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#### casey1999

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I am a confused as to how a chart plotter works.

Say I am in Hawaii and want to sail the great circle (shortest path) to Cape Horn. If I zoom out on my chart plotter where I can see both Hawaii (my location) and Cape Horn and point the boat in that direction, I will see a line from Hawaii to Cape Horn.

If I follow thin line is that the great circle route?

Similar situation, if you have a large scale chart of the pacific, and it shows Hawaii and Cape Horn- how do you determine the great circle route? I do not think if you draw a line between the two, and find the compass heading from the start location (Hawaii), that it will be the great circle route- but I might be wrong.

Seems you need to use a globe when navigating long distances.

#### knuterikt

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I am a confused as to how a chart plotter works.

Say I am in Hawaii and want to sail the great circle (shortest path) to Cape Horn. If I zoom out on my chart plotter where I can see both Hawaii (my location) and Cape Horn and point the boat in that direction, I will see a line from Hawaii to Cape Horn.

If I follow thin line is that the great circle route?
Depends on your plotter and plotter setup.
on my Furuno NN3D this is an configuration option I can set.

Similar situation, if you have a large scale chart of the pacific, and it shows Hawaii and Cape Horn- how do you determine the great circle route? I do not think if you draw a line between the two, and find the compass heading from the start location (Hawaii), that it will be the great circle route- but I might be wrong.

Seems you need to use a globe when navigating long distances.
There are formulas you can use if you know the lat/lon of the starting position.
Formula for distance and courses to steer.

If you sail true north /south or east/west on the equator you can steer one course all the time.

In your example, you must calculate a staring course and all the intermediate courses.
In other words to sail a great circle we normally do it in a lot of "straight line segments"
This is a fun website Great Circle Mapper

#### miatapaul

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Well chart plotting is generally not going to be a single plot half way across the earth. Obviously there are obstacles and likely stops you want to make in between. Chart plotters will not show anything useful zoomed out to show the whole Pacific and Indian Oceans, if they will even zoom out that much. Great Circle routing is generally done for aviation navigation as they are not worried about most islands getting in the way, and then you have to take jet stream into consideration. But for nautical navigation it is more of a vague notion, not a single plot. You will veer due to currents, and prevailing winds. You rarely worry about the "shortest distance" especially in a sailboat. A globe is good at getting an idea of what you might want to see along the way, but won't give much else useful.

While you can use a chart plotter to see your progress on such a trip, they are more useful in more confined waters, and making sure you are on the general path to get where you want. You can zoom in to see reefs and rocks when you are near land.

Other than that I am not sure what you are looking to accomplish. Yes you can draw a line on a globe to give you general distances. There are web sites to help with the calculations (Marine Great Circle Navigation Calculator is one) as they can get pretty complicated when trying to get a distance along a path on a sphere. (You should see the math wars that go on on geodesic dome forums!) But those are not anything more than general guides and will not even give an accurate path you will actual be sailing.

Oh and by the way those cheap (dollar store) beach ball globes are really not recommended for navigation! And really not good to use pins on them either! ;-) Actually I had one that was funny it had wrong continents, let alone countries.

Actually giving it some more thought, using a chart plotter, or even a large scale chart and drawing a line from HI to the tip of Cape Horn is kind of like me grabbing a map of the mainland and using a ruler to draw a line from New York City to San Francisco, saying that is how I am going to drive from one city to another.

#### knuterikt

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Great Circle routing is generally done for aviation navigation as they are not worried about most islands getting in the way, and then you have to take jet stream into consideration. But for nautical navigation it is more of a vague notion, not a single plot. You will veer due to currents, and prevailing winds. You rarely worry about the "shortest distance" especially in a sailboat.
Think I must get my school money refunded..
Part of the curriculum was great circle calculations using a calculator with trigonometric functions. (there are formulas that can calculate great circle using a "standard" calculator)

The equivalent of jet stream on the sea is called trade winds.
For ocean crossings it's quiet common to navigate into a favorable trade wind. No one will prevent you from steering the direct course but great circle can shorten the distance sailed.

I a merchant skipper ignored wind, currents and great circle he/she would quickly loose the job as these are factors that affect time & cost.

#### casey1999

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Ok, I am learning. One thing I learned if I do follow the great circle route from Hawaii to Cape Horn my compass heading will change along the way, otherwise I would be following the Rhumb Line which would spiral me to the pole.

I found this interesting site,
Mapping and Distance Tools
however I cannot get the cursor to work on the map. Would be cool to be able to "click and plot"

Edit- I got the above link to work by typing in the city (it will show on the map) then typed cape horn, it also shows, then cursor to Honolulu and cape horn and click these then map the route- takes some playing to get the thing to plot the great circle but it did finally work

As far as I know, there is nothing between Hawaii and Cape Horn, only water.

#### casey1999

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Oh and by the way those cheap (dollar store) beach ball globes are really not recommended for navigation! .
What the heck is that beach ball globe doing on the deck of Jessicas boat- Navigational purposes maybe?

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#### miatapaul

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Think I must get my school money refunded..
Part of the curriculum was great circle calculations using a calculator with trigonometric functions. (there are formulas that can calculate great circle using a "standard" calculator)

The equivalent of jet stream on the sea is called trade winds.
For ocean crossings it's quiet common to navigate into a favorable trade wind. No one will prevent you from steering the direct course but great circle can shorten the distance sailed.

I a merchant skipper ignored wind, currents and great circle he/she would quickly loose the job as these are factors that affect time & cost.
That was my point, as to the great circle not being close to an actual plot. You will want to consider the currents and trade winds. (actually the jet stream plays a role as well as it can push the trades around)

As to the calculations, it really depends on the accuracy you need. Geodesic Dome makers tend to fall into "close enough" and "it has to be exact" camps. I never really followed the nerdism as I did not really care, just wanted a structure to stand and be roundish. Not sure what Buckminster Fuller would think. But there was allways someone saying that they were figureing it all out wrong, and it has been 30 years since I took Trigonometry and Calculus so I have likely forgotten most of what I did know. And I don't use it in my line of work, so it has trickled out of my mind. I was very close to building a Geodesic Dome Home, then almost bought one, and decided against owning a dirt home. This was back in 2004, and given what happened to real estate prices it was a good choice.

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#### casey1999

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FWIW,
Found this on another forum:
GPS - great circle or rhumb

I am going to experiment with my chart plotter as the above suggest and then set course for Cape Horn.

#### hellosailor

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If you took a "dumb" chart plotter and said "OK, go to China" and it gave you a dumbline pointing straight to China...Wouldn't that become a rhumb line course, as the plotter kept refreshing the destination and giving you a corrected course every minute or so?

Or are they dumb enough to take a destination, plot a route, and then not update it constantly?

That is, at any given point and time, would "this minute's" course be the same?

#### casey1999

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If you took a "dumb" chart plotter and said "OK, go to China" and it gave you a dumbline pointing straight to China...Wouldn't that become a rhumb line course, as the plotter kept refreshing the destination and giving you a corrected course every minute or so?

Or are they dumb enough to take a destination, plot a route, and then not update it constantly?

That is, at any given point and time, would "this minute's" course be the same?
I have never used a chart plotter to chart a course. Either used sextant or the old GPS units that gave only long and Lat that I plotted on paper.

That said, if you chart plotter gives you a compass heading to steer to arrive at your destination that is far away (1,000's miles) and not direct north or on equator, then that is a rhumb line and you will actually be follwing the rhumb route which is longer. If the plotter is constantly changing your heading to arrive at the destination then it is giving you the great circle route.

#### Minnesail

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I've never used this feature for real, but while playing around with OpenCPN I've discovered that if you plot a course long enough that the great circle route would be significantly different* than the rhumb line it asks you which you'd like to plot.

*I have no idea what its criteria are to decide if the course is significantly different.

#### ChristinaM

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Similar situation, if you have a large scale chart of the pacific, and it shows Hawaii and Cape Horn- how do you determine the great circle route? I do not think if you draw a line between the two, and find the compass heading from the start location (Hawaii), that it will be the great circle route- but I might be wrong.
It depends on the chart (specifically, the projection). I have a North Atlantic passage planning chart that has a conical projection (IMRAY 100). On that chart straight lines are great circle routes. So if that's the kind of plotting that you're doing, then that's the kind of chart that you want.

#### casey1999

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Has anyone used a chart plotter for a long route (1,000's miles) and used to goto feature. Could you tell me how the chart plotter tells you to go? Are the compass heading directions constantly changing as you get closer to you destination (indication the gps is givng you great circle as opposed to rhumb line)?

#### jackdale

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I ran a Great Circle Route from Maui to Juan de Fuca. These are the bearings from OpenCPN.

#### Minnesail

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I ran a Great Circle Route from Maui to Juan de Fuca. These are the bearings from OpenCPN.

Do you know how often OpenCPN decides to change your heading? I see on yours it was every 565 nm, was that something you set?

#### jackdale

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Do you know how often OpenCPN decides to change your heading? I see on yours it was every 565 nm, was that something you set?
OpenCPN does that using its own algorithms, which I do not know.

I first noticed this when I set a course from BVI to Panama. It was not quite as dramatic. The long leg of the trip was set to a Great Circle in 198 M increments. The shorter legs were not.

casey1999

#### fryewe

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According to the help feature of OpenCPN (user's manual) the default setting for longitude intervals for computing great circle sailing routes is 5 degrees...

"To create a gpx file containing a great circle you have to enter the departure points lat & long, followed by the destination points lat & long and the[n] optionally a limiting latitude for composite sailing and the distance in long between waypoints, default is set to 5°."

The composite great circle route mentioned above is calculated when a limiting latitude (e.g. 40S) is entered to avoid sailing in certain waters where hazards may exist...at the expense of making the route a bit longer.

A great circle route can be calculated using spherical trig...or by laying out a route on a great circle sailing chart (gnomonic projection) and taking the courses to steer from the chart at the desired longitude intervals and entering them into the chart plotter as way points.

Don't know what my Furuno plotter does...I'll check it out tonight.

#### casey1999

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OpenCPN does that using its own algorithms, which I do not know.

I first noticed this when I set a course from BVI to Panama. It was not quite as dramatic. The long leg of the trip was set to a Great Circle in 198 M increments. The shorter legs were not.
Thanks for the input from someone with hands on knowledge. That chart plot is very interesting.

#### svHyLyte

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I am a confused as to how a chart plotter works.

Say I am in Hawaii and want to sail the great circle (shortest path) to Cape Horn. If I zoom out on my chart plotter where I can see both Hawaii (my location) and Cape Horn and point the boat in that direction, I will see a line from Hawaii to Cape Horn.

If I follow thin line is that the great circle route?

Similar situation, if you have a large scale chart of the pacific, and it shows Hawaii and Cape Horn- how do you determine the great circle route? I do not think if you draw a line between the two, and find the compass heading from the start location (Hawaii), that it will be the great circle route- but I might be wrong.

Seems you need to use a globe when navigating long distances.
Unless your plotter is equipped to utilize Gnomonic charts, upon which a great circle course appears as a straight line, your chart displays will be Mercator which become more and more distorted as the scale decreases. The Gnomonic/Mercator difference of course is the property that on a Mercator projection, a rhumb line crosses all Meridians at the same (planer) angle For planning purposes, you can obtain Gnpmonic charts relatively inexpensively, lay down your course lines, and then transfer the coordinates at primary Meridians as waypoints to your plotter using Mercator projection. Considering that on a small boat at sea, steering course differences of less than 5 to 10º is pretty difficult one would define ones primary Meridians on the basis of an interval where the heading variation between Great Circle waypoints and rhumb line waypoints is at least the same--5 to 10º. For more see "Sea Charts".

casey1999
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