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Old 25-01-2011, 07:29   #1
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Plotting Fixes With Navigation Software

I am concerned about discrepancies in the position shown on navigation software in some parts of the world. I would like to be able to plot a three bearing fix to verify my position. I understand that MaxSea had a great set of features for this type of DR plotting but that feature was not implemented in the new Time Zero version. Does anyone know of another navigation software that supports this? Any thoughts on my concern, if it's real, and (if so) how to deal with it another way? Thanks
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Old 25-01-2011, 08:24   #2
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This might be a case where paper charts and analog instruments make the most sense: Three Arm Protractor | Weems & Plath
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Old 25-01-2011, 12:34   #3
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This might be a case where paper charts and analog instruments make the most sense: Three Arm Protractor | Weems & Plath
Yep. This is very, very seldom going to be a problem with the fix. It is normally an issue with the charts. Which is why I've always said I can't really know how to use any electronic chart unless I first know how to properly use paper charts. And then I have to know the types of errors introduced by the electronic systems.

--> Inland Waters Resources - Chart Errors

-dan
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Old 25-01-2011, 12:48   #4
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I appreciate the responses I have received but I think they miss the point of my question. I would guess that most people are using electronic charts while cruising. Unfortunately many of these sailors don't know how to navigate with paper charts. I do. I have taught Advanced Coastal Navigation classes for the USCG Aux. That's why I am asking the question. I have talked to representatives of some navigation software products and they had no idea what I was talking about when I asked about plotting a visual fix. I have heard stories of electronic charts being off by a mile or more. I know this is because the paper charts they are based on are off. With paper you have to take visual fixes to validate your position but with GPS and nav software you just put complete faith in that little boat icon. I want to be able to validate that icon with visual fixes. Thanks.
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Old 25-01-2011, 13:08   #5
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FWIW, sometime ago a user asked for this exact feature - basically an ability to plot a few LOPs to get a fix on a chart. So now PolarView supports this by means of multiple "Quick Routes" (which are a universal plotting tool in PolarView). Essentially you can place a number of these lines, pointing with specific bearings to landmarks etc.

I occasionally use this feature myself to test accuracy of charts and my GPS - mostly just for fun, as charts are pretty much correct around here.
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Old 25-01-2011, 16:08   #6
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Originally Posted by Bill Shuman View Post
I appreciate the responses I have received but I think they miss the point of my question. I would guess that most people are using electronic charts while cruising. Unfortunately many of these sailors don't know how to navigate with paper charts. I do. I have taught Advanced Coastal Navigation classes for the USCG Aux. That's why I am asking the question. I have talked to representatives of some navigation software products and they had no idea what I was talking about when I asked about plotting a visual fix. I have heard stories of electronic charts being off by a mile or more. I know this is because the paper charts they are based on are off. With paper you have to take visual fixes to validate your position but with GPS and nav software you just put complete faith in that little boat icon. I want to be able to validate that icon with visual fixes. Thanks.
Sorry. I'm so used to the people who don't understand paper charts asking questions similar to this, that I just missed the point.

brak pointed out one package that allows this. But, I'm afraid that since most people just blindly follow their chartplotters, there doesn't seem to be much call for those features.

I heard tell of a brand of chartplotter that allowed the user to enter offsets to align charts. Don't remember which one it was, though. But I don't know if it allowed you to plot the adjustments on-screen. You may have needed to chart on paper and then just enter the offsets.

What you want makes perfect sense to me. Sorry I can't really help you with the answer.

At some point, maybe OpenCPN could add that. I think I'll think it through a bit and add a feature request.

-dan
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Old 26-01-2011, 11:24   #7
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With paper you have to take visual fixes to validate your position but with GPS and nav software you just put complete faith in that little boat icon. I want to be able to validate that icon with visual fixes. Thanks.
I understand your thinking. Pretty much in line with my own.
I have no plans what so ever, to give up my traditional skills because of a moving icon on a screen.
For a lot of reasons I'm very interested in OpenCPN, and while this chart-plotter has no dedicated tools for plotting traditional position lines, be it terrestrial or celestial, it is still possible.

For example to plot a bearing to prominent landmark.
Use the measure tool to drop two marks along the bearing to the land mark. Connect these marks to a route, and there is your position-line.

Similar thinking applies to plotting a celestial position line.

Thomas
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Old 26-01-2011, 13:42   #8
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<< I would like to be able to plot a three bearing fix to verify my position. >>

I agree, it is conspicuously absent in most programs. The NavPak demo is fully enabled for traditional navigation. If you feel comfortable finding your way around with a compass and sextant, then your software is free.

The first thing you would want to do when the GPS goes dead is to plot some bearings. In addition to point and click bearing plotting, the program should have some way to get the variation without GPS and apply it to the bearing. Some chart formats do not have a field for variation, such as BSB version 2 and maybe 3, so at the very least a program should have an easy way to manually set local variation.
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Old 26-01-2011, 14:55   #9
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The best approach to variation, imho, is to use a standard variation model and calculate the value specific to given location. User may not know what the variation should be or simply have no time to deal with that, considering that he's already dealing with loss of GPS and who knows what else.

It's one of those things that software has to "just get right".
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Old 27-01-2011, 11:50   #10
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Referring to Bowditch (1984), it says that variation "requires measurement of its strength and direction at many places (art. 707) before it can be defined accurately enough to be of practical use to the navigator." It then goes on to explain that extrapolating variation from existing annual change may result in errors (duh). Then it says "Magneticians have not detected a recognizable worldwide pattern in secular change." This is an old book so maybe they have perfected a math model for variation since then, but I don't know. It reminds me of calculating the GP of the moon, where you can do it precisely as long as you are using coefficients that are periodically updated.

I'll stick with what it says in the book, even if it is old, and get my variation from the chart. I still need to look at the chart to see if there are any local magnetic anomalies before plotting bearings. I routinely cruise with old charts, but next time I go somewhere, I think I'll get a copy of chart 42 (Magnetic Variation) for the current epoch.
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Old 27-01-2011, 12:09   #11
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You are welcome to download a copy of the Mag Var for epoch 2010, as a BSB/KAP chart from Pilot Charts for Chart Plotters | Official OpenCPN Homepage.

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Old 30-01-2011, 12:24   #12
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<< You are welcome to download a copy of the Mag Var for epoch 2010 >>

Thanks Thomas, I think the NGDC site has one also. I plan to convert it into a TIFF and use it as a background map for day to day charting, then nobody on the boat can claim that they don't know the local magnetic variation.

Happy sailing,
Pelican Pete.
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Old 30-01-2011, 13:12   #13
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Make your plot on a piece of plain paper. One should not be plotting on a valuable paper chart anyway. I do this constantly. By the end of a crossing there is a large pile of paper with LOP plots, wind/current triangles, and all manner of doodles. It's a littler easier to use the traditional plotting pads as there are some reference lines and a compass rose.

A OpenCPN plotting tool kit would be sweet, though.

LOPs can be reduced either graphically by making a suitable scaled drawing, or making an unscaled sketch and then trigonometrically solving the triangle(s). The information, usually a distance along one of the LOPs is then measured on the chart.

Bowditch would simply do the trig in his head....
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Old 30-01-2011, 13:56   #14
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Using MapTech Offshore Navigator on my laptop, I can plot fixes of all sorts using the A / B line feature.

I have also done set and drift using the same feature.

I works quite well.
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Old 01-02-2011, 08:11   #15
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This is an old book so maybe they have perfected a math model for variation since then, but I don't know. It reminds me of calculating the GP of the moon, where you can do it precisely as long as you are using coefficients that are periodically updated.
Indeed, and the coefficients are periodically updated - the most recent update actually was done mid-2010 and covers the next 10 years.

The changes from results calculated based on previous set of coefficients (which covered past 10 years) is small, so clearly it is incremental.

In any case - those magnetic variation charts are built from the results produced by the same model Except, of course, they are fixed in time so you don't get the benefit of getting the most current value which you would when using model calculation directly.
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