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Old 19-03-2012, 10:38   #1
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Datum Shift to WGS84 differences between OpenCPN ans SCII

Hi,

I have some strange behaviour difference between Seaclear II and OpenCPN according to the datum shift from older Charts to WGS84.

I used a scanned Chart from Croatia, which states that satellite obtained positions should be moved 0.28 minutes eastward to be plotted on chart.
I calibrated this chart with MapCal II entering this value as LongShift and after that I used mc2bsbh and tif2bsb to create a .KAP file of that chart.

When I use OpenCPN as Navigation tool, the shift can be seen as the eg. 1430,0' E Meridiane is shown here with the position 1429,72' E which I think is correct.

Using Seaclear II with the same chart (.Kap File) the mentioned Meridiane remains 1430,0' E.

I created two .KAP file from the same chart (in twoo different directories) and switching between those two shows the differences in OpenCPN but not in Seaclear II.

Any Ideas why?

Thanks a lot
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Old 19-03-2012, 11:48   #2
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Re: Datum Shift to WGS84 differences between OpenCPN ans SCII

I just did a similar test between SeaClear and OpenCPN, using a chart with a similar shift applied. SeaClear is not reading/using/understanding(take your pick) the DTM value in the kap-files header, it just ignores it.
Instead you are supposed to enter the correct geodetic datum(instead of WGS84), whatever that is for your Croatian chart. Both SeaClear and OpenCPN will then handle the corrections necessary, as long as they recognizes that datum!!
Thomas
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Old 20-03-2012, 00:38   #3
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Re: Datum Shift to WGS84 differences between OpenCPN ans SCII

Thanks for your answer, my problem is, that my chart references to datum "Bessel ellipsoid" or "Bessel 1841" which I cannot found in the MapCal datum list.

regards Andy
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Old 20-03-2012, 04:02   #4
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Re: Datum Shift to WGS84 differences between OpenCPN ans SCII

Now I found, that Croatia uses "Hermannshogel Datum" which is derived for "Bessel 1841" I will try that now.

Thanks again
Andy
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Old 20-03-2012, 11:57   #5
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Re: Datum Shift to WGS84 differences between OpenCPN ans SCII

...if I were you,Andy, I would shift the chart to WGS84 in MapCal Calibration and then call it WGS84 in the output.
I wouldn't bother with DTM corrections, I wouldn't call it Bessel 1841 as a datum....I would call it WGS84.
I would apply the corrections directly in MapCals Calibration Points...ie if your calmark is at the crossed lines of lat/long, I would enter the WGS84 corrected lat/long in the dialogue.
After 2 or 3 Calmarks are entered, apply the grid for a fast check of the shift and to help to do other Calmarks...
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Old 20-03-2012, 12:01   #6
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Re: Datum Shift to WGS84 differences between OpenCPN ans SCII

PS: an up-to-date "List of Lights" for your area can be a help to doublecheck the WGS84 correction.
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Old 20-03-2012, 12:45   #7
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Re: Datum Shift to WGS84 differences between OpenCPN ans SCII

Hi Seagull,

thanks for your suggestion, but my problem is, that I do not have only one chart but 76 and I already calibrated them.

So I did the following:
1) Edit the "Chartcal.dir" replacing all "GD=WGS84" to "GD=Hermannskogel Datum" and replacing all "DS=0,0" to "DS=-0.000166666666666667,-0.00466666666666667" which represents the alignement notivced in the chart itself.
2) Running the Utilities (I wrote a .CMD FIle for that)
bsb2tif chartname.kap chartname.tif
mc2bsbh chartcal.dir CHARTNAME
tif2bsb -c 127 chartname.hdr chartname.tif chartname.kap
3) rebuild the CHart databas in OpenCPN as well as in MapCal

And it works for both chartplotter Systems.

Regards and many thanks
Andy
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Old 20-03-2012, 15:48   #8
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Re: Datum Shift to WGS84 differences between OpenCPN ans SCII

mm. The reason I mentioned it was when I used SeaClear twith my old bsbs, it got them very wrong. but maybe it's not it's fault.-many of these bsbs have datum in the text-header written as NAD27 but were actually calibrated as wgs84....I can't swear to this reasoning because nobody else has reported the same error with these charts in Seaclear. and I haven't bothered with Seaclear much since.....

SeaClear apparently took the "NAD27" seriously showing a consistent error that roughly matched NAD27 but other program ( Opcn, Maxsea, Ozi, VisNav) showed me where I knew I was.
Since some of these bsbs were NAD83, and showed accurately in SeaClear, I would have to hassle with changing the gps output back and forth. (Oh, the good ol' (not-so) old days! )

It's a lot simpler to have em WGS84 in the calibration.
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