Computation of magnetic variation for any given location is apparently an inexact science.
My NOAA chart for Washington
, DC harbor has a compass
rose which notes that variation in 2010 is 10 degs 45 mins Westerly, with a yearly change rate of 0 degs 1 minute.
If you use NOAA's Geophysical Data Center computer to calculate the variation for the same location for the year 2010 you get 9 degs 28 mins Westerly, also with a yearly change rate of 0 degs 1 minute.
That's more than one degree of difference!!
Navigator beware :-)