GPS plots a Rhumb Line or Great Circle?

[Salty John]

Re: GPS plots a Rhumb Line or Great Circle?

From any great circle route calculator you will get 044T and 4050m for the two coordinates you give. Seeing as the great circle route is the shortest distance there must be a mistake in your numbers somewhere.
GPS on it normal default will not give the course to steer as 044T, it will give an intital course and this course will continually change as you proceed toward the destination.

 

[Richard10002]

Re: GPS plots a Rhumb Line or Great Circle?

Great Circle Distance = 3517.05nm 4047.35mls 6513.58kms

Initial True Course is 44.3 degrees(T)

I think your result is in statute miles, and my rhumb line is in nautical miles.

In this 3500 miles, the Great Circle saving is 130 miles so, (as has been suggested), fairly academic when compared with weather, (good or bad).

presumably, a GPS will give 044T as the course to steer, or course to go, when you set the route, i.e. this will be the course which the user will see. It would then change as the route is followed.

It will still be interesting to see what "our" GPS and plotters produce.

 

[wanderlust]

The garmin gps definately plots a great circle. I still have the chart with a perfect curve across the Atlantic from Cape Verdes to Barbados. You can try this for yourself if you like by putting in a route across the atlantic then following the route and plotting the positions on the route line onto the chart. Join the dots and you will see a curve

 

[sailormags]

Oh dear! It's all gobblydegook to me! What will Stevie_D think of me - I'm his better half. I was feeling quietly confident about getting to grips with the 'technical stuff' but now I need a sick bucket and a lie down. /forums/images/graemlins/frown.gif

 

[FAITIRA]

Re: GPS plots a Rhumb Line or Great Circle?

Certainly the distances are great circle, so I have always supposed the course must be too, but sadly the wind never lets me hold my course long enough to worry about that!

 

[temptress]

Most GPS sets can be set to either follow a Rhumb line or Great Circle course. For relatively short distances say a few hundred miles it make little practicle differance.

Your GPS is probably set to Rhumb line (in my experiance the default) unless you decide to change it.

 

[MapisM]

First principles, Clarice. Simplicity.

I was also driven nearly crazy ...by the complicated answers to your question!
Just trace any route staying on the same latitude, let's say W to E.
If the GPS gives you exactly 90°, it's working on rhumb line.
Less than 90° means it's calculating great circle.
Unless you are tracing the route in the southern emisphere - in this case, either 90° or more, respectively.

 

[bilbobaggins]

[ QUOTE ]
I'll award myself a pint then!

I did an O level in Air Navigation and got an 'A' when I was 16 (so pre gps) and have been working on it ever since

[/ QUOTE ]

If'n we ever find ourselves at a boaty 'do' together, I'll buy you a pint, you can buy me a pint, and we can alleviate the tedium of the speeches by discussing the 'Anomalies in the Calculation of Bands of Error Relating to Single Position Lines' or practical exotica such as 'The Three-Drift Wind'.....

( Hanzup all you 'Liveaboards' who have ever heard of the above pro nav equivalents of the Rocna vs OtherThingy debate, never mind actually used 'em? )

/forums/images/graemlins/smirk.gif

 

[Anonymous]

Part of the problem is a discrepancy between the thread title and your question which are two totally different questions - especially when a GPS is linked to a third party plotter or charting system.

My Navman 5500i integral plotter and GPS does not mention the issue - even in passing! I think from what other have said here and from some research online that GPS engines designed for marine use give the initial heading for a great circle track. I say 'GPS engine' because the essential GPS is a positioning system and the heading required is nothing to do with the GPS satellite navigation system per se. It is calculated by your GPS. The GPS does not plot.

The answer to your actual question (as opposed to the question in the tread title) is that your GPS will take you on a Great Circle. Your plotter display could be anything, but that's a different question and I certainly can't give the answer for my Navman.

 

[rivalranger]

Re: First principles, Clarice. Simplicity.

Following this simple test on my Lowrance 3600 plotter produces a nice arc for the bearing line.

 

[tgpt21]

A rhum line is defined as a line which intersects with every meridian of longitude at the same angle. So if you follow a constant course you follow a rhum line. A GPS therefore gives you the Rhum line course to steer to your waypoint. The only time you would be sailing a great circle is when the course is true north or south as you would be following a meridian of longitude which as we all know is a great circle, that is to say a circle on a sphere the plane of which passes through the centre of the sphere.

On a mercator chart a Rhumb line is a straight line passing the meridians of longitude at the same angle whilst a great circle is a curve. On Gnomic projection the great circle is a straight line (the meridians are depicted as curves) and the Rhum line is a curve.
To use your GPS thingy to sail a great circle you would have to plot your great circle (easiest using a gnomic routing chart) and then create a series of convenient alter course positions (waypoints I believe thay are now called) thus creating a series of tangents (Rhum lines) which best follows the Great Circle course.

EVERY projection assumes the earth to be a sphere (although the earth is in fact an oblate spheroid, a sphere flat at the top and bottom) I believe that some models used in the GPS gadget do takes this into consideration, so that when you're in the middle of the Atlantic a thousand miles from anywhere and the pencil line on the chart is over a mile wide, you know where you are to within 2 metres.
My apologies for this rant I was a Merchant Navy Deck Officer during the 70s and was taught that electronics were the root of all evil. I still have my sextant and tables which I use for celestial and terrestrial navigation. I love paper charts. I think I recall all the theory and stand to be corrected if my memory fails me.

God bless Mercator, Marc St. Hilaire Galileo and Copernicus. Long live long by chronom. John Harrison we love you. Oh the mysteries of the noon sight, the d and v corrections. Whetever happened to GHA and LHA and the first point of Aries?

PS I suppose rising and dipping distances of lights would mean nothing to anyone!

 

[jimbaerselman]

Just think of any commited rhumb line navigators down by the South pole. They'd be going round in circles if they used a rhumb line GPS to cross from one side of the pole to the other!

 

[tgpt21]

Actually at the pole (or anywhere else) the navigator, if following a rhum line sailing, would not go round in circles he or she would go around in a spiral towards the pole, his course cutting the meridians at the same angle all the way unless his course is 000, 090 180 or 270 (T). The difficulty of navigating the rhumb line at the polar regions is not travelling along the the course, but the problems with the compass giving you the correct heading and actually depicting the poles in the Mercator projection. At 90 degrees N or S the paper would have to be infinitely long.
Of course to get to the pole from anywhere it's just a matter of heading true north or south and you would be following a meridian ie a great circle.

 

[jimbaerselman]

Sorry, I forgot the /forums/images/graemlins/wink.gif.

The point is, polar navigation (in True, not Magnetic) shows the shortcomings of rhumb line navigation in an extreme way. This make it easier for anyone struggling with the concepts to see that rhumb lines at the equator are fine, but as latitudes increase, they become increasingly impractical, until one reaches the absurd polar example.

Within a some m (or km for that matter) of the pole, there is, in fact, no such thing as East and West. Compasses of any sort have no meaning. A different form of navigational grid is used.

 

[Anonymous]

Good point, Jim. How did/do travellers in the Arctic and Antarctic navigate around (without modern electronic aids)? Suppose Jack and Jill are in their tent some 50km north of the South Pole, and Jill decides to climb a hill to fetch a pail of ice-water. They have a PMR radio set and Jill makes a point of remembering her route in case she needs to call Jack for help in an emergency (e.g. falling down the hill /forums/images/graemlins/smile.gif) Jill falls. How does Jill tell Jack how to find her? (Hansel and Gretel technique is not permitted).

 

[BlueSkyNick]

He doesnt...... He should have gone with her in the first place, like he does in the nursery rhyme!! /forums/images/graemlins/smile.gif

 

[Anonymous]

[ QUOTE ]
He doesnt...... He should have gone with her in the first place, like he does in the nursery rhyme!! /forums/images/graemlins/smile.gif

[/ QUOTE ]The nursery rhyme was written years ago, when they were still in lust /forums/images/graemlins/wink.gif

 

[stephenh]

Re: GPS plots a Rhumb Line or Great Circle?

Not really too sure what I'm talking about (!) but a while ago I asked a mate this question and he attached my Garmin 12 ( very old..) to a computer and went into it to show me what was in there.

IIRC the Garmin GPS supports various NMEA sentences, two of which have the letters BWC and BWR in them.
Bearing to Waypoint for BW and Circle or Rhumb for the C and R
You have the option to change from the one to the other but apparently Garmin use BWC as the default.

Hazy memory - I will happily be corrected...

 

[jimbaerselman]

[ QUOTE ]
How did/do travellers in the Arctic and Antarctic navigate around

[/ QUOTE ] Longitude plus distance from pole will define position. Alternatively, when Pontius was a pilot (late 1960's) he'd carry a chart marked out in the UPS Grid (Universal Polar Stereographic), and assuming whoever else also carried one, they'd be able to transfer information. Personally, I always stuck with lat and long, which was a pretty good approximation for long plus distance until WGS 84 came along. So much for position info - but, which direction to travel if you deny me my trusty Garmin?

Pontius or his relatives (cunning sods) dreamed up Gnomic projections, which have the endearing quality that all straight lines plotted on them are great circles . . . so, of course, some Arctic navigators use these. They look just like 360 degree protractors (pole in the centre, of course) except the rings of latitude diverge as they move away from the pole. So scale increases as latitude decreases.

So, we plot Jill's position, and my departure point, and join the two with a straight line. The angle of this line relative to the pole is measured. If it's clear, set off towards some suitable geographic feature in that direction. Or some astronomic feature. Or set up your gyro with it's axis pointing towards Jill (don't ask where you plug it in) and march off into the snow following a straight line, allowing for earth spin in the latter two cases.

If you mounted a great big stick on the pole, you could watch it's relative bearing change as you marched up/down the hill along your straight great circle line, musing how useless compasses were up there. You could even double check that bearing relative to your course against your Gnomic wotsit to tell you how close you were getting to Jill.

[DISCLAIMER] All this information is approximate, and should not be taken as a lesson on how to enter a polar port, for which only the appropriate admiralty charts are suitable, but please note that even these do not plot the current positions of icebergs, polar bear threats, magnetic north poles, Jill's current position or patches of horse [--word removed--].

 

[jimbaerselman]

Or, of course, he just turns on his radio homing device