Great circle formulae, True course and actual heading

[Ed Williams]

>
> I am just surprised that they vary so much depending what model is been
> used.

Really? Mostly, I think they are within a degree.

>
> What model is the best? And also it seems that by the poles the calculation
> are somewhat wrong, is it just my imagination?

The compass is pretty much unusable near the magnetic poles, mostly
because of dip and also because the variation varies so rapidly.

Either the Defense department's WMM or International Geomagnetic
Reference Field models should be good enough for navigational
purposes. Mostly, you'd choose one to be consistent with some other
data.

Jepp used to use WMM, but when I asked them recently, they said they
were using IGRF. Most of the world's airlines use Jeppesen data.

Ed

[Ron McConnell]

If I remember correctly, the goal of the WMM is one degree accuracy
for magnetic variation (declination) for a given latitude
and longitude (WGS-84) for a given date within the 5-year
life of a set of coefficients. The current WMM coefficient
set is for 2001-2005. The 2006-2010 set will likely
be released by NIMA in late 2009.

The WMM does not account for local magnetic anomallies
like big iron ore deposits, big steel buildings, big ships, ...

I haven't checked the IGRF lately, but it is likely to be similar.

I've run calculations backwards nearly 50 years
for the WMM 1996-2000 coefficients and
the results were close to the records I had
for one lat/long which is 5º W in 2003
and was nearly 0º in 1957. So, extrapolations
for a few years into the future might still be useful.
I should try comparing the WMM-1995
results for 2003 compared to the WMM-2000 set
- if I get my honey-dos done. :)

Cheers, 73,

Ron McConnell
w2iol


N 40º 46' 57.9" W 74º 41' 21.9"
Magnetic Variation = 13.0º W in October 2003
FN20ps77GU46 [FN20ps77GV75]

http://home.earthlink.net/~rcmcc

[Ron McConnell]

Ron McConnell wrote:
>... The current WMM coefficient
> set is for 2001-2005. The 2006-2010 set will likely
> be released by NIMA in late 2009. ...
^
That should be "late 2005."
^
Cheers, 73,
Ron McC.
w2iol

N 40º 46' 57.9" W 74º 41' 21.9"
Magnetic Variation = 13.0º W in October 2003
FN20ps77GU46 [FN20ps77GV75]

http://home.earthlink.net/~rcmcc