I have put two waypoints into both GTN and SkyDemon.
A N51°20.13’ W000° 29.35’
B N51°21.86’ W000° 30.43’
I have checked carefully, and both boxes are set to decimal minutes (as opposed to seconds.)
I have then created a route OCK – A – B
You would think that both flightplans would show the same results, but SD has a total of 4.3 nm (2.6 + 1.7) and GTN 4.0 nm (2.5 + 1.5). The tracks are the same.
I know that GTN uses GC and SD uses rhumb, and rhumb (except in the limiting case) is longer than GC but surely that cannot explain a 7.5% difference in four miles?
I put the same numbers in https://edwilliams.org/gccalc.htm and got yet a third solution for the second leg of 1.86nm, and the track being 3.5° different.
Can anyone cleverer than me work out what’s going awry?
I could not replicate exactly because despite setting SD to decimals, I can only enter waypoints in seconds, and I have the opposite problem on the GTN (v6.40 iPad simulator). So I converted to seconds for the SD input. I get 2.4+1.9=4.3 on both systems. No idea why you get something different, sorry.
No idea which method Ed Williams is using to calculate the forward azimuth and geodesic but you could check GeodSolve — online calculator implemented at https://geographiclib.sourceforge.io/cgi-bin/GeodSolve
Charles Karney is one of the leading specialists on this topic today. He wrote the very beautiful Wikipedia entry for geodesics on an ellipsoid (which are rarely closed). An older but perfectly adequate method by Vincenty has been implemented by Karl Swartz at gcmap.com.
I have a spreadsheet for computing loxodrome distance following Bugayevskiy & Snyder (message if you want it) although the mathematics for this is quite straightforward — see eg https://geographiclib.sourceforge.io/html/rhumb.html
I assume both GTN & SD use WGS84 without taking an altitude argument?
denopa wrote:
I can only enter waypoints in seconds
Yes, an annoying feature of SD that hasn’t made this any easier!
I have worked it out. I have been looking at the GTN on the sim, while the aircraft is in flight. It is taking off distance for fly-by turns. Derr!!!
Doesn’t for me, fwiw
That’s really weird. Are you using the desktop sim, or is it plugged into a “real” sim, such as XP?
I’m only doing 120kts for the purpose of this exercise.
Ipad (that all I have right now) and going fast :-)
