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Old 04-10-2013, 04:55   #1
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CM93 chart offset

During cruising in Davis Strait, we found some charts do not agree with the gps position.
According the legenda of the official canadian paper charts that we also had onboard, positions might be off up to 4 miles.
Cm93 was out about 3.4 miles.
When I try to correct this the chart will only "shift" about 1 mile max.
Is there a "build in" max. correction distance?
Dirk
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Old 04-10-2013, 10:49   #2
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Re: CM93 chart offset

Quote:
Originally Posted by rooiedirk View Post
During cruising in Davis Strait, we found some charts do not agree with the gps position.
According the legenda of the official canadian paper charts that we also had onboard, positions might be off up to 4 miles.
Cm93 was out about 3.4 miles.
When I try to correct this the chart will only "shift" about 1 mile max.
Is there a "build in" max. correction distance?
Dirk
What I can see is that the max corrections is set to 10.000 m for the spin-controls both for x & y. That should cover your case, unless there are more build in restrictions.
At what lat & long are you trying your correction.

Thomas
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Old 04-10-2013, 11:01   #3
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Re: CM93 chart offset

I tried offsets of (10.000 , 10.000) on two places in Davis Strait.
Bot resulted in a slightly more than 3 miles shift in position.
Not quite what I had expected.

Thomas
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Old 04-10-2013, 11:52   #4
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Re: CM93 chart offset

Hoi,
position is 69 31 N 067 24 W (Artic harbour)
I Added two screenshots.
the firstone showing our track int to the bay without offset setting.
a clear offset of abt. 3.4 NM
the second with a offset (x) set to 6297mtr (is 3.4 NM)
shift of the chart is only abt 1 NM
Dirk
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Old 04-10-2013, 11:58   #5
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Re: CM93 chart offset

Further testing shows that what offsets off (10,000 m,10,000 m) means is latitude dependent. On the equator the result is as expected about 7.6 miles in a NE direction. This distance decreases with latitude. In the Svalbard area (79 N) it's down to 1,5 miles.
This looks like a bug!

Thomas
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Old 04-10-2013, 19:14   #6
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Re: CM93 chart offset

I went on the reef last night as I attempted a 10 meter wide pass in the dark which is only 2 meters deep. Obviously this is beyond gps capabilities.. but it would help to be closer.

The cm93 chart is off by over 1km! I used sculling oar to get through ok after probing the area with the kayak to find the pass, and no problems, but I notice all the cm93 charts in vanuatu are wrong by a large offset.

Rather than everyone enter the offset, can we have some server and database and maybe a special plugin to share corrections? Also the plugin could allow for users to share tracks, and notes as well as additional details like coral heads etc... What do you think?
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Old 05-10-2013, 09:27   #7
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Re: CM93 chart offset

Sean, A good "Big Idea", I am in favor, how? I won't ask ..entering unknown at night. It's lucky you have a solid hull, but be careful.

Thanks for the updates to Climatology and Weather_routing on github, will try compiling today.
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