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Old 22-04-2014, 19:04   #1
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Chart Work

While I have Maxsea, I became interested in OpenCPN while getting a new captain going in electronic navigation. I do piloting and coastal navigation on the US West Coast to Alaska. Many times in new dangerous waters. On paper charts I shoot bearings, take radar ranges and plot them, and have a fix in seconds. I don't see how that is remotely possible with OpenCPN using Chart Work. What am I missing?
The author of National Algorithims went to considerable work making his programs, but pre-entering every landmark that might possibly be used in a trip is unreasonable. Then shoot the bearing, then export to OpenCPN? Spur of the moment trips are out of the question. And yes, I know I could just rely on GPS. I'm too old to rely on one navigation aid.
It seems to me piloting needs to be part of OpenCPN. I need to click on a landmark and drag an LOP. Am I wrong?
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Old 22-04-2014, 19:47   #2
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Re: Chart Work

I don't know how "remotely possible" you can stand, but I just use the Route Pencil to draw a line from the bearing "target" and tap ESC key when the line is right....
by adding or subtracting 180 degrees to draw this line from the target. Then, right click that line and choose "Reverse" to check my math.

Also,
In the newer Betas, if my bearing is magnetic, I can have that too by setting it up in the toolbox ...using the "World Magnetic Plugin from this link
PlugIns | Official OpenCPN Homepage
But I have never had anything like a proper chart table. Jumping in and out of the cabin,,,pencils rolling around on the cabin sole, parallel rulers hanging up on the raggy folds of my damp paper charts....and all those nasty finger smudges on the charted lights...I think Opencpn is a luxury!
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Old 22-04-2014, 21:49   #3
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Re: Chart Work

Adding a plotting mode would be kinda cool!! That way if you lost both of your GPS's you could switch to plotting DR positions on the fly.

Roughly, I would think, something like the routing mode, with tools that allow plotting bearings and range arcs, creating and locating the fix, and fix editing operations.

Where creating a fix drops the fix symbol and names it for the time it was created. Locating the fix moves the last created fix mark to the current cursor position and draws a course line to the previous fix. Fix editing should include such things as a way to change the time (name) of the fix, and to make the fix look pretty on the chart.

Also, there should be a mouse over that displays the course, speed, and distance of the current leg.

And, of course this will need to support both true and magnetic bearings and courses.
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Old 23-04-2014, 02:12   #4
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Re: Chart Work

Happy Seagull:
Thanks for the the route pencil... Sometimes you don't see the forest for the trees.
Now how about an arc for a radar range?
I am a Navy trained navigator, set in my ways. The first thing I do with a new (to me) boat is set up a chart table. Old habits, you know. I also use a sextant and a 3 armed protractor. I don't quite have faith in electronic navigation. Like my father doing math by hand to check his adding machine.
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Old 23-04-2014, 12:50   #5
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Re: Chart Work

"radar arcs"? Well, firstoff, I need say I am using the Beta but I recall the Main version has similar.
I just goto Toolbox,Ships tab,and then "Show Radar Rings" and set up whatever I like!
but I hear you per the "Mark I eyeball" and the "Version 1.0 Brain".

Still,there's a ton of features in O I forget unless I'm on some sort of "jag"...drawing a new feature like a bw or whatever that is uncharted but I'm afraid I'll forget ...and then save it as an exported gpx to load when I'm back in the area.
Likewise you can make an icon for a mark that can inform in various ways. And even have direct links to files in the mark's "Properties" dialogue...say a pdf tidebook.
Stuff like that. Doing em once or twice, I remember they are there as features and the learning curve is much shorter than the first time!..and There always seems to be a way .You can change the colours of the lines, and even get rid of the arrowheads when you just want to have them as lines and so on....there's a bit of digging to do in the manual....
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Old 24-04-2014, 05:51   #6
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Re: Chart Work

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Originally Posted by HappySeagull View Post
Toolbox, Ships tab, and then "Show Radar Rings".
Beware that those rings are simply a bunch of perfect circles on your screen — they should not be used to measure distances, even with smaller ranges, even near equator.
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Old 24-04-2014, 10:01   #7
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Re: Chart Work

Quote:
Originally Posted by Lepke View Post
\
Now how about an arc for a radar range?
Just draw the LOP the appropriate length. If you need to adjust it, end the route and then move the mark.
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Old 24-04-2014, 12:02   #8
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Re: Chart Work

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Originally Posted by SamsonovAnton View Post
Beware that those rings are simply a bunch of perfect circles on your screen — they should not be used to measure distances, even with smaller ranges, even near equator.
You must live closer to the pole than I do . Using different projection charts? (And by the way, how well does the "route pencil" work up there? I haven't got any charts with better projections to test it....)

but yes, if I understand your meaning,I suppose it's best to call them "Mercator" Rings....
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Old 24-04-2014, 12:16   #9
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Re: Chart Work

Moscow 55N, Vancouver 49N...
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Old 25-04-2014, 05:21   #10
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Re: Chart Work

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Originally Posted by HappySeagull View Post
Using different projection charts?
It is the very basic property of any cartographic projection that its relative scale factors along meridians and parallels, when not constant, are valid in only an infinitely small range. So, unless your radar range is infinitely small , there will be scale inconsistencies almost in all directions.

To draw a real radar ring — “a set of points equally distant from a reference point”, — one should solve the task reciprocal to distance measurement. Either there should be some formulas for a specific projection, or first go from 2D (lat, lon) to 3D (X, Y, Z) and then back to 2D, and then to projection. With smaller ranges typical to leisure watercrafts, simpler sphere models must be replaced with ellipsoid.

Currently with OpenCPN, even if you place your boat in 0° N 0° E, the reported size may vary greatly with zoom level and direction (not only horizontal vs vertical, but even northward vs southward), depending on vessel dimensions and “GPS offset” parameter.
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Old 25-04-2014, 08:48   #11
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Re: Chart Work

Anton...
Not that I disagree, your math is of course correct. But do you really think that a real user cares about the difference in latitudes normally sailed, say 60N-60S, with radar rings set to a couple of miles?
I personally never did as the inaccuracies brought to any navigational calculation by my variable speed and not constant course are always much bigger.
But perhaps we just have a different perception of "vary greatly"

Pavel
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Old 25-04-2014, 10:52   #12
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Re: Chart Work

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Originally Posted by nohal View Post
But perhaps we just have a different perception of "vary greatly".
As I said, it depends on zoom level and GPS offset.
  1. Move your boat to 0° N 0° E.
  2. Set your boat size to 20 m long, 5 m wide, with GPS offset at 19 m.
  3. Set ship icon to “Vector, at scale” with a minimum size of 12 mm (for example).
  4. Enable 6 radar rings of 1 NMi each.
With this setup and a scale level of 1:50'000 (for example), so that the rings occupy the entire screen, the horizontal radius of the outer ring is 5.4 NMi in my case, and the vertical radius is 5.3–5.4 NMi. When zoomed out at the extreme, so that the ship icon is still symbolic, the radius is actually near 3.2 NMi (yes, almost half of expected). Once zoomed out so that the ship icon turns into crosshair, the radius will be exactly 6.0 NMi.

Change your GPS offset to 10 m (middle) — and the issue will go away… until you sail to Greenland with a 60-mile radar.
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Old 25-04-2014, 10:55   #13
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Re: Chart Work

@ Lepke ..
Quote:
a 3 armed protractor
I'm still puzzling how I'll do that in O without invoking a lot of bother...but it occurred to me you might enjoy the "Celestial Navigation" plugin -that might serve somehow.
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Old 25-04-2014, 11:35   #14
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Re: Chart Work

Quote:
Originally Posted by SamsonovAnton View Post
It is the very basic property of any cartographic projection that its relative scale factors along meridians and parallels, when not constant, are valid in only an infinitely small range. So, unless your radar range is infinitely small , there will be scale inconsistencies almost in all directions.

To draw a real radar ring — “a set of points equally distant from a reference point”, — one should solve the task reciprocal to distance measurement. Either there should be some formulas for a specific projection, or first go from 2D (lat, lon) to 3D (X, Y, Z) and then back to 2D, and then to projection. With smaller ranges typical to leisure watercrafts, simpler sphere models must be replaced with ellipsoid.

Currently with OpenCPN, even if you place your boat in 0° N 0° E, the reported size may vary greatly with zoom level and direction (not only horizontal vs vertical, but even northward vs southward), depending on vessel dimensions and “GPS offset” parameter.
Maybe you have found! a bug in Opencpn
Otherwise, an accuracy that involves changing the gps offset on my small boat is negligible compared to the gps's advertised accuracy (or using a pelorus, handbearing compass,radar et al)and the errors introduced by my charts' projection as you noted. But the objective is to place me on an "erroneous"chart -merely a diagram of the "real world" which is itself merely a construct of my brain in a universe that seems to defy a Unified Theory.
"Navigators" have always had a unique handle on the Uncertainty Principle..
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Old 26-04-2014, 04:53   #15
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Re: Chart Work

Re: Mercator circles I noticed this issue when implementing the Anchor Alarm, but it was too tempting to reuse the Radar Rings code. It is one of the reasons for putting in a limit for the Alarm radius...
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