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3 Answers

Skyvector’s Magnetic correction

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General Aviation

On skyvector, the correction, for magnetic heading, that comes up doesnt always match what the agnostic lines dictate.

 

For example a direct, true course, from KBOS to KALB is 282. Both when I plot it and when I do it on skyvector. However on the chart the magnet var is about 14.5W. Skyvector says magnetic course is 298, a 16 degree difference.

Its not a big difference, but I'm planing my first cross-country (not from KBOS to KALB lol) solos and its a bit confusing.

3 Answers



  1. John D. Collins on Apr 12, 2013

    Jordan,

    I don’t use Skyvector but on my flight planner it also came up with 298 degrees for the initial course. Remember, this is not the same as the course along the entire route, just the initial value. As you fly to the west, the charted variation changes, so your magnetic course also changes. In addition, all direct courses use a great circle route, so when you are going to the west, the initial course is a small amount northward and it continuously changes along the route eventually curving back slightly to the south as you approach your destination. If you lay a straight edge on a map, the course will approximate the great circle for short distances, but the greater the distance, the more error that is introduced with a straight line course plotted on a map that is a projection of the earths spherical shape.

    Variation also changes over time as the position of magnetic north moves. Magnetic variation used by chart makers and GPS navigators is based on a model of how it changes over time and is never precise. When VOR’s are sited, they are aligned to 360 being magnetic north. But over time the variation changes. The airways and instrument procedures using VOR’s are based on the original radial values that were in effect at the time of the installation of the VOR. Some of these were last aligned back in 1960. In the case of the BOS VOR, it was last aligned in 1995, 18 years ago and the variation at the time was 16 degrees West. Now it is close to 15 degrees west. Some I know of are more than 5 degrees off the current charted variation.

    Since electronic navigation is either based on VOR or GPS, as long as you keep the CDI in the center, the actual magnetic course needed to fly the route under zero wind conditions will usually be within +/- 5 degrees of the calculated course. Keep the CDI in the center and you will get to where you intend to go. Small differences of a few degrees really aren’t operationally relevant, I know I don’t hold a heading to +/- 1 degree.

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  2. Jordan Markov on Apr 12, 2013

    Thanks for the help John.

    I cant hold a heading to 1 degree either, I just wanna make sure I can explain the difference if the FAA guy asks on my check-ride.

    Is it safe to say skyvector is more accurate in correcting for Mag Var?

    Would it be better to calculate a new heading for each (10 mile) leg? I’ve noticed the magnetic correction is different for the different legs.

    Thanks again

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  3. Dave on Jul 08, 2013

    There’s a detailed description of how SkyVector does magvar here:

    http://skyvector.com/forum/magnetic-variation-weirdness-flight-plan-box

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