Heading discrepancies in flight plans by Plan-G and ForeFlight

[WalterT]

Tim, I want you to know how much I admire your Plan-G program.  The breadth of the feature set is astonishing.  And the user interface is intuitive and a pleasure to use.  The program installed easily on my machine and even the SimConnect feature works flawlessly. 

As I write this a pilot friend of mine is flying a real Cessna 182 on a 1000 mile flight through the Pacific Northwest.  He sent me his ForeFlight flight plan and I recreated it exactly on Plan-G.  The plan has 8 rows and the distances in each row agree exactly in the two flight plans.  But in each row there are discrepancies of a few degrees in the Heading column. For example, the first leg is KBLI to 4WO, a distance of 93 nm.  Plan-G says the heading is 134, and ForeFlight says the heading is 138 degrees.  (Yes, both programs are using magnetic north headings.)  The second leg is from 4WO to KELN, a distance of 139 nm.  Plan-G says 98 M, and ForeFlight says 102 M. Similar discrepancies occur in every row of the two flight plans.  Neither of us is clever enough to think of an explanation.  Do you have any ideas?  Thanks for your time, and for the wonderful program.  Walter

[tim arnot]

Most likely: The world magnetic map drifts over time, and Plan-G's magnetic map is set to 2005, the same as FSX. There have been at least two WMM updates since FS was released. Consequently if your friend is using up to date charts and planners (as he should be), they will have diverged somewhat.

[WalterT]

Thanks for your prompt reply to my question, Tim.  Your answer makes good sense and I will forward a copy to my friend, who incidentally made a sightseeing flyover of Glacier National Park today.  Unfortunately there was so much haze and smoke in the air that visibility was very limited.  There are lots of forest fires in the western United States right now.  Walter