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Old 09-08-2011, 04:05   #1
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Problems with Rotated River - Maps

Hi guys, interesting work you're doing here!

I've been using the free navigation program from N.V. Verlag so far, but I'm looking for alternatives that support AIS, logging, instruments etc.

However - I see what appears to be a bug concerning odd-angle rotated maps, like maps of rivers. My example is Göta Elv, maps bought from "svenska solsidan/solkraft", but I also see the problem on other maps covering the same river/channel. The maps load fine in the N.V. Verlag program.
A screenshot is attached.
Best Regards, Eskil

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Old 09-08-2011, 06:12   #2
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Re: Problems with rotated river-maps

Eskil

Without testing the charts you have one can't be sure.
I have access to Solteknik charts for the area from 2001 though.
The same problem, but I would hesitate to blame OpenCPN. I have never seen such crap charts ever before.
The bsb header is all wrong. The charts are skewed but SK=0.
The projection is stated as "SWEDISH GRID".
Garbage in garbage out.

If you open your charts in a text editor, what value do you find for SK?
It should be somewhere in the beginning around line 7.

Thomas
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Old 09-08-2011, 06:39   #3
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Re: Problems with rotated river-maps

The long and short of it is, someone need to update those charts with google earth.

I have found many inconsistancies with charts in several areas that are immediately apparent with a satelite picture. A lat long marked satelite picture would be ideal for navigation, but unfortunately marine charts just haven't caught up to that yet. I can put two charts on the ICW, (a manmade canal), that are off by several inches on the chart, (equate to 100ft). I have also cruised up a river according to the chart, (GPS) apparently on dry land 100 yards away from the river.

The problem is these charts were made before GPS, and haven't been updated. I know this doesn't help your problem, just realize river charts are more likely to be older and less accurate than frequntly traveled coastal areas.
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Old 09-08-2011, 12:46   #4
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Re: Problems with Rotated River - Maps

Hi Thomas

The header for one of the maps look like this:
!Copyright Solteknik HB 2011 All right reserved
BSB/NA=1352d Gota alv Trollhätte kanal KG1504:Eskil Joergensen
NU=1352d Gota alv Trollhätte kanal,RA=2124,8478,DU=300
KNP/SC=12500,GD=WGS84
PR=TRANSVERSE MERCATOR,PP=15.000000,PI=UNKNOWN
P1=0.000000,P2=0.999600,P3=0.000000,P4=0.000000
SP=UNKNOWN,SK=0,UN=Meter,SD=Medelvattenstånd,DX=1. 06,DY=1.06
CED/SE=1,RE=1,ED=03/31/11
VER/1.1
OST/1
REF/1,0,0,57.992994433,12.126126906
REF/2,1062,0,57.987094801,12.141529135
REF/3,2124,0,57.981193254,12.156926325
REF/4,0,4239,57.960274362,12.081897976
REF/5,1062,4239,57.954380002,12.097293096
REF/6,2124,4239,57.948483726,12.112683190
REF/7,0,8478,57.927539501,12.037749155
REF/8,1062,8478,57.921650402,12.053137149
REF/9,2124,8478,57.915759389,12.068520132
PLY/1,57.991440747,12.126512807
PLY/2,57.980981693,12.153534807
PLY/3,57.917396346,12.067761968
PLY/4,57.927697056,12.040794457
DTM/0.000000,0.000000
IFM/4
RGB/1,31,32,38
RGB/2,0,255,0
RGB/3,43,46,49
RGB/4,64,64,255
RGB/5,106,62,93
RGB/6,255,0,0
RGB/7,198,189,156
RGB/8,239,231,189
RGB/9,129,159,255
RGB/10,255,239,132
RGB/11,193,213,255
RGB/12,255,255,255

So SK=0. But it works in other programs??
It's quite funny/worrying how the map moves in the frame!!?, when I use the arrows and move the map.
Eskil
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Old 09-08-2011, 13:52   #5
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Re: Problems with Rotated River - Maps

Eskil

OpenCPN handles these kind of charts as long as they follow the bsb convention and states the skew angle with SK. If you want to see it in action, download some of the strip charts for the US ICW.

It should be possible to calculate SK internally from the reference points (REF) however. This is probably what the other chart plotters do. I will enter this as a feature request in the OpenCPN tracker. I guess nobody was aware of the problem (?), even though "Göta Älv" charts has been discussed before.

Thomas
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Old 09-08-2011, 14:32   #6
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Re: Problems with Rotated River - Maps

There is no need to wait for this feature to appear in OpenCPN, as the charts are easily fixed.
1 Measure the angle.
2 Change the SK value with a good text editor (notepad is no good).
3 Download "libbsb" from opencpn.org and run "bsbfix" on the chart file.
4 Go to the chart tab and tick "Force Full Database rebuild" + OK

Thomas
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Old 09-08-2011, 14:49   #7
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Re: Problems with Rotated River - Maps

Amazing help! thanks!
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Old 09-08-2011, 15:36   #8
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Re: Problems with Rotated River - Maps

Welcome Aboard Eskil
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Old 10-08-2011, 01:04   #9
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Re: Problems with Rotated River - Maps

Eskil, Cagney et al...

This was discussed in the charts thread a year or so ago...
Read
Charts
and a few pages onwards...

I talked to some people back then and as far as I understood SK is not mandatory in BSB so it can (and should be) possible to omit for the person doing the georeffing of a chart. The reason, it was said, was that SK should be possible to calculate from the georef points.
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