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Old 17-01-2015, 13:56   #1
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Magnetic Variation (WMM) when boat is off the chart

I just installed OpenCPN and am impressed with the work you folks have done. I have used electronic charting software since 1999 during tens of thousands of miles from Alaska to southern Mexico and Annapolis to the BVI.

However, I am confused about Magnetic courses and bearings in OpenCPN 4.0.0 and WMM 1.0 (with the 2015-2020 wmm.cof installed). When I use the OpenCPN measure tool to calculate a Magnetic bearing, heading, or course the bearing I see is incorrect and often dangerously wrong compared to using a paper chart and the Magnetic Compass Rose/variance.

I tested this using Measured Nautical Mile courses on many charts with a variety of magnetic variations. I also compared measured OpenCPN Magnetic bearings with those calculated by Visual Suite using the same chart.

My computer is NOT connected to a GPS during this testing which is the normal case when I am doing route planning.

I have read the entire WMM thread and find nothing about this issue. If this issue is known and has been discussed please point me in the right direction.

When I display the extended WMM information I see that the BOAT variation is INCORRECT (for the current chart) but the cursor information is CORRECT (for the current chart). However, the displayed bearings and course info is based, (apparently?), on the boat variation.

The reason for the erroneous Magnetic course, heading, or bearing is that my boat was not placed anywhere on a chart and WMM uses (apparently because I just installed it) 15 E as a default. In my first test case the variance should have been 11 46 E but the WMM icon showed 15.0.

When I move the boat onto the chart the variance is then measured and displayed correctly and Magnetic bearings, headings, courses are calculated correctly.

When the CURSOR (but not the boat) is moved to a chart with a different variance, then the measured course and bearing on EVERY chart is calculated with the variance where the boat is located NOT based on the current chart!

This makes the software less than useful for planning.

Why in the world would you knowingly use the wrong magnetic variance? The chart data contains the variance appropriate for the chart.

The WMM documentation does not state that the boat must be on the current chart to display accurate magnetic information.

The OpenCPN documentation says only to install WMM for magnetic information.

I think the requirement that the boat be on the chart in use should be documented in BOLD RED letters.
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Old 17-01-2015, 14:02   #2
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Re: Magnetic Variation (WMM) when boat is off the chart

TacomaSailor...
Congratulations, you have found a bug. This does not have to be documented, this has to be fixed.

EDIT: Oh, and BTW, the chart contains no *data* about magnetic variation, that's why we use the WMM plugin to get it.
Sure it is less than usual for your particular usecase, but I'm wondering what *planning* in magnetic is good for. When you sail there, the boat variation is obviously correct, when you are not there at all, what is the magnetic heading good for?
As a workaround, use "Move boat here" functionality for now.

Pavel
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Old 17-01-2015, 15:55   #3
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Re: Magnetic Variation (WMM) when boat is off the chart

"Sure it is less than usual for your particular usecase, but I'm wondering what *planning* in magnetic is good for. When you sail there, the boat variation is obviously correct, when you are not there at all, what is the magnetic heading good for?
As a workaround, use "Move boat here" functionality for now."


I started my computing career at NASA reducing satellite data then progressed to real time laboratory measurements and onto real time refinery process control. I eventually got into automated real time control of operating systems and applications. 30+ years writing and supporting application and OS code to crunch important numbers.

The computer geek in me can not let minor details go by.

The magnetic variation for route planning is not very important because, as you say when the boat is in the water the data will be correct, but I just can't led such an undocumented "feature" go by un-noticed.

Again, I am very grateful for the work everyone has done. It is a fantastic effort and result. I managed a lot of big software projects involving multiple companies, developers and analysts from multiple sites, and management from advertising, marketing, IT, and financial. So, I can appreciate the coordination that was required with such an opensource document.
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Old 17-01-2015, 16:51   #4
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Re: Magnetic Variation (WMM) when boat is off the chart

TacomaSailor...
The whole OpenCPN documentation is created in a wiki. If you feel anything is insufficient, confusing or wrong, just log in to opencpn.org and fix it...
I have also created FS#1686 : Change the logic for magnetic headings while NOT underway so if you have any comments, don't hesitate to add them there.

Pavel
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Old 03-08-2015, 13:50   #5
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Re: Magnetic Variation (WMM) when boat is off the chart

Folks,

I think I've discovered a little additional information related to this issue, though it may be a completely separate issue, so please let me know if I should re-post elsewhere.

In short, using OpenCPN 4.0 on a macbook pro, checking the box to use magnetic headings and bearings in OpenCPN initially made no difference anywhere, even when inputting my local appropriate magnetic declination (-14.4). The headings and bearings do display an (M), but they are exactly the same as true headings. Installing the WMM plugin didn't change anything. Note that this testing was without any GPS input.

Once on the boat, I'm using OpenCPN without a dedicated GPS input or dongle; instead I'm using an app on my iPhone called GPS2IP to wirelessly feed GPS data to the laptop. It works pretty darn well actually- but putting the boat on the map using the iPhone GPS coordinates initially didn't make any difference in the magnetic headings either. However, the app has checkboxes for which elements of the NMEA sentences you'd like to send, one of which is heading. This has the unfortunate effect of showing the boat pointing whichever way the phone is pointed, however... all magnetic headings and bearings suddenly display appropriately. Using the phone as the GPS input without HDG turned on in NMEA calculates appropriate COG, which is all one would need, so I would prefer to leave it turned off so that the phone can be carried around.

So- does the presence of the HDG variable in the NMEA sentences have something to do with OpenCPN being willing to honor the request for magnetic display??
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