mc2bsbh - MapCal Template to BSB Template Conversion

[dacust]

Originally Posted by chartnut

...

Sinbad7: I am new to graphics, but I thought the jpeg format was not a lossless format, especially when compressed. If my understanding is correct, using jpeg in the conversion process will diminish the quality of the end product. I have been sticking to GIF, TIF, or PNG during the conversion process because all these formats are lossless.

You are correct. I think Sinbad7 was starting from a JPG.

-dan

[GPS-Marco]

Originally Posted by chartnut

Thanks for all the quick responses!

SP field: I agree with dpons comment - "Specifies the coordinate system of the chart and the interval in feet". This field contains 2 parameters separated by a comma. Sorry, I am a syntax freak! I do believe that mc2bsbh should be updated to remove the UNKNOWN from the "SP=" entry and leave it blank.
...

Hello,

Do you have any SW that complains with the value SP=UNKNOWN?

I have different BSB chart samples from different producers (USA-NOAA, Brasil, etc.) and all of them use different conventions for the unused/not applicable fields: blank, 0.0, UNKNOWN, no field at all.

We do not have the real BSB specifications since BSB is a proprietary format but fortunately the intent of mc2bsbh is not to prepare official BSB charts but just to prepare a kap header that works fine with "most" of the SW out there and that definitely works fine with OpenCPN and SeaClear.

So, if no SW complains with the mc2bsbh header, I think we should not be worried.

Ciao, marco.

[jonasaberg]

Originally Posted by dacust

You are correct. I think Sinbad7 was starting from a JPG.

-dan

Absolutely, jpg should be regarded as pest and cholera. Never ever use it during scanning and/or conversion of charts.

/Jonas

[chartnut]

GPS-Marco said "Do you have any SW that complains with the value SP=UNKNOWN? --- 8< --- So, if no SW complains with the mc2bsbh header, I think we should not be worried."

No, I do not know of any SW that complains, but I will continue to manually override this in my chart conversions.

Googling this afternoon, I got this off a gdal posting relating to what PI= may be all about:
"I think KNP/PI= has something to do with roughly how much distance 1 pixel covers, anyway here is a scale : PI ratio for a number of NOAA charts:
SC=400000,PI=30.000
SC=80000,PI=10.000
SC=40000,PI=5.000
SC=15000,PI=1.000
SC=10000,PI=1.000 "

[chartnut]

I am converting a chart that is based on WGS72. The chart indicates that WGS 84 positions should be moved 0.11 min southward and 0.04 min westward. In MapCal, I set the Datum to WGS72 and I put in Lat Shift: 000 00.110 S and in Long Shift: 000 00.040 W. This generated the following entry in ChartCal.dir:
DS=-0.00183333333333333,-0.000666666666666667

I do not see this information showing up in the .hdr file created by mc2bsbh.

[GPS-Marco]

Originally Posted by chartnut

I am converting a chart that is based on WGS72. The chart indicates that WGS 84 positions should be moved 0.11 min southward and 0.04 min westward. In MapCal, I set the Datum to WGS72 and I put in Lat Shift: 000 00.110 S and in Long Shift: 000 00.040 W. This generated the following entry in ChartCal.dir:
DS=-0.00183333333333333,-0.000666666666666667

I do not see this information showing up in the .hdr file created by mc2bsbh.

Hello chartnut,

you should find a non zero DTM/ field. The two values in the DTM field are shifts in seconds.

Ciao, Marco.

[Netsurfer]

Hi ...

I am actually working on a script which should create a BSB header (in txt format) for a Google Map screenshot.

It is intended to be processed by imgkap (M'dJ replacement for tiff2bsb - see http://www.cruisersforum.com/forums/...tml#post686229 ).

So here are my questions:

1. Which values do I absolutely need to add?
   BSB/
   NA= name of the map / OK
   NU= number / OK
   RA= width,height in pixel / OK
   DU= dpi/ppi shouldn't matter but could be 72 / OK
   
   KNP/
   SC= scale, is it really needed for OCPN to display the kap? / ??
   GD=WGS84 / OK
   PR=MERCATOR / OK
   PP= projection parameter, any needed for Mercator? / ??
   PI=UNKNOWN, / ??
   SP=UNKNOWN, / ??
   SK=0.0, / ??
   TA=90.0 / ??
   UN=METERS, / ??
   SD=UNKNOWN / ??
   
   OST/
   what is this one good for? needed? / ??
   
   CED / ??
   
   CPH/0.0 / ??
   
   DTM/0,0 / ??
   
   KNQ / ??
   
   REF / OK
   
   PLY / OK
2. Is the order important?
   If so, which one is the correct one?
3. Would it have any advantages if there are more REF points than just the 4 corners of the image?

Any help is much appreciated!

Many thanks in advance,
Gunther

[HamishB]

Originally Posted by Netsurfer

I am actually working on a script which should create a BSB header (in txt format) for a Google Map screenshot.

maybe it helps:
https://trac.osgeo.org/grass/browser...t.kap_template


Hamish

[HamishB]

oh yeah, and for correctly calculating the local map scale based on Web Tile zoom level and latitude, see line 1140 here:

SourceForge.net Repository - [gpsdrive] Contents of /trunk/scripts/gpsfetchmap.pl

(This same formula is used by OpenStreetMap tiles, Google Map tiles, Microsoft Map tiles, ... the cartographic abomination that is spherical mercator)


Hamish

[cagney]

Gunther
See this post:http://www.cruisersforum.com/forums/...tml#post591685

Thomas

[HappySeagull]

ABSOLUTELY? but why not have it all?Imgkap writes a lot of it automatically...because
Unfortunately,OpenCPN is very rugged and opens just about anything,while other apps are more sensitive.You saw this post?I didn't get into it much but some charts won't open in anything but OpenCPN,which is ok!but still a little disturbing.
http://www.cruisersforum.com/forums/...tml#post677075

"KNP/
SC= scale, is it really needed for OCPN to display the kap? / ??"

It tells a program how to quilt ie in what order.OpenCPN uses this to line up along the chart bar,so it's important.I've tested this by changing it.Imgkap does it automatically quite well.There's just a few home-calibrated images that needed moving...ie small graphics covering large areas don't look well

"PP= projection parameter, any needed for Mercator? / ??"

Mercator charts I have use the Middle latitude as in"BSB/NA=APPROACHES TO/ APPROCHES A TSEHUM HARBOU,NU=3476,RA=8170,11658,DU=254..KNP/SC=10000,GD=NAD27,PR=MERCATOR.. PP=48.68333, PI= 0.50000,SP=UNKNOWN,7 SK= 0.00.. UN=METERS,SD=LNT,DX= 1.00,DY= 1.00..CED/SE=3476,RE= 347601,ED=01/24/96..VER/1.0..OST/1..IFM/4..REF/1,246,11285,48.6375944,-123.4352000..REF/2,7740,11287,48etc.etc"
...I think it's better to have it as it relates to the Lat-versus-Long geometry in the projection...
and you see that SP is UNKNOWN,so that's an answer per Mercators.

OpenCPN likes to have the UN value, of course.It's a very useful feature where charts are a mixed bag of Fathoms and Meters like sailing betwixt USA and Canada

SD..I don't know what uses this in display but while UNKNOWN works ok and Google Earth charts don't need it...UNKNOWN is probably better than putting something in that another app wouldn't understand...I can't remember just now if MSL for MeanSeaLevel worked outside of OpenCPN,but OpenCPN didn't care.
Actually what Open CPN can display on the hover over a kap in the chart bar might be a good indicator of what it'd be nice to have in the chart header outside of what's "necessary"?

[HappySeagull]

about as simple as it gets from my pile of junk..from imgkap to see in OpenCPN.RA,DU,KNP/SC are written by imgkap but can be rewritten but Scale is needed though.It can't be UNKNOWN

! 2011 imgkap 1.1 - at 30/06/2011 from rectangletest1EX.tif..!SeagullTest..VER/2.0..CED/SE=1,RE=1,ED=30/06/2011..BSB/NA=.. NU=UNKNOWN,RA=1030,616,DU=254..KNP/SC=30064,GD=WGS 84,PR=MERCATOR,PP=0.00.. PI=UNKNOWN,SP=UNKNOWN,SK=0.0,TA=90.. UN=UNKNOWN,SD=UNKNOWN,DX=3.00,DY=3.01..REF/1,0,0,49.516666,etc

If I make scale UNKNOWN it's nogo.Likewise skew is needed as something ..

I'm still uncomfortable with the idea of charts that only work in OpenCPN.What's Open about that?

[Netsurfer]

Hamish, Thomas and HappySeagull ...

thanks for your support!
Sorry, was still busy with the basics ... .

Well, AFAIS the only thing I (the script) has to calculate is the scale value.

@Hamish
I had a look at the linked script but I do not really understand where all the values in the formula come from?

I have the following values:

• Lat/Lng values of every point of the map
• Lat/Lng for every pixel and its position

Is it possible to calculate the scale from these values?
BTW: The screen capture is meant for small areas like a harbour plan/view. Not for large maps. So it is unlikely that the latitude degrees span more than 1 degree at a maximum.

Which latitude value should be taken for the calculation?
The most northward one (if in the Northern hemisphere)? Or the most southward one? Or the one of the center of the map?

Thanks

Gunther

PS: Just to avoid any misunderstandings - Google Maps uses WGS84 and Mercator projection. So I guess nothings else to put to the header, correct?

[HappySeagull]

Hi,Netsurfer,
yes,Mercator and WGS84 work for me all the time in imgkap at the large scales I use but
I stumbled on this one day- GE isn't actually Mercator.. it's "Cylindrical Projection".

quoting from the link below..."Google Earth uses Simple Cylindrical projection for its imagery base. This is a simple map projection where the meridians and parallels are equidistant, straight lines, with the two sets crossing at right angles. This projection is also known as Lat/Lon WGS84."

link is About Datums and Projections - Google Earth Help

Does it matter?My own homemades work well enough matching waypoints but I bear this detail in mind when they don't..so GE is just a useful picture that is in the same location...and(just thinking), maybe,the best way is to centre the dangers(rocks reefs) if that's the purpose of the GEmap being made...hmmm.

[Netsurfer]

HappySeagull ...

you are totally right, but ...

I am talking about Google MAPS not Google Earth!

And the biggest advantage imho of GM over GE is that it uses WGS84 and Mercator.

Gunther