Isogonic Lines, Magnetic Variation - Which one do you use!! Options

[BMeister]

Hi there,

Okay Going back to basics because I can't remember everything and I couldn't find the answer when looking it up!

So As usual I pose here, learn the knowledge and post it back and help others!

Isogonic Lines, they are the lines on ie. a VFR sectional chart, that help calculate the variation between the true course you drew from point A to B to Magnetic course seeing as we fly using our MAGNETIC compass

My question is,

During my course, I have 2-3 Isogonic lines and I really can't remember which one I use for which side of the line Im on!!!

For example: If I was here /W15' If I was here /W16' If I was here

which one do you use the /W15 or the 16 when do you swap over which do you start with?


Im tempted to stay you use the Isongic line closest to your current position, but if your in the middle of two which do you use either?


Another Fun Question

[GolfrGuy7]

Hmm.. interesting question. I personally would just go with whatever is closest to your position at the time. For example, when you are to the left of the /W15, use 15. And then when you get to the halfway between the /W15 and /W16 you switch to the 16. When you think about it, you are always between two different lines, and you use whatever you are closest to. So why would it be any different when you are flying across multiple lines? That's my take on it anyways..

[Kilrah]

Well logically you just use the one you're closest to... and if you're exactly inbetween, you use whatever you feel like, is easier to add, flip a coin... or you decide to add 15.5°... which just makes no sense.
Firstly the lines aren't straight, then there's nothing that tells you that the variation between 2 lines is strictly linear and that the middle will be exactly 15.5°, there are uncertainties and factors that can skew the lines on a short term basis - all ways to say that it's not something absolutely exact and precise and is "hard-coded".

One thing you learn in physics quite early - it's no use giving a precise result with lots of digits like 12.433 when you know that due to uncertainties in the source data your calculations can not give you precision better than the unit level...

Sooo.. I guess you can do with +/-1° of error, you'll already have a hard time finding a magnetic compass precise to that level anyway.

[Fast Jet]

I agree with Golf Guy, just use the one you are nearest to.

[BMeister]

I agree with Golf Guy, just use the one you are nearest to.

Thanks Gentlemen!!!

I was unsure and I didn't want to pass my knowledge on to someone if I didn't know for sure.

so I guess when your in th middle between two lines you use either!! and then the one your closest too

great thanks

anyone have any advice on 'Back to Basic's' topic?