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post #1 of 14 Old 10-28-2009 Thread Starter
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Canadian and US ENC charts

Probably a stupid question (hey, I'm new to it all ), as I believe, Canadian and US charts differ in a number of ways, one of which is a different chart datum for depths. Does this also apply to ENC charts, and if so, how do GPS/plotters deal with it?

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post #2 of 14 Old 10-28-2009
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The plotters from any of the manufacturers primarily use variously digitized versions of that government's official charts. They do not want the liability of producing their own. So whether paper or electronic, you get the same picture (except for colour vs monochrome depending on the display)

Which is fine in Canada and the US, as long as you understand what each chart system is telling you. In Mexico, however, there's been no updating for literally hundreds of years and the charts (paper and electronic) are notoriously bad... everytime we anchored within a 1/2 mile of the beach the plotter showed us 1/2 mile inland!. Sure wouldn't want to make a night time landfall there without radar!

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post #3 of 14 Old 10-28-2009 Thread Starter
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Hmm, so if (for example) I'm cruising off around the San Juan, Haro strait type areas and I go between the US and Canadian charts, I have to try and figure out which chart is which and therefore what the depths are telling me (ENC, not paper, it's pretty obvious with paper ones!)?

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post #4 of 14 Old 10-28-2009
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first it would depend on *which* ENC charts you are using. if these are proprietary charts from someone like Navionics or Garmin, they have converted everything to the same datum.

If you use government-issued charts such as S57 NOAA charts or S63 charts from Canadian Hydrographic Agency, then they theoretically can differ. Datum is encoded in the chart information record, though not all tools or viewers will show it. NOAA charts, in particular, use "mean lower low water".
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post #5 of 14 Old 10-28-2009
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Actually, this brings up excellent point - PolarView does not expose this information to a user. Just added this feature to my TODO list for the next release.
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Hmm, so it's perhaps worth buying the proprietary kit after all given that you could easily go from a MLLW datum chart to a LAT datum without really noticing.

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post #7 of 14 Old 10-28-2009
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Quote:
Originally Posted by PaulinVictoria View Post
Hmm, so it's perhaps worth buying the proprietary kit after all given that you could easily go from a MLLW datum chart to a LAT datum without really noticing.
That's one consideration, in particular if you switch charts a lot.

That said, around here the difference in numbers if they were based on a different datum is negligible. You are much more likely to encounter more significant differences based on which charts you use and how good a data source they are built from.

If a bad chart is off by a wild number (not particularly uncommon) 10% difference due to datum is pretty much inconsequential.

Which charts are best for which location is something that you really only find out from experience. I know of at least one proprietary chart source that tends to be quite good and another that tends to be so far off it ain't funny (you will be sailing on shore quite a bit based on those charts ). No names - this may be my particular experience and yours may vary.
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post #8 of 14 Old 10-28-2009 Thread Starter
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Well, most of my sailing for a few years at least is going to be around the south/east coast of Vancouver Island so there's a fair amount of switching between NOAA and CHS. The NOAA charts cover most of it but obviously they don't have any detailed charts on the harbours etc on the Canadian side.
Also bugs me that the NOAA allow access to their charts for free and the CHS want to charge me $600 per set!

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post #9 of 14 Old 10-28-2009
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NOAA is unique and their decision to provide acess to (taxpayer paid) charts for free is something to be applauded.

Virtually every other country and hydrographic authority uses S63 format for ENC charts and charges for access.
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Yep, "taxpayer paid" alright, twice if I buy them!

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