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Old 13-01-2010, 04:26   #466
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Position error:

At this point, it would be good, if there were some official reference points at known locations around here, so we could verify our results.
Perhaps they exist, anyone?...

/Pete.

By using the Lon/Lat lines on the chart, I see, as Marco suggested, that this error changes depending where you are on the chart. The center being the most accurate. Near the chart edges, then the error increases to 120 meters here in Sandhamn, or there abouts!

/Pete
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Old 13-01-2010, 05:05   #467
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IALA SEA MARKERS

Would it be possible to include this official IALA list of sea markers in the Mark Properties list in OpenCPN? Would be very useful for local updating of personal charts and in future perhaps also for the updating of a central chart server. A '//' delete mark would also be handy.
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Old 13-01-2010, 05:08   #468
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Ok, since the log is clean, the georef algo did not find any major problem. The issue should then be related to the display of non mercator maps. As far as I saw, OCPN dispays the map making calculations related to its center (and then the center of the map presents minimum error).

It should be feasible to calculate instead the center of the actual dispayed screen (real lat/long, as red by the mouse) then calculate the related x:y on the image map and then display the image centered on the calculated x:y with the extent (W:H) calculated applying the right display scale. Or something like that...

I think this would minimize the display errors for non-mercator maps (or for non perfectly scanned mercator maps...).

Ciao, Marco.
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Old 13-01-2010, 11:50   #469
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Pete, Jenne, Marco,

Good job all!

Thank you for identifying that OpenCPN displaying of non-mercator charts is an issue. I suspect that with a number of this group working on converting charts to BSB format for display in OpenCPN there will be other similar issues in the future.

This issue is similar to the one of displaying charts with datums other then WGS-84. I have suggested that a set of manual controls be added to OpenCPN that would allow the removal of local errors based upon direct observation. This would allow the local correction of these kinds of datum and projection errors. Anyway, Dave is a smart guy and it is up to him to find a solution to these issues.

Best regards to all,

Paul
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Old 13-01-2010, 12:03   #470
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This issue is similar to the one of displaying charts with datums other then WGS-84. I have suggested that a set of manual controls be added to OpenCPN that would allow the removal of local errors based upon direct observation. This would allow the local correction of these kinds of datum and projection errors. Anyway, Dave is a smart guy and it is up to him to find a solution to these issues.
Best regards to all,
Paul
Hi Paul. Actually for non wgs84 charts the solution is already in. A couple of months ago we identified the parameter DTM/0,0 inside the BSB header as the datum shifting controller. The two numerical values are float numbers for northing and easting in seconds of degree. OCPN handles those shiftings (thanks Dave...).

You can use something like notepad++ to open the .kap, modify the DTM values, save the .kap. When the chart is perfectly aligned in OCPN (based on your observations) just use bsbfix to make the .kap chart readable to other BSB compliant SW too.

This solution does not work for correcting projection errors since these shifts apply with the same extent to the whole chart while projection errors vary moving over the chart.

Ciao, Marco.
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Old 13-01-2010, 14:22   #471
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Originally Posted by GPS-Marco View Post
Hi Paul. Actually for non wgs84 charts the solution is already in. A couple of months ago we identified the parameter DTM/0,0 inside the BSB header as the datum shifting controller. The two numerical values are float numbers for northing and easting in seconds of degree. OCPN handles those shiftings (thanks Dave...).

You can use something like notepad++ to open the .kap, modify the DTM values, save the .kap. When the chart is perfectly aligned in OCPN (based on your observations) just use bsbfix to make the .kap chart readable to other BSB compliant SW too.

This solution does not work for correcting projection errors since these shifts apply with the same extent to the whole chart while projection errors vary moving over the chart.

Ciao, Marco.
Hi Marco,

My concern is that a sailor approaching a harbor will discover that the GPS position displayed on the chart does not match what is happening in the real world. At that point it is probably unrealistic to expect someone to edit the header file to make the necessary corrections. Thus, I believe that it would be a good thing if it were possible to make these corrections from within OpenCPN.

I must disagree with you on the use of the datum correction parameters to locally correct projections errors for navigation. As you stated before, the minimum projection error is at the center of the chart. So, if you apply a datum correction to minimize the projection error at a position away from center of chart you have effectively moved the center of the chart to that position. Thus, the corrected position and positions in the surrounding area well be more accurate.

Regards,

Paul
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Old 13-01-2010, 14:29   #472
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I think anyone who starts worrying about the chart datum as they're approaching the harbour deserves whatever happens next

Perhaps the accepted chart type should be advertised clearly in the manual instead..
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Old 13-01-2010, 14:47   #473
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Hi Marco,

My concern is that a sailor approaching a harbor will discover that the GPS position displayed on the chart does not match what is happening in the real world. At that point it is probably unrealistic to expect someone to edit the header file to make the necessary corrections. Thus, I believe that it would be a good thing if it were possible to make these corrections from within OpenCPN.

I must disagree with you on the use of the datum correction parameters to locally correct projections errors for navigation. As you stated before, the minimum projection error is at the center of the chart. So, if you apply a datum correction to minimize the projection error at a position away from center of chart you have effectively moved the center of the chart to that position. Thus, the corrected position and positions in the surrounding area well be more accurate.

Regards,

Paul
1) Agreed. I did not mean a "real-time" correction. The work for aligning maps shall be done in advance. Info can come from the map itself (very often non wgs84 charts declare the shifts w.r.t. wgs84) or can come by comparing known positions (other maps, precisely known points, etc.). And when you know a precise shift value of a chart, there is this forum to share it so everybody will sail with a better map.

2) I disagree: a map shal be precise in its whole extension, not only on the lower left corner... we must work a little bit more on OCPN to achieve this.

Ciao, Marco.
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Old 13-01-2010, 16:32   #474
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1) Agreed. I did not mean a "real-time" correction. The work for aligning maps shall be done in advance. Info can come from the map itself (very often non wgs84 charts declare the shifts w.r.t. wgs84) or can come by comparing known positions (other maps, precisely known points, etc.). And when you know a precise shift value of a chart, there is this forum to share it so everybody will sail with a better map.

2) I disagree: a map shal be precise in its whole extension, not only on the lower left corner... we must work a little bit more on OCPN to achieve this.

Ciao, Marco.
Marco,

With so many people converting charts how are you going to guarantee the precision of every chart? What happens when someone comes along and converts a chart without knowing the significance of NAD-27 datum or transverse mercator projection? Who is going to take on the responsibility to verify that the charts are accurate?

Even the commercial electronic charts are not always that accurate. Each user of these charts is required to decide if the navigation information being provide is accurate enough. For OpenCPN it is no different. What is different is that we can provide suggestions on how to improve the navigational accuracy of the displayed charts.

Regards,

Paul
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Old 13-01-2010, 17:01   #475
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With so many people converting charts how are you going to guarantee the precision of every chart? What happens when someone comes along and converts a chart without knowing the significance of NAD-27 datum or transverse mercator projection? Who is going to take on the responsibility to verify that the charts are accurate?
Paul
Absolutely right.

Q. With so many people converting charts how are you going to guarantee the precision of every chart?
A. Nobody will guarantee. Many people checking these free maps will give you more confidence but nobody will guarantee. I always invite people to use official charts for navigation.

Q. What happens when someone comes along and converts a chart without knowing the significance of NAD-27 datum or transverse mercator projection?
A. The chart is maybe wrong. But see above: sharing free charts will help in catching errors.

Q. Who is going to take on the responsibility to verify that the charts are accurate?
A. You. Who is required to update official owned paper charts with latest notice to mariners? You. Who is going to take the responsibility to verify the engine, the sails, the instrumentation, the mast, the weather, the safety equipment, .... you again, captain.

I've never believed that charts we are georeferencing can substitute official charts. But think it's a great chance to improve OCPN (many bugs have been highlighted), to understand georeferencing(!), to plan routes sitting on the armchair, to spend some time dreaming on the south seas...

Fair winds. Marco.
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Old 14-01-2010, 08:04   #476
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... The CM_93 Ed 2 2009 charts available here work wiith OpenCPN and cover the World ...
CNF.SeaSoft...
Thanks for this link, where I see charts I will never need that are still very interesting. They all open in OpenCPN, but I have not yet tried _all_ formats.

Question: In the charts section and elsewhere, it seems there are duplicate downloads. E.g., when I download MapMedia charts, there are often five links, and when I expand and decode them, they all appear to be charts of the identical area repeated five times. Is this what others have observed, and if so is it necessay to download all parts? If so this could be a full time project. I now have the CM93 charts, which are downloaded but not yet viewed.
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Old 17-01-2010, 16:27   #477
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I seem to suddenly have lost the GRIB function in OpenCPN. The weather button does not stay depressed and creates no response. I re-installed v. 1.3.6 but it is still not functioning. What has happened I wonder? The show grib icon is ticked.
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Old 17-01-2010, 16:49   #478
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With so many people converting charts how are you going to guarantee the precision of every chart?
Right! This may not be the proper place for a feature request, but it may be the proper moment. We want a crosshair cursor, with double chrosshairs, just like the mapcal one, to check a chart's calibration with greater ease and accuracy.

Please?

- D.
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Old 17-01-2010, 17:07   #479
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And anyway, I agree with just about all Marco says, and idpnd, "anyone who starts worrying about the chart datum as they're approaching the harbor", the chart plotter is just another thing you use, and you shouldn't trust it more than necessary anyway.

As I approach the harbor, or the anchorage, IRL, just about the last thing I look at is the plotter. I have paper charts but usually they're not updated. The electronic ones are even less updated. What I always have, which is always the latest edition, is the pilot book for the area.
If I look at anything for the approach, it's that. I look at the plotter too, but, sort of, distantly. That's why I'm happy to keep it downstairs.

And if I trust anything, it's not even my eyes, or my calculations, it's more my nose.
So ENCs, great, I love them, just use them for what they're worth, which is quite a lot, but they're just a tool like any other, no more, no less.

- D.
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Old 18-01-2010, 10:36   #480
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I seem to suddenly have lost the GRIB function in OpenCPN. The weather button does not stay depressed and creates no response. I re-installed v. 1.3.6 but it is still not functioning. What has happened I wonder? The show grib icon is ticked.
Just tested this. No the Grib button never stays depressed. The popup just comes and goes. Perhaps your popup is outside your window? I have the following values in my opencpn.config file
Quote:
GRIBUseHiDef=0
GRIBDialogSizeX=300
GRIBDialogSizeY=540
GRIBDialogPosX=858
GRIBDialogPosY=211
What are your values?
I think that it is a bug that the button doesn't stay depressed when the gribs and the grib popup are active.

Thomas
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