Great Circle Sailing v Rhumb Line

[scotty123]

I've just been looking at this.
Bit confused.
Is it simply the difference between one projection & another?

If I compare the Earth to an orange, is a Great Circle Route a line joining start & finish?
Is there any difference between N/S & E/W route?

What does my GPS read?
Is a waypoint such as Azores, a Great Circle sailing from say Bermuda?

I have seen suggestions that a GPS datum can be changed from eg WGS 84 to 'Great Circle Sailing', but can't seem to find that on my Garmin handheld.

 

[landaftaf]

ina nutshell, a great circle is the shortest distance on a sphere (or spheroid as our globe is)

most charts in common usage are made using the mercator projection, this stretches latitude as well as makes lines of longitude appear parallel (which of course they are not)

if you draw a great circle on a mercator chart it will appear to bend towards the poles, as ppl usually draw a straight line on their mercator chart (rhumb line), it looks the shortest distance but in fact is the longer.

in fact a great circle has best mileage savings on a e/w or w/e route, and the saving is greater in high latitudes.
n/s is following a line of longitude which is a great circle already

wgs84 refers to the internationally accepted oblate spheroid shape (world geodetic scale) (spellign is prob rong ) adopted by cartographers (in 1984 believe it or not), so all sat nav instrumentation can be referred to that shape - there are others /forums/images/graemlins/grin.gif

it has nothing to do with a great circle

not looked but wikapedia probably has loads of data on this sort of stuff /forums/images/graemlins/cool.gif

 

[dom]

Q. Why did the equator win the Most Valuable Parallel award at the Latitude Super Bowl?
A. Because it was a great circle. /forums/images/graemlins/laugh.gif


......oh dear. Sorry

 

[landaftaf]

its the way you tell them /forums/images/graemlins/grin.gif

 

[johnalison]

You could try sailing on 45 degrees on a Mercator course and try to reach the North Pole. After infinite miles you still won't be there. (ignoring land, perhaps in a plane)

 

[Twister_Ken]

A good illustration is a flight from say, London to Los Angeles, which passes over Greenland.

Hardly seems intuitive, but it is the shortest way to get there.

Usual way of demonstrating this is to stretch a string between two points on ball. It will follow a curve.

 

[DaveS]

I've thought about this a bit, but I give up. If your heading is maintained at 045 then you'll progress northwards at about 0.707 * your speed (call that V knots). (Ignoring currents, land, etc.) If you started X miles south of the North Pole then surely after X / 0.707V hours you'd reach the Pole. N'est pas?

 

[dom]

A man walks out of his house, puts on his coat and kisses his wife goodbye. He heads due south for a while and meets some friends. After this he travels west to pick up some shopping. He finally sets off due north home.

Where does he live? ... not too hard!

 

[Helina]

Does he live with his wife ?

 

[JonJon]

North Pole? stupid place with all this global warming could sink

 

[Salty John]

The great circle route is only of significance for distances over about 600 miles, and if you are in high latitudes and you want to travel east or west. If you were travelling across the Pacific, for instance, you could save, perhaps, 200 miles on the 5000 mile journey from California to Japan. Unless you particularly want to understand the theory from an academic point of view just ignore great circle routing.

 

[Twister_Ken]

>Unless you particularly want to understand the theory from an academic point of view just ignore great circle routing.<

Not if you want YM Ocean!

 

[dom]

Good point. I forgot the north pole was afloat even though was part will soon take on a new meaning!

 

[Salty John]

Correction:
Unless you particularly want to understand the theory from an academic point of view, or you want YM Ocean, just ignore great circle routing.

 

[bilbobaggins]

A ( single ) rhumb line course is one which cuts all meridians at the same angle. Thus, apart from the special case of 0º, the path that is described on the spheroid is a 'loxodrome'.

Or, as one Singlehanded Transatlantic Race yachtsman said to another, 'Did you follow the Great Circle Route, or did you come straight here?'

/forums/images/graemlins/grin.gif

 

[Richard10002]

From The Caribbean to UK, we used to alter course at noon everyday if we were following a great circle, (pre-GPS), i.e. a series of Rhumb lines.

IIRC, once GPS arrived, we would alter course by 1 degree every time the Course To Waypoint changed.

 

[machurley22]

[ QUOTE ]
I've thought about this a bit, but I give up. If your heading is maintained at 045 then you'll progress northwards at about 0.707 * your speed (call that V knots). (Ignoring currents, land, etc.) If you started X miles south of the North Pole then surely after X / 0.707V hours you'd reach the Pole. N'est pas?

[/ QUOTE ]
Hmm. Would that be 045T or 045M?

 

[DaveS]

Must say I was assuming 045T. If we used M we'd get a course which sort of curved up to the Magnetic North Pole instead.

 

[Helina]

When sailing from New York to Falmouth we followed the weather forecast instead of the great circle.

 

[bilbobaggins]

[ QUOTE ]

Hmm. Would that be 045T or 045M?

[/ QUOTE ]

After an appropriate interval, it would need to be 045 'Grid' to continue being useful....

....But do you really want to go there? The really worthwhile pubs are well south of the 80th Parallel.....



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