geomagmodels/IGRFWMM.jsp
#1331 - 2009.09.09 Scott Nelson - Magnetic north pole moves.
Yes, the magnetic north pole does move around fairly sporadically. In fact, every so often the magnetic north and south pole completely reverse. Use the new declination, http://geomag.nrcan.gc.ca/apps/mdcal-eng.php, and ignore what your map says.
#1330 - 2009.09.08 Andrew Wong - Online declination likely more accurate
I would trust the online declination info over your map, especially since it was printed over 30 years ago. I assume advances in technology since 1965 has given scientists more accurate measurements of the magnetic north and better understanding of how it changes. That should account for the differences between the map's annual declination estimate and the online one. My $0.02.