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Old 31-03-2018, 08:49   #1
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self calibrated charts not quiling correctly

hi, I'm new to openCPN and to electronic charts and so forth. Old school with sextant, tables, printed charts...that is my world, but I'm a linux expert so I've decided to give openCPN a try on land....just to see how it works. Since Uruguay has no bsb charts available for free I got some old jpg scans I had. Converted them following the instructions on your website. And here comes my problem... they are on different scales (1:20000, 1:10000, 1:4000, 1:5000) some of them when installed on openCPN quilt wonderfully in another, others won't. Some are some millimeters higher or lower on the chart, others quite off the place where they should be... my friend google has been of no help as to how to solve this, and my linux expertise can't bring me through this. Do you have any ideas of how to solve such a problem? Otherwise, great software this openCPN... a pity I'm on land and have no connection to navigational instruments as to really see its full potential.
Alejandro
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Old 31-03-2018, 09:08   #2
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Re: self calibrated charts not quiling correctly

How exactly did you convert them? If you scanned paper charts, you likely need many reference points to make them accurate.
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Old 31-03-2018, 09:17   #3
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Re: self calibrated charts not quiling correctly

I did not scan them myself. I downloaded them years ago from the website of HidrografŪa. No the offer only some pdfs with low resolution, but those jpgs I had have a very nice resolution.
I used MapCal II and all the linux tools and scripts as explained in https://opencpn.org/wiki/dokuwiki/do...on_using_linux
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Old 31-03-2018, 09:53   #4
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Re: self calibrated charts not quiling correctly

Just learned from another thread that you may need to make sure that OpenGL is turned on. It's under the tools menu, in the section on advaved graphics. Hope I got that right, you can search for the thread I started this past week.
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Old 31-03-2018, 10:14   #5
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Re: self calibrated charts not quiling correctly

Quote:
Originally Posted by SFS View Post
Just learned from another thread that you may need to make sure that OpenGL is turned on. It's under the tools menu, in the section on advaved graphics. Hope I got that right, you can search for the thread I started this past week.
I have OpenGL activated. That can not be the problem. Thanks anyways.
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Old 31-03-2018, 10:23   #6
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Re: self calibrated charts not quiling correctly

I'll paste 3 screen captures so that it is clear what is happening.
The one is a general view of Montevideo, the other one, the one making problems, is the Puerto del Buceo (where the Yacht Club is), and the third is the original jpg.
That small Puerto del Buceo chart is a square, and I'm pretty sure I got it correctly calibrated, but it shows as a rectangle in openCPN and slightly off position.

Take a look:
Attached Thumbnails
Click image for larger version

Name:	carta_Montevideo.jpg
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ID:	167411   Click image for larger version

Name:	cuarteron_buceo.jpg
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ID:	167412  

Click image for larger version

Name:	ROU24.jpg
Views:	47
Size:	447.9 KB
ID:	167413  
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Old 31-03-2018, 10:58   #7
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Re: self calibrated charts not quiling correctly

candombero...

1. The chart appears to be skewed, according to OCPN. But the original image looks North-up to me. So some trouble with calibration.

2. Please post the header of the offending chart, including the reference points. We can take a look together.

Thanks
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Old 31-03-2018, 11:14   #8
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Re: self calibrated charts not quiling correctly

Quote:
I'm pretty sure I got it correctly calibrated
Nope. In the case shown, the latitudes at least are plainly wrong. Double-check it.
Also, do not calibrate a homemade chart as anything except wgs84.
Is the original chart's datum compatible with WGS 84?
Old charts and maps can be drawn with some pretty eclectic local datums and will need shifting to suit WGS84.

And, there's more possibilities beyond just getting the lat long correct- simplest-you need check that the original was DRAWN as a Mercator, If not, the image must be reshaped to suit the proportions of mercator's latitude vs longitude. This can be done pretty well in many image editors using the proportion of the central latitude's cosin. There's even a way to calibrate an image by interpolation from a single "known" pixel but that's a bit involved...."Transverse mercator"..you won't likely need there.
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