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Old 30-11-2013, 05:41   #1
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New Zealand Raster Charts now BSB

I just had a look on the NZ LINZ site and found that since Nov 22nd they are releasing their raster charts in BSB format. Thanks NZ

NZMariner - Official Raster Navigational Charts (RNCs) | Land Information New Zealand (LINZ) - Toit

I have tried the sample in OCPN and all seems well.
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Old 30-11-2013, 09:35   #2
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Re: New Zealand raster charts now BSB

Please read this post :

OpenCPN Beta Version 3.3.1117 Released

and try ...
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Old 30-11-2013, 10:28   #3
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Re: New Zealand raster charts now BSB

Great move by the Kiwis. Congratulations!

Unfortunately the small scale charts shows clear signs of bad geo-referencing. Something has gone wrong ......?
The larger scal charts seems OK.

In the log there are heaps of lines like:
Quote:
Skew override on chart /home/thomas/Charts/ChartsII/BSB3/NZ_release/BSB_ROOT/NZ14600/NZ1460001.KAP is 117.125 degrees
Click image for larger version

Name:	NZ_release.png
Views:	147
Size:	249.8 KB
ID:	71316

Look at all the charts in the chart bar that is marked as "Skewed". There are no skewed charts in NZ anymore, as far as I know. There used to be a few, a long time ago, around Tauranga, for example.

Thomas
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Old 30-11-2013, 10:59   #4
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Re: New Zealand raster charts now BSB

Nothing wrong with the Kiwi charts. They work fine with 3.2.2.
So it looks like some trouble with the internal skew calculation patch.
This is one for the tracker!

Thomas
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Old 19-12-2013, 09:49   #5
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Re: New Zealand raster charts now BSB

The previous reported problem is fixed in Git and will be included in the next beta release.

Meanwhile.....
If you are cruising the Tonga/Samoa area or are planning to do so. The official NZ chart release is missing a number of charts in this area, that are available in Marco's version.

Thomas
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Old 19-12-2013, 13:08   #6
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Re: New Zealand raster charts now BSB

Hello,

if you are planning to go to the south pole please note that the LINZ NZ charts representing the south seas and spanning many degrees in latitude (like the NZ14065) present a little problem:

since LINZ charts include third order polynomials, OpenCPN will uses those polys; unfortunately those polys cannot approximate correctly the Mercator projection for such charts and there will be an error up to some tens of pixels in the Latitude direction.

Maybe OCPN should check such cases, avoid using the polys and rely on REF points only.

Ciao, Marco.
Attached Thumbnails
Click image for larger version

Name:	LatitudeShift.png
Views:	156
Size:	98.1 KB
ID:	72410  
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Old 19-12-2013, 14:37   #7
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Re: New Zealand raster charts now BSB

Thank you Marco!
I have entered this issue in the bug tracker.
FS#1265 : Charts: Third order polynomials leads to BSB chart display error in high latitudes.

Thomas
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Old 20-12-2013, 04:14   #8
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Re: New Zealand raster charts now BSB

Hello GPS Marco,

Quote:
Originally Posted by GPS-Marco View Post
Hello,
.....
Maybe OCPN should check such cases, avoid using the polys and rely on REF points only.

Ciao, Marco.
What do you understand by "REF points" ,

Thank by advance. Gilletarom.
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Old 20-12-2013, 06:14   #9
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Re: New Zealand raster charts now BSB

I prepared a very simple empirical method to estimate the poly error:

Given of a chart:
MaxLat, MinLat , HeightInPixel

the third order poly error in pixels (RMS) is about:
1) MaxNorthing = ln (tan (MaxLat/2 + Pi/4))
2) MinNorthing = ln (tan (MinLat/2 + Pi/4))
3) RMSPixelError = Abs(HeightInPixel/1000*(MaxNorthing - MinNorthing )^3)

So: if the RMSPixelError of a chart is 2 or more pixels, it is maybe better to use the BSB REF points instead of the polys.

Ciao, Marco.
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Old 20-12-2013, 09:40   #10
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Re: New Zealand raster charts now BSB

Quote:
Originally Posted by Gilletarom View Post
Hello GPS Marco,



What do you understand by "REF points" ,

Thank by advance. Gilletarom.
Gilletarom

Open a .kap bsb chart in a text editor, and you will see how the key word "REF" is used. For an explanation have a look at Chart Conversion Manual | Official OpenCPN Homepage. There are also numerous threads on this forum that discuss the subject of making and manipulating bsb kap charts.

Thomas
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Old 23-12-2013, 09:32   #11
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Re: New Zealand Raster Charts now BSB

Marco...

I do not fully follow your estimate for RMS pixel error
Given of a chart: MaxLat, MinLat , HeightInPixe
l the third order poly error in pixels (RMS) is about:
1) MaxNorthing = ln (tan (MaxLat/2 + Pi/4))
2) MinNorthing = ln (tan (MinLat/2 + Pi/4)) 3) RMSPixelError = Abs(HeightInPixel/1000*(MaxNorthing - MinNorthing )^3)

What is the constant "1000" coming from? What about chart scale, semimajor_axis....?

I suppose this is truly empirical, but I would like to understand it before I code it....
Dave
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Old 24-12-2013, 05:00   #12
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Re: New Zealand Raster Charts now BSB

Hello,

I attached a simple excel to evaluate the 3rd order poly error and the empirical formula error estimation on a mercator chart. The objective of the empirical formula is just to switch OCPN beteween the poly usage (that is sometimes wrong) and the REF usage (that should be always rigth). So it does not whant to be "precise".

Anyway, another switch condition could be to read in the BSB header the ERR values (should be always present in conjuction with the presence of the polys). These ERR rows report the error at the REF points due to the usage of the polys: again if the RMS of the errors read from ERR lines is grater than say 2 pixels, don't use the polys. Unfortunately, these ERR reported errors can be due to a real distorsion of the chart and not only to the polys error in fitting mercator exact latitude formula.

Ciao, Marco.
Attached Files
File Type: xls LatError.xls (20.5 KB, 48 views)
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Old 26-12-2013, 06:37   #13
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Re: New Zealand Raster Charts now BSB

I am continually impressed with the chart expertize in this forum. It is quite remarkable. I think back to the massive effort to convert charts that turned into real disapoointment, but I think that experience and effort shows itself every day in some way. Thanks guys for your time and expertize!

Sent from my GT-N5110 using Cruisers Sailing Forum mobile app
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