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Old 23-02-2009, 01:38   #1
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Bluechart error?

I find it difficult to believe that I would be the first to notice, so either I've buggered something up, or I have been unable to find it reported on the web.

On Bluechart v9.50 all the obstructions that cover that I've looked at are reported 6 times too tall north of Apple Cove point just north of Kingston in Washington USA.

For example just north of Kingston, there are two obstructions that cover that on Bluechart are reported as 5 feet and 3 feet, which matches NOAA.

Just north of Apple Cove point is one that is reported at 24 feet, NOAA shows it as 4 feet. Everything around Colvos rocks is reported 6 times too tall as well and up at Protection Island and San Juans.

Around Sidney B.C. seems to be correct again.

I've flipped the depth/elevations view from feet to meters and back, and it is consistent.

At this point I've just noticed it on the computer, tomorrow I'll go to the boat and see if it is the same on the GPS, not that I expect it to be different.

Tides are bigger than I remember here if a 24 foot rock above the datum can cover at high tide.

John
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Old 23-02-2009, 05:22   #2
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Quote:
Tides are bigger than I remember here if a 24 foot rock above the datum can cover at high tide
That would be a lot of tide even for the NW. I've found obstruction data on charts is pretty unreliable. How it gets there is based on the bathymetric study done to make the chart. That data is not updated. If it has errors, there is no established process to fix them and they don't go to the field to research obstructions nor consider them for update. Only when they do a full bathymetric update would something like that be fixed.

Charts are not that accurate when considering the detail of one obstruction. Bluecharts are Garmin private label material. They update the aids to navigation using the NOAA data but not the depth. There really is no way to assume that one set of discrepancies is more accurate than the other when it comes to charts. Close to shore the errors magnify. The primary navigation channels are the only data that gets a lot of attention.
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Old 23-02-2009, 07:02   #3
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That would be a lot of tide even for the NW. I've found obstruction data on charts is pretty unreliable. How it gets there is based on the bathymetric study done to make the chart. That data is not updated. If it has errors, there is no established process to fix them and they don't go to the field to research obstructions nor consider them for update. Only when they do a full bathymetric update would something like that be fixed.

Charts are not that accurate when considering the detail of one obstruction. Bluecharts are Garmin private label material. They update the aids to navigation using the NOAA data but not the depth. There really is no way to assume that one set of discrepancies is more accurate than the other when it comes to charts. Close to shore the errors magnify. The primary navigation channels are the only data that gets a lot of attention.
I should have put a smiley on the last statement, I was making a joke about how high the tides get. My real issue was presented earlier in my post. I was pointing out that for about 10 obstructions that cover (all that I have looked at in a quick search) on the chart between Apple Cove point and Canada, all are exactly a factor of 6 bigger than what is reported on NOAA raster charts.

John
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Old 23-02-2009, 08:23   #4
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You should report the error to Garmin. Just so they can correct the data in the future.

If things are exactly 6 times off, then I would guess somebody referenced feet data as fathoms...
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Old 23-02-2009, 08:37   #5
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As Scottie mentioned, the descrepancy is probably because bluechart gives you feet, while the US Charts are usually in fathoms (I think). I know that in Canada all the charts are in meters, but one always has to check the legend to make sure.
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Old 23-02-2009, 10:26   #6
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I talked with Garmin support, they think it is a chart error problem and have reported it to the appropriate section.


John
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Old 24-02-2009, 08:36   #7
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The Error is Unfathomable

It sounds like a logic error in the translation of the original charts to Blue Charts. If the originals were to be read in fathoms and the Blue Charts were to be listed in feet, a 6x multiplier would have been applied to all depth readings. However, as objects projecting above the datum are reported in feet--regardless of whether depth is reported in feet or fathoms--a 6x multiple applied to the listed reading would result in the error you describe.
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Old 24-02-2009, 11:00   #8
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It sounds like a logic error in the translation of the original charts to Blue Charts. If the originals were to be read in fathoms and the Blue Charts were to be listed in feet, a 6x multiplier would have been applied to all depth readings. However, as objects projecting above the datum are reported in feet--regardless of whether depth is reported in feet or fathoms--a 6x multiple applied to the listed reading would result in the error you describe.
That's what I was thinking. A friend pointed out that where the error starts is on a chart boundary, and that the NOAA chart to the south with no error has depth in feet, and the north NOAA chart with the error has depth in fathoms. He has v10.0 and the error does not show on his charts.

John
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Old 24-02-2009, 23:08   #9
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I did not upgrade beyond 8.0 so far, I guess 10.0 will be my next upgrade.
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