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post #1 of 12 Old 01-19-2008 Thread Starter
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Inaccurate Charts Mexican West Coast

Around Banderas Bay (Puerto Vallarta), on the west coast of Mexico, the charts and the GPS disagree by up to a mile sometimes. Apparently the charts were drawn long ago and have never been updated. Does anybody know the story, the history, of who drew these charts and of where the mistakes crept into them?
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post #2 of 12 Old 01-19-2008
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Not an unusual occurrence by any means!

We have regularly sailed in areas where either the charts are based on surveys last done in the early 1800's or have never been fully surveyed.Good fun when your GPS actually shows you anchored in the middle of an island!

Nothing beats good old eyeball navigation!

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post #3 of 12 Old 01-19-2008
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Lots of commercially and militarily unimportant areas are less that recently updated. The governments pay for most of the updating, and without commercial or military reasons to do so... they don't. You really should check the chart information of any chart you use to see how old the chart really is. Anything more than 20 years old is going to be less than stellar in accuracy.

When your measure of accuracy is limited to the tools of celestial navigation, which may well have been the case in many of the more remote areas, they are as accurate as was possible for the time... but the charts may not have any real congruence with the reality of GPS-based navigation.

That is why GPS is only one of a dozen or more tools you're supposed to be using... not the only tool. When you're doing dead reckoning and using coastal landmarks to position yourself... you will at least know where you are relative to any immediate land... which GPS can't tell you in areas like this.

The icon of your boat's position on a chartplotter has very little to do with where your boat is in reality. It is an estimated position of your boat in an interpretation of the real world as translated by various explorers and cartographers that happens to coincide with your boat's actual position in the real world some of the time. While your actual position is very accurate most of the time—your relationship to land in the real world and that of your icon to land in the GPS chart are not always going to mesh.

Using a GPS on electronic charts from the 1800s is kind of like measuring a board with a micrometer, marking with a can of spray paint and then cutting using a jackhammer—with much the same results.
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Around Banderas Bay (Puerto Vallarta), on the west coast of Mexico, the charts and the GPS disagree by up to a mile sometimes. Apparently the charts were drawn long ago and have never been updated. Does anybody know the story, the history, of who drew these charts and of where the mistakes crept into them?

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post #4 of 12 Old 01-19-2008
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I believe that "Charlie's Charts" are the standard for "eyeball" Navigation in Mexico rather than government charts.

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post #5 of 12 Old 01-19-2008
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Luckily the Mexico mapping issues are not much of a bother - but in other regions of the world the lack of mapping is used for piracy. Old world technique to move bouys and other navigational aides - let vessel run aground and then raid...

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post #6 of 12 Old 01-19-2008
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I heard that the Caribbean was notorious for that in the days of yore.

And today in some areas in and around Indo and the Phillies............
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And people say I'm paranoid for having a 15" draft.

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Luckily the Mexico mapping issues are not much of a bother - but in other regions of the world the lack of mapping is used for piracy. Old world technique to move bouys and other navigational aides - let vessel run aground and then raid...

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And people say I'm paranoid for having a 15" draft.

Nah I think you just like being paranoid

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Jody-

It isn't paranoia if they're actually out to get you, it is a survival mechanism. Now, where'd I put the tinfoil hats???
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Nah I think you just like being paranoid

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Jody-

It isn't paranoia if they're actually out to get you, it is a survival mechanism. Now, where'd I put the tinfoil hats???
See and here I was thinking you say something like "now where did I put my favorite [insert bev here]"

-- Jody

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