I42
WGS-84 actually treats the Earth as an ellipsoid (a body of rotation defined by an ellipse rotated about its minor axis). A more accurate model flattens the South pole end rather more, and makes the North pole end a bit pointier (hence, 'everything has gone pear shaped') but that make the spherical trig calculations far too difficult and the ellipsoid is good enough for government work.

To just elucidate a bit further, WGS-84 is a gravity model, so the density of the different components (mantle, core etc)
is modelled rather than just the shape.

Then, of course, the Barycentre has to be taken into account..

Oblique! Maybe last week... but rather more oblate of late.

Slightly OT, but related - I'm reminded that for many years one of the first of the enthusiast ADS-B receivers (RadarBox) suffered from a 5 million square mile equatorial "black hole", whereby it was incapable of decoding coordinates between 2°S and 2°N, so if you wanted to use your shiny new box to track aircraft in Singapore, Nairobi, etc you were in for a nasty surprise.

Couldn't agree more! Spent all of my time in the Navy flying on True.

I took an RAF Britannia into Resolute Bay, Northern Canada in 1972 when the Magnetic North Pole was only 30 NM northwest of the airfield. I remember the compasses were all over the place on the ILS/visual approach!


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Quote:

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I remember the compasses were all over the place!

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As a consequence of that, headings on airways and approach plates in the Canadian Arctic (the "Area of Compass Unreliability" - shown on charts as the Northern Domestic Airspace) are True bearings.


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India Four Two, we did the approach and landing on what was Rwy 35. I am sure we used an ILS with broken cloud giving some contact with Resolute and the Smiths Flight System compasses were showing almost 90 degrees out. The runway was packed ice and snow. Being October it was also very cold!


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Resolute Bay
 

https://skybrary.aero/bookshelf/acci...da-20-aug-2011

A sad example of the confusion that can come from high declination values.

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Just bought a truly beautiful 25000 of part of Italy with this panel.

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These are very useful.

I agree, we used to carry navigators with sextants on the RAF Britannias. I did a polar trainer in 1971, with SpecN navs at the end of their Manby Course and a Litton 51 on board. We flew from Thule to the North Pole, orbited for about 20 minutes while checking compasses etc then set off for Brize Norton and landed 9 hours later.. One thing that came out well was the Smiths Flight System and our magnetic compass.

The NZ Antarctica enroute charts (Transverse Mercators) have tracks in True & Grid (Grid is aligned with 180E/W) - or they used to!