Working out Magnetic Deviation

[CoombellKid]

I think this was touch on here somewhere, but cant seem to find it...

Anyways, how do I work out the magnetic deviation for my local
area. I think I remember there was a sinple formula to do this, anyone
know what it is?

thanks in advance

regards,CS

Rob

[Vermin]

http://img162.imageshack.us/img162/3821/magaustr3dr.gif
(http://img162.imageshack.us/img162/3821/magaustr3dr.gif)

[JimmyH155]

it's called Magnetic Variation - that is what is on the chart from Bureau of Met. Magnetic deviation is completely different and is the effect that nearby metal has on your compass. If you put your scope up near a shed for instance, there might be a bit of "magnetic deviation" as well as variation. Try moving a few metres away from the shed (pointing in same direction) and see if the compass swings a degree or 2. :D

[Vermin]

It can also be called magnetic declination.

http://www.google.com.au/search?hl=en&q=define%3A+magnetic+declination&btnG=Google+Search&meta=

[JimmyH155]

i found my original essay. here it is
dumb question magnetic deviation
There are two different types of magnetic "anomolies" The way a magnetic compass points to somewhere other than true north )(or south) is correctly called "Magnetic variation" or sometimes nowadays it is also called magnetic declination.
Magnetic "Deviation" is something completely different as I know from past experience. Whilst serving Queen and Country in the old days before gyroscopes and satellites - every time before we left on an overseas assignment, we would take our ship to a specially selected remote area of the harbour and we would "Swing the compass" What that meant was that we would line ourselves up (with help of tugs and buoys) on every position of the compass and would measure the difference between our true heading and what the compass said. The deviations would be noted on a "Deviation Card" which would be subsequently used for every change of course - along with the correction for "Variation".
Deviation is caused by the metal surrounding you on the ship and also maybe on the seabed, thats a lot! And it varies with the direction you are pointing.
As far as us astronomers are concerned, you may have a little bit of "Magnetic Deviation" due to your proximity to say your metal shed. This may cause you to set up not exactly pointing South.
To remember which direction to steer by compass, we remembered the adage "Variation East, steer least, Variation West, steer best"
Hope this lecture has been interesting.;) :thumbsup:

[CoombellKid]

Actually magnetic variation was what I was after, just used the wrong
words sorry, So I kinda know what they're about just wanted to know
how to work it out for my local area, thanks for the map Vermin

I'll read your lecture L8er :thumbsup:

regards,CS

Rob

[Vermin]

Could you explain that\? It seems very cryptic to me.

i.e. If add the variation (Variation East) then I have to make very few corrections (steer least). But If I subtract the variation (Variation West) I steer best...? So which is it, add or subtract?

[Dennis]

We used to remember this as follows:

"Grid to mag add"
"Mag to grid, get rid"

So, going from the map bearing of say 240 deg, we would add the magnetic deviation (usually written on the map with the date and rate of change) and set the compass accordingly.

If plotting from the compass to the map, we would subtract the mag dev from the compass bearing to enter it on the map.

At least I think that's what we used to do in the days of my hill walking in the UK.

Cheers

Dennis

[JimmyH155]

answer to your question: Suppose you want to steer a TRUE course of 180 degrees (same as point telescope to due south.when lining up)
The Magnetic variation (declination) in your area is (say) 12 degrees East
Using the rule "Variation East, steer least" you would set the COMPASS course to read 180 - 12 = 168 ie when the compass is reading 168 degrees, it is really pointing due south 180 deg.
If your area like around Perth is say 4 Degrees West, "...Best" then to point due south (TRUE) you would set COMPASS to read 180 + 4 = 184 degrees. Unless you are on a big ship or bang next door to your shed which is full of scrap iron, and old car bodies forget the adjustment for "Deviation."
As you can see from the chart, magnetic variation makes a BIG difference, and can easily explain why some people may be way out with polar aligning their scopes.
Lucky if you don't live on Magnetic Island - it completely confused poor old Captain Cook - sent his compasses haywire!!!
Hope this helps;)

[OneOfOne]

I live in Melbourne and use a figure of 12 degrees to the East. What I do to set my scope up is to face south, according to the compass. I point the polar axis of the tripod about 12 degrees to the left of this magnetic south. If you look at the map provided by Vermin, you can get an idea of your deviation.

My understanding is that the magnetic pole does not coincide with the axis of the earth's rotation. The deviation is related to this as well as some local affects, like underground mineral deposits etc., and of course if you are standing next to a big lump of metal. I also find if I move my compass too close to the tripod the compass will swing toward it, so be careful of that too.

[Shawn]

I use the simple "True virgins make dull company approach" when navigating
Where true is "True"
Virgins is "Variation"
Make is "Magnetic"
Dull is "Deviation" which dont count, based on JimmyH's post.
Company is "Compass"
Working from left to right in this equation using Var east compass least etc.
You get the correct result. of course the whole equation can be reversed to go the other way.. On a nautical chart the info for var is in the compass rose, the year the chart was printed with the amount to add for every subsequent year. usually several roses all slightly different placed appropriately on the chart, the differences on a large scale chart are very small. local anomolies often are the consideration. And deviation as per above is not to be ruled out, unless you are out in the middle of a paddock somwhere, you would be supprised by what upsets a good compass...
This is info gleamed from my 5 years of sailing the south pacific...

hope it helps.

[dennislowe]

I use the Sun to check my compass first. To do this, put a stake in the ground and make sure it is vertical. Work out your local time for solar transit and at that time run a string line along the shadow line. This is a true North South line. You can now use this line to check your compass or to set up your mount. The accuracy of this method depends on how well you can locate yourself (a GPS helps here) and how well you can work out the local solar transit time (most astronomy ephemerides can do this for you).
Dennis.