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Old 07-01-2009, 10:11   #16
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Yea, that one error is huge, on the other hand I never saw fog down there, so it's pretty obvious....... night is another thing..... I cant remember if there is a note in Charlies Charts. I still have those for Mexico and the west coast of US.
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Old 07-01-2009, 14:45   #17
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Let me know if you find anything. I still can't believe I'm the first person to find the problem with that chart. Even though I've checked it a dozen times, I keep thinking someone is going to say '...dummy ..you forgot to...'.

Anyway, it turned out when we were down there that Puerto Morelos is a great place to visit. We met an American couple who have a condo there and they showed us how much of a fun place it is, especially on the weekends. Turns out this guy invented the breathalyzer when he worked for 3M. Wonderful people.

BTW I've recorrected that chart (280201) and now looks pretty good. If anyone needs a copy PM me and I'll send you a copy or the calibration data or my updated Google Earth version.
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Old 13-12-2009, 14:11   #18
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You can't just take the Lat/Lon from your GPS, which will probably be referenced to the WGS-84, and plot it on a random chart. Instead, you need to determine the chart datum and set your GPS to it. Many Mexican charts use the NAD27-Mexico datum. I don't know whether that's the problem with using the Mexican charts, but it may be.
What is the difference between WGS-84 and NAD27? What specifically is meant when we refer to datum?
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Old 13-12-2009, 15:09   #19
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We watched the GPS position as we sailed south out of Bandaras Bay on the west coast and the plot walked right up on land for over a mile before merging with our position later down the coast. We were using DR and were right in the middle where we wanted to be. We had friends who would go into bays and harbors at night using GPS coordinates and we never could summon the nerve. This episode reinforced that nervousness.

I wasn't missing a chart plotter like I was earlier in the trip.

Jim
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Old 13-12-2009, 15:21   #20
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Get a good guidebook like the Rains Mexico Boating Guide and there are GPS waypoints for most places cruisers actually go. And you can add a few more based on these. OK once you get used to being on land.
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Old 13-12-2009, 15:35   #21
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What specifically is meant when we refer to datum?
Geodetic system - Wikipedia, the free encyclopedia
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Old 13-12-2009, 18:48   #22
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Oh well that clears everything up...not!

What I did glean from this Wikipedia entry is that a datum is an interpretation of the shape of the earth and that throughout history different datums have been used to estimate that shape. Currently we use WGS-84, the Mexicans use something a tad older. Does that make sense?
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Old 13-12-2009, 19:01   #23
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We're in Mexico right now with charts purchased from Bellingham Charts and new Garmin charts. About 2 times out of 5, our chartplotter shows us being anchored on land. The La Paz charts has everybody driving up on to the sandbar mid-harbor. The survey for Bahia de La Paz and the east cape was done in 1897. We rely on guidebooks and local knowledge. Any hint or rumor of a reef and we stand clear several miles.

We've been using Om Shanti's Sea of Cortez book and its been fantastic. The GPS waypoints have all been spot on so far.
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Old 13-12-2009, 19:26   #24
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Oh well that clears everything up...not!

What I did glean from this Wikipedia entry is that a datum is an interpretation of the shape of the earth and that throughout history different datums have been used to estimate that shape. Currently we use WGS-84, the Mexicans use something a tad older. Does that make sense?

Your on the money. Many countries still used old datums, it's not just Mexico. Another fact a GPS is still not legally concidered a "navigational instument" it is an aid and should only be used as such.
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Old 13-12-2009, 20:24   #25
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There are well known similar issues, 1/2 mile not 2, on the south coast of Newfoundland.

I have also used Google Earth images to make charts.

Not had the opportunity to use it yet, but for similar reasons.

But.... you have to be very careful or you can introduce your own errors.

PS - using Fugawi
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Old 13-12-2009, 21:20   #26
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A better than Wikipedia explanation is in:
Amazon.com: How to Read a Nautical Chart : A Complete Guide to the Symbols, Abbreviations, and Data Displayed on Nautical Charts (0639785802174): Nigel Calder: Books

I need to know about datums for my work; I've taken a look at Calder's book, and the explanation is pretty good.

Y'all should also be aware that NAD-83 datum and WGS-84 are identical, for our purposes (and even mine, when I'm at work).
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Old 13-12-2009, 21:41   #27
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In the end, I guess it is all about piloting. You can not expect ANY chart to dead on accurate. You have to use your eyes, your ears and your nose to figure out where you are. You must not approach land at night unless you are familiar with that stretch of land. You must use coastal piloting procedures when approaching the coast. Take bearings, compare it to a chart, and if in doubt use your eyes, and depth sounder. If still in doubt call someone one on the VHF for help.
Stay off the coast until you know for sure that you are clear to enter a area.
Its just common sense.
Bob
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Old 14-12-2009, 01:54   #28
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Oh well that clears everything up...not!

What I did glean from this Wikipedia entry is that a datum is an interpretation of the shape of the earth and that throughout history different datums have been used to estimate that shape. Currently we use WGS-84, the Mexicans use something a tad older. Does that make sense?
What we generally refer to as a datum is actually a geodetic system - comprising a shape, a grid, and a datum. The Earth is not a perfect sphere, but is squashed and lumpy, like a hand-made snowball. The irregular shape means that gravitational forces and therefore sea level varies around the world - the shape of the theoretical sea-level globe is called a geoid. The geoid is too mathematically complex to be used in geodesy, so an ellipsoid is used. An ellipsoid(the shape) is a mathematical model that approximates the geoid. The grid (lat and long) is superimposed on the ellipsoid and this is aligned with a datum - a benchmark or reference point on the earth. Before satellites it was only possible to use an ellipsoid that matched the local mean sea-level. NAD27 was based on the Clarke ellipsoid that was calculated by manual survey around North America and the datum was Meade's Ranch Kansas, which approximates the centre of the continent. With the advent of satellite-mapping and more powerful computers it was possible to calculate an ellipsoid that is a closer approximation of the geoid over the entire planet. WGS84 uses these newer ellipsoids (the ellipsoid has been improved since 1984) and several datums. NAD83 is equivalent to WGS84 - the difference between it and NAD27 is not a constant figure. They used different ellipsoids, so while there might be only a couple yards difference in New York, they could differ by hundreds of yards in California. Bowditch has more details: http://www.irbs.com/bowditch/pdf/chapt02.pdf
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Old 14-12-2009, 10:01   #29
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It's not just Mexico

The west coast of Vancouver Island still has some charts based on an earlier datum. Some friends got into a big discussion as to whether to trust the GPS in fog before they discovered this fact. Good thing they didn't. They came in to a port in visible conditions then discovered the datum problem at the dock with the GPS stating they were on land.

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Old 14-12-2009, 12:18   #30
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Many of you have already seen this, but for those of you that haven't: HERE is my take on chart errors. Note that it doesn't even get into the discussion of datum. There are so many ways it can be off, that datum may be the least of your worries. BUT, I really should at least add a mention of datums.

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