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Old 03-07-2007, 22:42   #16
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Originally Posted by NavPak ECS
The clue to why Seaclear is free is in the Seaclear help file:


Calculations and accuracy

Heading and distance calculations are done using the rumbline....
//// end excerpt from Seaclear help file.

This means that the coordinate routines are based on plane trigonometry instead of spherical trig so that if you measure a distance, it will only be an approximation and the range and bearing to a waypoint will also be an approximation, and you can't use it across the International Date Line (180 longitude).

Pete,

Your conclusion about the problem with rumbline calculations is not accurate, or at least not very precise. Whenever a straight line course is laid out on a standard Mercator chart it is a rumbline course and Seaclear would calculate range and bearing along that course as accurately as you could from the chart (or hopefully better).

What Seaclear can NOT do is calculate great circle routes. But there are not very many recreational navigators who can do that, either. And unless you are sailing across oceans without stopping great circle routes aren't much more than a geometric curiosity.

When was the last time YOU did your navigation calculations using spherical trig?

Bill
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Old 04-07-2007, 01:13   #17
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Bill,
Regarding Plane Sailing, The American Practical navigator says: "this method should not be used for more than a few hundred miles"

I agree with that and it is recognisied as a standard. If you are using rhumb line calcs for more than few hundred miles then you could be way off. Do you want to be 8 or 10 degrees off course for a passage of only a few hundred miles, or do you want the passage to be 10% longer than expected? Not me, I want the correct range and bearing regardless of the latitude and distance. Do yourself a favor and try a few comparisons or rhumb line vs great circle calcs. You will be surprised at the big descrepancies, even for short distances. As the book says, anything over a few hundred miles should not be used.
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Old 04-07-2007, 05:21   #18
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rhumb line calcs won't work?

I'm following this but 'not getting it'. Without getting real technical, could you explain why those rhumb line calcs won't work?

Let's say we create a real life working scenario.

I have a known GPS coordinate for a destination some 1000 miles distant. I use the Seaclear charting software and NOAA charts to plot my course from here to there. The course is converted to waypoints and uploaded to my GPS. If the GPS is updating my Seaclear chart position it stands to reason that my chart position will be updated, right? The GPS is also showing the way to the next waypoint.

What I'm confused about is how you're saying that this will not get me to my next waypoint and eventually to my destination GPS coordinate?????

Although the method described above may not be the shortest route between two points, I can't imagine how this can't work and I'm interested in your response. If this doesn't work, we might as well toss all our GPS's and all charting software.

I believe that American Practical navigator article might have been written before GPS technology was created. GPSs have changed the way we navigate.

Rick in Florida
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Old 04-07-2007, 05:53   #19
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Many modern GPS receivers are capable of delivering the bearing to a waypoint as either a rhumb-line or a great-circle course. Be aware that on some units where you have the option of selecting the mercator or great circle, that selecting one option will calculate the entire route in that mode.

Other GPS units only calculate using the great circle, without the option of calculating a mercator rhumb line between the positions. I’m unaware of any GPS that only calculates rhum line bearing (desired track).

When sailing along the coast or making a North-South offshore passage, the two are synonymous, for all practical purposes.

However, when making an east-west ocean passage, well away from the equator, following a great-circle course could result in a considerable sailing time/distance savings.

As long as you have specified great-circle course on the GPS setup page (I believe Garmin normally defaults to great circle calculations for distance and desired track.), you need not indulge in tortuous calculations involving spherical trigonometry or plot new courses repeatedly from a gnomonic chart - you have only to create a destination waypoint, hit the go-to button, and read off the desired course.
The figure you get will be the (ever changing, as you move) great-circle heading from your current position.
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Old 04-07-2007, 09:08   #20
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Thanks for explaining that Gord.

Waypoints are wonderful things!!

Rick in Florida
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Old 04-07-2007, 12:19   #21
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Originally Posted by NavPak ECS
Bill,
Regarding Plane Sailing, The American Practical navigator says: "this method should not be used for more than a few hundred miles"

I agree with that and it is recognisied as a standard. If you are using rhumb line calcs for more than few hundred miles then you could be way off. Do you want to be 8 or 10 degrees off course for a passage of only a few hundred miles, or do you want the passage to be 10% longer than expected? Not me, I want the correct range and bearing regardless of the latitude and distance. Do yourself a favor and try a few comparisons or rhumb line vs great circle calcs. You will be surprised at the big descrepancies, even for short distances. As the book says, anything over a few hundred miles should not be used.
Pete
Pete

I have a couple of minor quibbles with what you say, but I think we really are on the same page just approaching things from different directions. I agree (of course!) with Bowdich that using the PLANE sailings is a bad idea for calculating long range distance and bearing. But SeaClear uses MERCATOR sailings (mathamatically and practically different) which are useful and accurate for long range calculations of rhumbline distances and bearings.

As you suggest above the use of rhumbline calculations varies greatly depending on the distance and direction of the trip. From San Francisco to Hawaii the difference is only about 20 NM out of 2000. From San Francisco to Japan, the difference is HUGE, something like 3X the distance by rhumbline over great circle. For someone who is using a computer (or a mercator chart!) to blindly do all of their passage planning this would be a rather hopeless case to use the rhumbline!

Since I do all my planning and navigating on paper charts, how my program calculates long distances is irrelevent to me. If I need a great circle route I dig out the gnomonic chart and transfer waypoints from that to my mercator chart that I use for day to day navigation. My electronic toys are just a handy cross check. But I guess my approch is very quickly becoming old fashioned on such things, and I shouldn't let my luddite tendencies color my recommendations to more modern sailors.

One of the things I like about SeaClear is that I can scan in the exact paper charts (corrections, notes and all) I am using and see the same thing on the computer and on the chart table.

Bill
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Old 04-07-2007, 13:46   #22
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Bill

I'm still confused.

Using NOAA charts, Seaclear not only shows my boat in the marina, but shows it exactly in my slip. I'd say that's pretty accurate, right?

As Gord Pointed out, GPS navigation is a great circle calculation. GPS is the input to Seaclear. My confusion is this: If you agree that the GPS will navigate between two points several thousand miles apart using the great circle calcuation. And take my word for it that Seaclear is pretty accurate in displaying boat position on NOAA charts. If it does this using GPS input, then how is this combination inaccurate? In what way?

Rick in Florida
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Old 04-07-2007, 16:41   #23
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Hi Rick,

We picked up SeClear from some mates in SE Asia. Works great for us. I liked how simple it was. We use MaxSea, CMAP and The Captain too. All work fine. I did love the fact that is was quick and easy to print from SeaClear.

Long ago we ran outta paper charts and were printing 8.5x11 sheets out of our $70 inkjet and putting them in a binder. Easy, light, takes up little room and you can just flip through the book as needed. I think we dumped 100lbs of paper charts 'cause I was getting sick of digging around them.

The only thing I think one would have to consider is if someone scans in a proper chart for using in a program like SeaClear. There's the set-up part of the program where you have to tell it the lat/long ref on the chart. If someone messes up or is lazy your info is off from the chart. If someone gives you files go in and double check them.

best - J
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Old 04-07-2007, 17:52   #24
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Bill,
<< I agree (of course!) with Bowdich that using the PLANE sailings is a bad idea for calculating long range distance and bearing. >>
Yes, it says anything over a few hundred miles, whatever that is.

<< But SeaClear uses MERCATOR sailings (mathamatically and practically different) which are useful and accurate for long range calculations of rhumbline distances and bearings. >>
Depends on how close you want the result.

<< From San Francisco to Hawaii the difference is only about 20 NM out of 2000. >>
Yes, but the course difference is about 8 degrees. That is huge. You will be way off course when you leave. As another example from Jacksonville Florida to Bermuda, I get a course difference of 4 degrees over a distance of 858 miles.

Another example at a higher latitude is from Hobart Tasmania to Resolution Island in New Zealand. Using another program based on Plane trig, it shows the distance and direction from Hobart to New Zealand (Resolution Island) as 828 miles and 106 degrees True. The actual distance and direction is 843 miles and 110 degrees True. That's about the same course descrepancy (4 degrees) as the Florida to Bermuda example, but the distance descrepancy is greater.

For an east-west course, you can estimate the descrepancy by using the cosine of the latitude. For example the cosine of 45 is about 0.7, so where a minute is a mile on a great circle, a minute on the 45th parallel is 0.7 miles. That is a big descrepancy.

<< Since I do all my planning and navigating on paper charts, how my program calculates long distances is irrelevent to me. >>
That's fine if it serves your purpose, but most people expect the correct answer when they measure a distance on a chart plotter.

<< If I need a great circle route I dig out the gnomonic chart and transfer waypoints from that to my mercator chart that I use for day to day navigation. >>
Everyone has their own favorite requirements and method. The Gnomic chart is good, but on a sailboat going to windward, you will probably tack dozens or hundreds of miles off the original great circle, so that the original great circle is obsolete, and the shortest distance to the destination is a new great circle from your present position. When trying to decide which tack to favor at a given moment, it is helpful to have a continuously updated VMG to the destination, and to get that, you need the correct range and bearing to the destination, continuously updated.

As you said, if you are just planning on paper charts and you don't care about the accuracy of long distances, then Seaclear may be OK, but if you always want the correct distance, direction and VMG to your destination then you need something based on spherical trig.

Pete
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Old 06-07-2007, 16:54   #25
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<shrug>

The great circle route from Cape Flattery to Yokohama is cool on the chart software. The only problem is the prevailing winds and currents would make it effectively stupid to try to sail there direct.

Using an appropriate tool which accomplishes the job is all that is required. Magnetic ompasses are inconsistently wrong, and my ability to helm a precise course is probably even worse, but I seem reasonably able through a collection of tools and techniques to make the landfalls (or more usually these days to pilot) and make my destinations. An 8-degree error in predicted course is not nearly the error induced by any big ocean current, and as a navigator I adapt and adjust, attempt to plan for all sorts of variables, and then I guess. An informed guess, but a guess.


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Old 12-07-2007, 21:27   #26
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<< Using an appropriate tool which accomplishes the job is all that is required. >>
Of course, I never suggested that great circle sailing was mandatory, however I did forget to mention that it is required to calculate accurate current set and drift. So the great circle capability gives current set and drift, VMG, plus the correct range and bearing to the waypoint or other object. None of it mandatory, but nice to have.
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Old 13-07-2007, 08:10   #27
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Complex systems

Having your log/depth/wind/compass/gps all interconnecting with your computer navigation software is cool, and could probably make the navigator feel really pampered and secure knowing the recent history of set and drift. I've never actually been aboard a boat where such a system has been installed, but I'm sure there are some of them out there as they seem to be talked about a lot in shiny magazines. The reality for most sailors, however, is going to be somewhat less complex, and they'll need to calculate their own set, probably on a paper chart.
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Old 13-07-2007, 18:57   #28
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take a shot on Seaclear

Amgine

I'm going to take a shot on Seaclear which will display log /depth /wind /compass /gps. From what I've read you can only have one "talker" on NMEA 0183 without a multiplexer. I would imagine that's the GPS. The manual is a little vague about connections, but as long as a laptop is involved with lots of ports, there is hope. Sure cuts down on required displays at the Nav station.

Now if I can find a way to get my engine instrumentation on to the display..... hmmm...
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Old 13-07-2007, 19:54   #29
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Give us a report

Tell us how things work out for you. I can't even try SeaClear (well, I could, if I set up WINE or Parallels on any of my non-Windows boxen, but I can't justify it for any single program...) so if you manage to use it in application maybe you can tell us what you like/hate about it.

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Old 02-08-2007, 11:23   #30
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Great thread, I like the info. I picked up (downloaded) SeaClear II while in Port Townsend and have been stumbling around with it an paper charts plus my small Garmin "chart plotter" GPS. I am in Eureka, CA and decided to get an antenna that will plug into my laptop via USB port. The SeaClear program won't recognize the antenna, has anyone dealt with this issue and have any ideas on it. I opened the Comm tab in the software but don't see any way to direct it to the USB port, I tried all of the Com ports that are listed there with no success. The software that came with the antenna recognizes the antenna and it is working for that but won't work with the SeaClear. My Garmin GPS has a data cord with serial port plug and that will work with SeaClear but then don't have a way to keep the small GPS powered unless I keep feeding it batteries.... Any help is certainly appreciated. I think what I need is a way to direct SeaClear to use the USB port but don't know how to do this.
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