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post #1 of 19 Old 06-14-2010 Thread Starter
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Chart Plotter Accuracy - Be Careful

My neighbor just came back from a cruise in NC waters that took him up the Alligator River. He told me that he met the crew of a Catalina 42 that was using a chartplotter to navigate and ran aground in the river because the electronic chart failed to show a shoal that was clearly shown on the paper charts. After losing several hours trying to kedge off, wherein the captain badly hurt his hand on the anchor chain, the boat was towed off the shoal at the cost of $1,800. My neighbor, who also has a electronic chart plotter, confirmed that the shoal was not shown on his electronic charts either (don't know if the two plotters were the same make, nor which makes). He confirmed that the paper charts showed the shoal clearly. Just a note of caution, the computer is not always correct.

Last edited by NCC320; 06-14-2010 at 11:14 AM.
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post #2 of 19 Old 06-14-2010
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Why would anyone rely on a chartplotter in these waters? The ICW marks are very good in the river. And if you're venturing out of the channel you really have to watch the depth sounder carefully. No chart can be relied upon. The depths change frequently and the waters are certainly not surveyed annually.

If you really want to see inaccurate electronic charts try the Bahamas and Navionics charts. Their charts will take you over the tops of islands (if you're dumb enough to rely on them).

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post #3 of 19 Old 06-14-2010
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Total reliance on a chartplotter is as big a mistake as total reliance on a printed chart.

It is not correct to imply that chartplotters can be inaccurate and charts are not. It may be true in this case but leaving out variables such as the age of the chart vs the age of the chartplotter databases makes any comparison useless. I've seen inaccuracies in both.

I'd rather take information from as many sources as reasonable, including eyeballs, depth sounder, radar etc., and combine them to make course decisions.
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post #4 of 19 Old 06-14-2010
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Its EASY to get in trouble with a chartplotter.
Especially when using HIGH mignifications ... called 'resolution error' - using inappropriate 'magnifications' based on old survey data that was established with sextants, theodolites, 'chains', etc.
An example of a 'resolution error' would be driving an automobile while looking through a very highpower binocular.

Plus, moving/shifiting bottoms, newly forming shoals, recent changes of the 'marks', etc. simply arent 'automatically' updated on chartplotters. I do a lot of long distance cruising and have the 'gut-feel' that the best chartplotter with the latest software is only accurate about 90% of the time (and when operating on 'medium' magnifications values.)
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post #5 of 19 Old 06-14-2010
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We entered Spanish Waters in Curacao recently and the chart plotter showed us to be in the Hyatt hotel swimming pool...

The Bahamas have not been surveyed since the Brits left 30-40 yrs ago so why would anyone think the charts might be accurate.

Rich is absolutely right...higher magnifications just increase the delusional nature of believing chart plotters.

# 1 eyeball at all times in restricted waters and preferably with a high sun over your shoulder!!!
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post #6 of 19 Old 06-14-2010
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In some ways the raster charts are even better than the vector charts, at least with the raster charts when you magnify them too much they get blocky, but if you magnify those vector charts they have that nice clean edge no matter how big you make them.

I think part of the danger with chart plotters is just a kind of techno-faith people seem to have, they put a certain amount of belief in it just because it is an electronic gadget.

What are you pretending not to know ?

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post #7 of 19 Old 06-14-2010
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Quote:
Originally Posted by Yorksailor View Post
The Bahamas have not been surveyed since the Brits left 30-40 yrs ago so why would anyone think the charts might be accurate.
Wrong! The Bahamas have been surveyed extensively in the past few years and the Explorer Charts are very accurate. However you still need to use your eyes and proper seamanship! Prior to the Explorer Charts some of the Admiralty charts were from the eighteen hundreds.

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post #8 of 19 Old 06-14-2010
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I'd have to agree with Vasco. The Explorer charts are very good and cover the Bahamas quite well.

I'd point out that in many areas, shoaling is a problem and that only recent local knowledge is really helpful. The Alligator River area is one that has a lot of shoaling and the charts are not dependable.

Sailingdog

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post #9 of 19 Old 06-14-2010
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I agree on Explorer charts, I use them, but what legitimate governmental charting organization charted the Bahamas and produced comprehensive charts.

However, even though I use Explore Charts I would not trust them to do night entries...In the Bahamas it is 'Good visibility eyeball navigation' only

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post #10 of 19 Old 06-14-2010
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Quote:
Originally Posted by Yorksailor View Post
I agree on Explorer charts, I use them, but what legitimate governmental charting organization charted the Bahamas and produced comprehensive charts?
More and more private enterprise is taking the place of government hydrographic services, especially in smaller countries that have few or no resources. That charts are produced by private companies does not make them any less "legitimate". The test is whether they are accurate or not. Some of the best charts in the Bahamas and the Caribbean are from private, non-governmental organizations. All the sailors in the Bahamas that I have met would far prefer the private Explorer Charts to those put out by the British Admiralty, a "legitimate governmental charting organization" as you put it.

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