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From KEWB to 0B8 isogonic lines is 15 degrees. In flight plan the true course is 243, so it's 243+15=258 but it shows 259.

KEWB has a magnetic variation of 16W defined in the National Flight Database (NFD). When present, we always use the values published in the NFD. The NFD is a data file published by the FAA in ARINC-424 format for use in navigation systems, GPS's, etc. It is published every 28 days and SkyVector regards it as the canonical source for all navigation data. I'll refer you to this thread for more reading on this topic http://skyvector.com/content/magnetic-course-numbers-few-degrees#comment-569 All that said, you raise a very valid point. The NFD value for EWB's variation is dated from 1995. Both the New York Sectional and the Boston TAC have Isogonic lines based on 2005 models. (Isogonic lines are drawn based on the 2005 World Magnetic Model and geologic anomolies can cause localized differences near the surface.) The current (18-Nov-2010) airport diagram PDF it indicates a value of 15.0W dated to January 2010. 11 months is a long time for a new value to be incorporated into the NFD. I'll file a change request with the NFDC to point out that there exists a newer, different variation value for EWB. If they accept the change, it should be in effect in 2-3 months. I hope this satisfies your curiosity. I welcome your opinion on the correctness of our decisions. Thanks for keeping us honest and have fun up there! -dave