Another stupid question.

[Bundy]

Hi guys, oh boy I know I'm gonna get it for this one ha ha.

I really stuffed up my dual cross country by not working out my Magnetic variation correctly. Now that I am nearing my solo cross country, I REALLY want to get it right!

I have just bought a 2010 1:500000 map of Johannesburg. At the bottom, it says that its a 2004 edition. A little higher up, it says that "magnetic variation for 2010 annual change 4' westwards." now this is where I'm confused...

Do I add 4' every year from 2004? IE 6 years X 4' = 24 Degrees?

Please help, I can find nothing in my MPL handbook

[D2O]

Not really. For what I figured out (anyone, please correct me if I'm wrong) the map was printed in 2010 with the latest 2010 aeronautical information, but the map edition is from 2006. While I'm not sure what the edition refers to in terms of the information, I can assure you that the magnetic variation information on that map is for 2010. You need only apply the 4' westwards for each subsequent year after 2010, although that method is not really true IRL. Also keep in mind that the map is based on true North. The small offset compass symbols on the map indicates where magnetic North is.

Best is to consult Google for latest magnetic variation (declination in astronomy terms).

Johannesburg is currently -17 degrees.

Check the link here:

http://www.saao.ac.za/public-info/sun-m ... clination/

or for the 2010 variation chart:

http://upload.wikimedia.org/wikipedia/c ... D_2010.png

[andrepieterse]

Dear Alan

Never a stupid question.

Your calculation is correct except that your degrees should be minutes. Then it will make more sense.

So, according to your calculation the variation for your area in 2010 (on a 2004 map which specifies 4 minutes change West per year) would be the value on the isogonal (line joining areas with same variation) nearest to your area plus 24 minutes (West in our case).

Kind regards

[Bundy]

Thanks Andre!

Now I've got it.
no wonder I was so far out on my heading ha ha.

[D2O]

Now I'm confused

Hope your nav goes well