View Full Version : Great Circle Routings
Hi
I am sure I saw a thread on great circle routings
(LHR-SEA as I recall)
but now cannot locate it
I am interested as I will be going LHR-CPT in a few weeks time
(for the first time!)
and having taken a look at Great circle, would like to know what the normal routing might be ( and as somebody. no idea wo, detailed the LHR-SEA routing, can anybody show me a typical LHR-CPT routing please?
tks in advance
HEATHROW DIRECTOR
25th Jan 2010, 14:35
Aircraft do not necessarily follow great circle tracks. In busy areas like Europe they follow established air routes between cities. Over the Atlantic under ideal conditions they might fly GC but the tracks to be flown are defined daily and provide the best routes to take account of weather systems. E.g A low pressure area in the Atlantic might mean that traffic to North America will fly to the north of it to gain benefit from tail winds whilst opposite direction traffic may be routed to the south.
London to Cape Town is just about great circle all the way, although I don't know the precise routes across Africa and how they may take account of weather. Stand by for a possibly bumpy ride though!!
HTH
Opsbeatch
25th Jan 2010, 15:34
Little website that will help with a route that I've used for planning fuel stops etc. Great Circle Mapper (http://gc.kls2.com/)
OB
Two-Tone-Blue
25th Jan 2010, 18:24
I thought we'd done this in the last couple of days as well. Perhaps we aren't allowed to talk about these things?
This may explain some of it (http://squall.sfsu.edu/crws/jetstream.html)
fincastle84
25th Jan 2010, 20:55
A great circle is the shortest route between 2 points on the Earth. The easiest way to visualise it is to get hold of a globe & a piece of string. Put one end of the string on your point of departure & the other on your destination. Pull the string tight. The path described by the piece of string is the great circle track. The easiest great circle to envisage is the Equator.
The LHR-CPT track heads down across France, Libya or Algeria & then across the Sahara to Nigeria. It then runs just west of the African coast until you get to CPT. Because the track is over the sea this route is a lot smoother than the LHR-JNB route, particularly if it's a night flight. (I think VS have a daytime CPT-LHR.)
Regardless, have a great time in SA; we've just returned after 3 glorious weeks.
spiney
25th Jan 2010, 21:53
So what's the point of the thread? - a Great Circle route on long haul would presumably be selected as an optimum but may vary considerably dependent on established air corrirors, weather factors, diversion options and prohibited areas...
I suspect airlines would rather not expend any more time or fuel than they need to get from A to B, but I suspect there's a bit more to it than the shortest distance between two points on the surface of a sphere... :bored:
fincastle84
26th Jan 2010, 07:20
The point of the thread is the same as any other on these forums; a quest for knowledge. As for my reply, I was just being helpful in explaining in layman's terms the meaning of the term 'Great Circle'.
I didn't have time to cover Rhumb Lines!
Spiney...
Point of the question was..
as a matter of interest ( just like many other points and threads)
as I will be travelling the route for the first time, I would like to know the route and which countries we will (proabably) overfly.
It would be nice to know.. No more... no less.
nothing sinister .
Fincastle.. tks for your reasoned response..
fincastle84
26th Jan 2010, 10:18
You're welcome. Glad to be of help. I spent many happy years as an RAF Navigator flying great circles all over the place (well, more or less great circles!).
Now I've been replaced by a GPS; cheaper to operate but never buys a beer!
Hartington
26th Jan 2010, 21:06
Some years ago I got involved in a project where it was necessary to calculate the great circle distance between any 2 airports. There were 5 companies involved; the airline and 4 suppliers and the nature of what was going on meant we all had to get the same answer for any 2 points. On the first run none of the results were the same! Some of the differences were minimal (1 mile) but some were in 10s 20s and 30s which could be critical. We never got complete agreement but we reduced the error rate to an acceptable level.
Some of the difference depended on where you measured from/to - the airline hadn't specified so if you measured from the control towers or the end of the runway (which one?) or.... But the main difference came from which model of the earth you use. We tend to think of earth as a perfect sphere even though we know it isn't. A quick Google can take you here :Javascript Great Circle Calculator (http://williams.best.vwh.net/gccalc.htm) - take a look at the dropdown "Earth Model".
I still don't really understand why and it doesn't make any difference to the OP - just a thread drift!
fincastle84
27th Jan 2010, 18:59
A lot has to do with the different map projections. eg Lamberts Conical Orthomorphic, Mercator, Transverse Mercator etc.
Spherical trig should give a pretty good answer but I have to be honest it's lost in my dead grey matter, along with most of my hair.
forget
27th Jan 2010, 19:22
gsky, Use Google Earth measuring tool. Et voila. This without any dog legs of course.
http://i21.photobucket.com/albums/b270/cumpas/zzz.jpg