GPS and great circles
 

Just got back from a flight. Was cleared just out of Carins (Aus) direct to Maleny (just north of Brisbane). Distance 704 nm.

The direction to MLY was brg 141 according to the GPS. As I got closer and closer to MLY the brg progressively dropped to 133. The CDI was centred (+/- 1 dot)

This seems like a great circle track; Convex towards the nearer pole.

But this 8 degree change seems like a lot for such a short route. Is it?

Somewhere recently was a link to a good great circle calculator...does anyone have it?

CS

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Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake

[This message has been edited by compressor stall (edited 04 June 2001).]

About right I would say Comp Stall. Without working out the initial and final great circle tracks and using the formula for the convergence of the meridians to give an approximate value of track change it comes out as follows:-
Since I do not have the exact Lat/Longs of CS and MLY near at hand I have used CS and BN with off the top of my head figures.

Cairns 17.00 South 145.00 East
Brisbane 27.30 South 153.15 East

Change of Longitude is 8.15
Mid Latitude is 22.15 South

Convergency = Change Long times Cos Mid Lat
= 8.15 * Cos 22.15
= 7.38 approx
Someone may have more accurate data but the above suffices. The trip distance is not the main variable if there is no Change in Longitude such as Directly North/ South tracks then there is no convergency as you are tracking along a meridian. Greatest convergency and hence track change occurs on East/ West tracks at higher Latitudes.

From CS (S 16° 50' 59.8 E145° 44' 39.0 Mag. Var. 7°E Elevation: 23') to MLY (S 26° 44' 41.3 E152° 45' 41.0 Mag. Var. 11°E Elevation: 0') should be a course of 147°43'39.320", reverse course of 325°06'33.481" without magnetic variation, distance 709.1714331 nm.

Using the IGRF2000 model, the magnetic variation at CS (16.849944 S, 145.744167 E )at an altitude of 0 km on 6/4/1 is 7.18°E.

Using the IGRF2000 model, the magnetic variation at MLY 26.744806 S, 152.761389 E at an altitude of 0 km on 6/4/1 is 10.89°E.

Therefore your OBS out should have been 140°37'39.320", OBS in should be 134°13'09.481" assuming the navaids have the correct magnetic alignments, but more than likely they are 5 years out of date.

:rolleyes:





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It is possible to fly without motors, but not without knowledge and skill.

— Wilbur Wright

Zeke,

are you trying to wind everyone up with your numerical significance ?

Cheers all.



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Those who restrain desire do so because theirs is weak enough to be restrained.
William Blake

JT,

I just did the calcs the GPS would do, to the same accuracy.

What gets displayed...that’s another story.

:rolleyes:

Sorry CS forgot to give you a lead on a Great Circle calculator.
The link below will take you there.
http://www.best.com/%7Ewilliams/gccalc.htm
You can use the international earth model which will give a practical result.
Edited for correct url!!!

[This message has been edited by pterodactyl (edited 04 June 2001).]

Zeke,

Just stirring the pot a little my friend ....