Isogonic disagreement?

R

Rolfrv

Filing Flight Plan
Joined
Nov 2, 2012
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5
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Petaluma
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Display name:
rolfrv
On the Sectional and the WAC the Isogonic line near Petaluma (O69) shows 14*30" E. The airport info in the Garmin Pilot app, the Wing X Pro app, and the Variation at AirNav.com shows 16*E. AirNav shows it as 16*E(1985).
So why the variation? (pun intended)
Enquiring mind need to know!

Rolf
N9EY
 
The Earth's magnetic axis is shifting; currently the north magnetic pole (NMP) is drifting westward (not sure whether NW or SW) so all the isogonic lines are shifting with it. Easterly variations in the US are decreasing, westerly variations are increasing. I think that even the sectional is based on information that's a couple of years old (it says somewhere on the chart, but I'm too lazy to go out to my car to check my latest charts), but that's probably the most current information you'll find through aviation sources. I doubt that any values that show a larger easterly variation than the sectional have much chance of being more recent.
 
Rolfrv said:
On the Sectional and the WAC the Isogonic line near Petaluma (O69) shows 14*30" E. The airport info in the Garmin Pilot app, the Wing X Pro app, and the Variation at AirNav.com shows 16*E. AirNav shows it as 16*E(1985).
So why the variation? (pun intended)
Enquiring mind need to know!

Rolf
N9EY
Click to expand...

The difference can be due to on the ground variation vs airborne variation. On the ground magnetic variation can be affected by the local terrain composition. While in-flight the terrain has a lesser effect.

José
 
The model is updated every 5 years. Current is WMM2010. My guess is that some of these are still using WMM2005, probably the uncertified ones....
 
Thank you Jim!
That ref to the long post by Bob Gardner, from a year ago, helped a lot.
 
The raw data from the FAA has the variation and the reference year for that variation for that airport or navaid. All the apps should show that(but probably don't). I looked up O69, and the 16E is listed as 1985.
 
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