Known Electronic Chart Errors in ME or MD - Page 2 - SailNet Community
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post #11 of 13 Old 05-19-2010 Thread Starter
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Quote:
Originally Posted by wwilson View Post
LinekinBayCD,

I smacked into a rock that may have been either incorrectly charted or uncharted. Have a look at the NOAA chart below (#13324).

We were poking very slowly around the northern end of Dyer Island looking for anchorage in Dyer's Northeast Cove. We had rounded G"3" in the Dyer Narrows and were in the proximity of the Green/Red mark off the west side of Strout Island.

Dyer I. Northeast Cove


Our depth sounder was in agreement with the chart plotters that we were in 25-foot depths at 44˚31.037'N x 67˚48.389'W. Those coordinates, from my Garmin 3205 chart plotter using its built in BlueChart cartography. When I plot those coordinates on electronic charts today they appear right up against the western shore of Strout I. - just across from the Green/Red mark. That was certainly not the case! Our DR navigation had us mid channel. The nav station's Raymarine chart plotter using C-Map cartography was in agreement with the Garmin, they are independent systems.

The chart symbol just NW of the Green/Red is, "dangerous underwater rock of uncertain depth" according to NOAA Chart #1 definition. Given its surrounding depth of 19 - 26 foot spot soundings - I believe we may have hit that rock. The distance from the coordinates that I saved to the outer edge of the rock is 0.11-nm or about 668-ft. NOAA's stated cartography accuracy for a 1:40,000 scale chart is ~40-meters or 131-feet. It is possible, maybe even likely that the charted rock was not what we hit. I know that the tide was flooding - because it lifted us off the rock within minutes. It would have been pushing us in the general direction of Shag Island.

NOAA Chart #13324


By the time of impact we were well into "VFR", steering from mark to mark using visual DR and proceeding at dead slow through a sea of lobster trap floats on a clear sunny afternoon with unlimited visability.

Here is our Maine Memento (now repaired) which our Maine friends tell us initiates us "officially" into the Cruising Club of Maine.

I plotted the same coordinates on my Mapsource software which put the mark right on the west side of Stroudt I like you. Where did the chartplotter have you in relation to the Stroudt. If you had your tracking turned on and still have the track log in either of the chartplotters you should be able to figure out if there are any inconsistencies between your mark and the track log.

Last edited by LinekinBayCD; 02-16-2011 at 09:16 AM.
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post #12 of 13 Old 02-16-2011 Thread Starter
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Thought I resurrect this thread given some other recent threads touching on the accuracy of GPS. Although there are a lot of frequent admonishments about the dangers of relying too much on our electronic toys there really has not been much in the way of specific errors with charts pointed out.

I'm probably starting to push the 20 year mark in terms of GPS use and maybe 15 with chartplotters (the first being a Garmin GPSMap 12, a pretty basic unit). In all those years, most in Maine and some on the Chesapeake I haven't come across any serious errors with either the position given by the GPS unit or chart info loaded onto a unit. Maybe some changes with some ATN's being moved but that is about it.
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post #13 of 13 Old 02-16-2011
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I have a problem with wwilson's story about Dyer Island and the Strout Island Narrows, and that is that he is apparently taking the Lat/Lon coordinates reported by his GPS as "the truth" and thus appears to be suggesting that his electronic charts are out. But has he considered the possiblity that the lat/lon coordinates reported by his GPS were not actually correct, or the possibility that there is a datum difference between his Garmin chart -- or his GPS coordinates -- and the BSB raster chart he appears to be plotting on now?

Some GPS's and chartplotters, I think particularly those that use WAAS to improve their accuracy/precision, can show an estimate of their level of precision by drawing a circle around your boat on their plot. If you have that feature turned on, and study it from time to time, you may be surprised at how often and how much it changes.

Peterson 34 GREYHAWK, West Boothbay Harbor, Maine

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Last edited by catamount; 02-16-2011 at 10:43 AM.
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