(am I the only one that didn't know this??)
I stumbled on an old post in the Skyvector forums by the owner/creator of Skyvector. It cleared up some questions I have had about how magnetic variation is dealt with in Skyvector and in various navigation applications. It also explains an observation that Keith referred to in viewtopic.php?f=5&t=3915.
As Dave explains in his post, the magnetic variation for a VOR is established when the VOR is installed and it is rarely updated. The variation was established by site survey at the time of installation. Airways to/from this VOR are determined using this variation. The variation for the VOR is included in the dataset used by Skyvector and most navigation equipment.Keith Smith wrote: Case in point. ask Skyvector to show you the magnetic course from AMTRA to POM, it gives you an answer of 076 (equates to the 256 radial from POM, you would think). Actually, AMTRA to POM is a victor away defined by the 254 radial from POM (2 degrees different).
The variation for most named waypoints is also included in the dataset but it is an estimated value from a mathematical model which will change over time as database cycles are released. So if you plot a route from a waypoint to a VOR, your inbound leg will be based on a recently estimated magnetic variation at that waypoint but if you reverse that course and fly from the VOR to the waypoint, the magnetic variation will be the value which was established when the VOR was installed. This explains why in Keith's example above, the inbound heading was 76° but if you reverse the course it gives you and heading of 254°.
David also explains that his application applies a current mathematical estimate of variation for any GPS waypoints which you create. So if you create a GPS waypoint at the exact LAT/LONG for POM (N34°4.70' W117°47.22') and use that instead of the POM VOR, you will get a 257° heading to AMTRA instead of 254°.
Side note: Maybe I was the only one that didn't know this, but waypoints can be entered in Skyvector as:
- By distance along a VOR radial - i.e. 12NM from SLI on 215 radial by entering SLI215012 (must be 3 digit radial and 3 digit distance)
- By LAT/LONG by entering:
- Degrees Lat Long: N34.0783333W117.7870000 (no spaces)
- Degrees Minutes: N34d4.70W117d47.22 (no spaces)
- Degrees Minutes Seconds: N34d04m42.0000W117d47m13.2000 (no spaces)