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Mag, Grid & True North Align

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Mag, Grid & True North Align

Old 7th Nov 2022, 12:25
  #41 (permalink)  
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As PAXboy noted the earth is an oblate spheroid - but only just.

WGS-84, used as the default spheroid by GPS, has an equatorial diameter of 12,756 km. The polar diameter is 43 km less - a difference of 0.3%.
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.
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Old 7th Nov 2022, 12:45
  #42 (permalink)  
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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..
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Old 7th Nov 2022, 16:16
  #43 (permalink)  
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Oblique! Maybe last week... but rather more oblate of late.

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Old 7th Nov 2022, 17:32
  #44 (permalink)  
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Originally Posted by Ninthace
In the days of Flight Systems training at Cosford, we taught the basic theory of inertial navigation. All examples involved ac flying E or N or even NE. Any other direction involved negative numbers which the would-be techies, being products of the then educational system, had trouble coping with.
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 2S and 2N, so if you wanted to use your shiny new box to track aircraft in Singapore, Nairobi, etc you were in for a nasty surprise.
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Old 7th Nov 2022, 22:43
  #45 (permalink)  
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Originally Posted by Fortissimo
There is a team actively working on moving the entire aviation system to True. Canada is shifting its entire airspace to True in 2030 and ICAO has just surveyed state responses to the idea of a global change.
Why arent we doing this now?
Couldn't agree more! Spent all of my time in the Navy flying on True.
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Old 8th Nov 2022, 07:36
  #46 (permalink)  
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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!

Last edited by brakedwell; 8th Nov 2022 at 08:43.
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Old 8th Nov 2022, 08:37
  #47 (permalink)  
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I remember the compasses were all over the place!

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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Old 8th Nov 2022, 09:39
  #48 (permalink)  
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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!

Last edited by brakedwell; 8th Nov 2022 at 10:40.
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Old 8th Nov 2022, 20:12
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Resolute Bay


A sad example of the confusion that can come from high declination values.
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Old 25th Nov 2022, 09:05
  #50 (permalink)  
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Just bought a truly beautiful 25000 of part of Italy with this panel.
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Old 25th Nov 2022, 15:34
  #51 (permalink)  
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These are very useful.
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Old 25th Nov 2022, 18:25
  #52 (permalink)  
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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.
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Old 28th Nov 2022, 02:17
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The NZ Antarctica enroute charts (Transverse Mercators) have tracks in True & Grid (Grid is aligned with 180E/W) - or they used to!
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