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Old 23-05-2013, 12:00   #1
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Join Date: May 2013
Location: UK - South East
Boat: Beneteau Oceanis 393
Posts: 26
OpenCPN Tide data errors

Hi,
I'm a new user of OpenCPN which I find is great.
Being in the UK I decided to try to improve the tide data coverage for the UK and was able to add secondary port data with some success from WXtide IDX files (more on this later).

What I found is that for some secondary ports, particularly drying ports, where the low water data is unknown (such as Rye on UK south coast), data for low water offsets is often entered in the IDX file as 111.1.
This seems to cause erratic tide graphs - silly HW figures - clearly in error.

OK, so I went back to the OpenCPN installed IDX file (usr/share/opencpn/tcdata/HARMONIC.IDX) and had a look to see if any similar data is in the installed file.
I found 3 instances.
64.1333N 45.0W Minas Basin Windsor.
63.3333N 45.3667W Minas Basin Truro.
64.5833N 45.8667W Peticodiak River Hopewell.

Looking at the tidal curves in OpenCPN, the first of these doesn't seem to look too bad at first glance, the other two clearly show the problem.

Hope this makes sense.


The more info I promised later.
In adding my additional data I found that simply importing a new IDX file is not enough, there also needs to be a harmonic data file of the same name with no extension in the same directory as the IDX file.
I also found that the offset heights of data in WXtide files is in feet, but this shows as meters in OpenCPN (which uses meter offsets).

Brian
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Old 23-05-2013, 12:19   #2
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Re: OpenCPN Tide data errors

Screenshot of problem
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Surely nowhere has a high water of 112.19m !
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Old 24-05-2013, 14:15   #3
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Lightbulb Re: OpenCPN Tide data read errors

Whoops, made a boo boo.

Quote:
=
64.1333N 45.0W Minas Basin Windsor.
63.3333N 45.3667W Minas Basin Truro.
64.5833N 45.8667W Peticodiak River Hopewell.
Got the Lat & Long's mixed up.
Partly because in the IDX file the Long's are given before the Lat's, whereas in normal position references Lat's come first - well that's my excuse.

The correct positions are:-
45.0N 64.1333W Minas Basin Windsor.
45.3667N 63.3333W Minas Basin Truro.
45.8667N 64.5833W Peticodiak River Hopewell.

The first (Windsor) displays correctly, I've subsequently found that this is because it's also defined as a primary tide in the IDX file aswell as a secondary (The secondary subsequently gets ignored).
The other 2 have the 111.1 offset applied to the primary "Saint John, New Brunswick".

Whilst it would be relatively easy to modify the IDX file to replace the 111.1 offset with 0, this would be incompatible with any other IDX files.
The correct solution would be to detect a high offset value (say over 50 - or whatever Wxtide uses) to determine an "unknown" offset (Wxtide displays <unk> for values of 111.1 for height offset and for time value differences of 1111.

Sorry for any confusion this may have caused.

Brian
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