Path used to compute bearing and distance by GPS? - Page 3 - SailNet Community
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post #21 of 49 Old 11-12-2019
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Re: Path used to compute bearing and distance by GPS?

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post #22 of 49 Old 11-12-2019
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Re: Path used to compute bearing and distance by GPS?

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Originally Posted by mcast View Post
As a theoretical physicist, there is a question that I have been asking myself for a while: I would like to know how bearing and distance are precisely calculated by the GPS.

As a practical one I suggest you chuck that old Garmin over the side. Buy a new one. That one is ancient.

In the set-up menu it has an option to use Great Circle or direct routes.

If more than 100nms you will be using the Great circle Route otherwise your trip from the USA to UK will take you through the centre of the earth, or at least, a long way in the wrong direction.

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post #23 of 49 Old 11-12-2019
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Re: Path used to compute bearing and distance by GPS?

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Originally Posted by CrispyCringle View Post
Ive flown transport category aircraft for decades using both INS and GPS nav systems. I feel like I should know the answer to the OPs question. Im sure Ive glanced over it in reading the minutia of a particular nav system. Im fairly certain (most?) GPS uses the WGS84 model and will always plot great circle routes for course and heading.
That is also my experience: Great Circle Routes. Every GPS I've seen allows you to select a geodetic model other than WGS84. DON'T! There are still a few areas where the charts aren't based on WGS84, but unless you know what you're doing, never deselect WGS84. Whenever crew have unsupervised access to my GPS, I always check to make sure they didn't change the geodetic system. Bad things can happen if you don't notice that change (the young people these days seem to like to twiddle every setting on everything when they're bored on deck watch.)

If we want to argue minutia, the earth is a "geoid." That's the cartographers way of saying it's a lumpy, irregular, slightly distorted sphere with variable gravity that bulges just a bit around the equator. That's a long description, so they just say "geoid."

Nature seems to hate perfection as much as she hates a vacuum.

(Please don't tell the Flat Earth Society about this thread or we'll be inundated with comical descriptions of how we are all deluded by our conspiratorially-derived beliefs.)
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Last edited by patrickbryant; 11-12-2019 at 11:26 PM.
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post #24 of 49 Old 11-13-2019 Thread Starter
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Re: Path used to compute bearing and distance by GPS?

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Think we need someone who knows more about the engineering of gps use to answer this question. It remains a good question. How does the software work? Understand how you get a position from it but donít understand how the software uses that.
I agree, one should really see how the software works and talk to someone who knows about the GPS engineering. CrispyCringle's answer is along these lines, I like it.
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Re: Path used to compute bearing and distance by GPS?

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Originally Posted by CrispyCringle View Post
Ive flown transport category aircraft for decades using both INS and GPS nav systems. I feel like I should know the answer to the OPs question. Im sure Ive glanced over it in reading the minutia of a particular nav system. Im fairly certain (most?) GPS uses the WGS84 model and will always plot great circle routes for course and heading.
Cool answer, thank you.
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post #26 of 49 Old 11-13-2019 Thread Starter
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Re: Path used to compute bearing and distance by GPS?

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As a practical one I suggest you chuck that old Garmin over the side. Buy a new one. That one is ancient.
Mark
I don't have that GPS. That is just a picture I took from the web to give you an idea.
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post #27 of 49 Old 11-13-2019
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Re: Path used to compute bearing and distance by GPS?

I see more value in understanding how the math/navigation works. Understand the trigonometry and the software is just a fancy calculator with the hardware providing a visually appealing, easy to understand display.

With higher end chart plotters, you select which read out you want, rhumb line or great circle and the chartplotter performs the calculation for you.

Most people find great circles difficult to steer so the default is generally rhumb line. If you are going longer distance, you switch to great circle and find a way to steer the great circle, traditionally it was done by squaring off the curve, but I am guessing newer autopilts can do it on their own.

The software/hardware will constantly change, but the fundamental math and navigation principles don't.

Last edited by Arcb; 11-13-2019 at 04:38 PM.
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post #28 of 49 Old 11-13-2019
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Re: Path used to compute bearing and distance by GPS?

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Most people find great circles difficult to steer so the default is generally rhumb line. If you are going longer distance, you switch to great circle and find a way to steer the great circle, traditionally it was done by squaring off the curve, but I am guessing newer autopilts can do it on their own.
This confuses me...

In the real world... say I am sailing from Montauk to English Harbor.
I determine a waypoint for Antigua enter it into the plotter as I am clearing Montauk. The GPS plotter software tell me what the bearing/heading to that waypoint is. It will report in integer degrees I believe. I can't hand steer to an integer. But I can set that value into the AP and as I make way... assuming I can sail that course.... my GPS will tell me the course I am making (as well as the ship's heading)... course takes leeway etc into account. I choose to then correct the heading so the course is actually the heading I wanted to steer.

I WILL have to alter the AP as conditions changed... and I likely can keep that original heading despite the changing leeway etc. These are not usually momentary changes. I monitor the TRACK like to see that it appears smooth and close to the original heading line.

For sure I can't carry a course without leaving a narrow "cross track road"... a bit. But over the two thousand miles... what is the implication of this? How many extra miles would I likely sail?

And lets not forget that having favorable wind direction for 2000 miles is likely not going to happen.

pay attention... someone's life depends on it
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post #29 of 49 Old 11-13-2019
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Re: Path used to compute bearing and distance by GPS?

You don't sound confused to me. That's what I meant by squaring off the curve. You steer a series of straight courses to acheive what is ultimately a curving course on a mercator projection chart, but due to other naturally occuring circumstances and practical considerations, it doesn't matter that much. On a ship or a power boat it can be more noticeable. Unless you are steering the rhumb line on a long offshore passage, in which case, you could potentially be adding hundreds of miles onto a long passage.

Last edited by Arcb; 11-13-2019 at 05:23 PM.
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post #30 of 49 Old 11-13-2019
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Re: Path used to compute bearing and distance by GPS?

Here is an article that explains it. It describes the squaring off I was describing. I was taught the same way the article describes. Change course once daily. So on a 30 day crossing you could make 30 course alterations, once per day. So during a given 24 hour period you are steering a straight line, but the resultant course on a mercator projection chart, is curved.

Some one specifically used the New York to London example because its a higher latitude, long distance run where the difference between the great circle and rhumb line is more pronounced.

https://astrolabesailing.com/2014/10...ircle-sailing/
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