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Old 01-14-2021, 01:09 PM   #1
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Nautical Charts horizontal datum

On our trip last summer to the BC mid-coast, I was challenged a few times with out-of-position charts. Some of the charts for the BC coast are still using the NAD27 horizontal datum, which varies quite a bit from the actual GPS position.



My Navionics charts are ten years old, and I have been trying to find out if updating to the latest edition of electronic charts will correct the position errors. I have not been able to find any detailed information on the Canadian Government website as to whether the horizontal datum on older charts has been updated in the electronic versions.


Wondering if anyone knows how to find out about the horizontal datum on the new ENC vector charts. Or if anyone has hands-on experience with a recent edition of Navionics to know whether the charts are accurate.



As an example, the paper chart # 3744 Queen Charlotte Sound, is listed as using NAD27. My question is - If I buy the electronic version CA270084, is it still going to be NAD27, or will it have been adjusted to NAD83?
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Old 01-14-2021, 02:31 PM   #2
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Not Navionics but...

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Above is from Google maps and the 'marker' is where the data for the lighthouse is on Pine Island.

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Next is from apple maps and I think I can see the lighthouse better.

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Last is from my TZ iBoat app with up to date enc info with a google maps overlay.

The outlines of the island on the enc also don't match the satellite map overlay.

To me the data seems to tell me that the underlying enc data probably still needs an offset to match the real world.

Be careful in the fog! Verify with radar.
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Old 01-14-2021, 05:16 PM   #3
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Over the years I've seen quite a few horizontal position errors in the less heavily traveled by commercial traffic areas of BC. I've put it down to the age of original surveys. Canada has an enormous coastline. More than 10 times the US and a much smaller hydrgraphic budget. Understandable they have not yet corrected all areas.

As said further up thread. Verify your position with radar. But not just in the fog.
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Old 01-14-2021, 05:25 PM   #4
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How about navigable waterways....


USA -4th
Canada 77th


https://en.wikipedia.org/wiki/List_o...terways_length
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Old 01-14-2021, 05:43 PM   #5
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GPS isn't perfect, but 100x better than what came before. When I need a really accurate position or a chart of a difficult or dangerous anchorage, I use sextant angles and replot on a zoomed in chart. GPS is an average, verified by ground stations. It's not absolute.
Pic is my boat, 83', with the track left on for a couple weeks. Boat is tied to a dock, dock position is controlled by pilings and the pilings were driven to bedrock. The position varies over 300', but the vast majority of positions are probably within 20'. Apparently the boat picked up and flew off to the south for 4 fixes, then flew back. I live aboard and don't remember the flight. Too much Scotch, maybe.
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Old 01-14-2021, 07:40 PM   #6
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Quote:
Originally Posted by psneeld View Post
How about navigable waterways....


USA -4th
Canada 77th


https://en.wikipedia.org/wiki/List_o...terways_length
Where did that come from?

"77 Canada 664 2008"

I am sure I have been on at least 664 km and I haven't been very many places.

This about the St. Lawrence "From the east, access to the Great Lakes is through the St. Lawrence Seaway from Montreal, on the St. Lawrence River. By the time a ship reaches Duluth, at the head of Lake Superior, it has risen 600 feet, the height of a 60-storey skyscraper, through a series of 15 locks, and traveled 2,342 miles from the Atlantic Ocean – more than the distance across the Atlantic itself." from this website:https://greatlakes-seaway.com/en/rec...ng-the-seaway/

Or maybe they thought that was all on the US side?
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Old 01-15-2021, 02:28 PM   #7
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Personally, I don't think this is about "whose is bigger"?

What has been explained is a valid reason as to why Canada may not be able to conduct hydrographic surveys as frequently as the US. Couple the size of the problem with the greatly reduced resources (US has about 10X Canada's population), (maybe even "political choices" as well - priorities) and it makes sense.
What I got out of this, is the need for great care, and to verify by multiple sources (visual, radar, etc.). A good reminder.
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Old 01-15-2021, 02:57 PM   #8
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When I got my new plotter, it came with a Navionics chip. Comparing it to my iPad with a Navionics subscription, the plotter was at least 1 to 200 meters wrong, verified by passing known landmarks and holding the iPad and plotter side by side. The iPad was bang on, not the plotter. I replaced the plotter chip with a Cmaps chip and now the plotter agrees with the iPad. There was something wrong with the plotter chip - another gotcha to look out for.
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