Known Electronic Chart Errors in ME or MD
Not sure where this is more appropriate for Gear and Maintenance or Seamanship. Might be appropriate for a sticky for a place were we could post know chart errors for different manufaturers or providers and locations.
I have fairly extensive experience with Garmin charts for both the coast of Maine and the Chesapeake (15+ years). I have not come across any areas where I have found errors on the charts or areas where I did not get a good GPS signal. I'd be interested to hear from anyone who has found errors either in Maine or on the Chesapeake.
Roger Long recently wrote an article about a ledge in Whitehead passage (Maine's Casco Bay) that appears on paper charts, but is missing from many vector charts used in gps chart plotters. You can find it in the 2010 mid-winter issue of Point's East Magazine, (p. 26) which you can access from Points East archive.
The location is shown below in this snippet from NOAA 13290 using their online chart viewer.
His article has screenshots of gps displays which are missing the ledge around the daymarker. A fireboat belonging to the city of Portland suffered $90,000 in damage while returning from a rescue in this area in Nov. 2009. (see related news article)
While you are there, check out the cover photo on that Points East Midwinter issue. That's one on my photos!
Here is a photo of the same area (zoomed in) I took off the screen of my brand new [purchased last week of March 2010] GARMIN GPSMAP 546, This shows the difference between the vector chart, and the above raster image.
This one has the 18 ft contour running through the daymarker, which is not in agreement with the raster image in the above post showing the 6 foot contour outside of the daymarker.
If you go back to the Points East article, Roger Long says the daymarker was moved a few yards east-northeast in 2009, but not enough to put the daymarker outside the 6 ft contour.
Bottom line: Don't blindly follow the glowing cursor.
i have been though white head pass a thosand times in all conditions including 0 visability,anybody that tries to cut inside that marker is a gonna ,there is no excuse for the fireboat to be in there!!!!even the 10 yearold budding lobsterman has seen the weeds on that ledge drying out..
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.
Downright scary! Could have been charting error. The NOAA chart, BTW, shows the position you cited as being right up against the shoreline obstructions E of the red/green mark, just as do your two chartplotters.
IMHO, chartplotters and GPS are not the proper aids to navigation for places as tight as this. The Mark I eyeball and, especially, radar are the tools of choice for this situation.
Glad you got off with nothing more than you did AND with an initiation to the Cruising Club of Maine :-)
Thank you Bill,
You are a gentleman, as always. A crueler man would have felt compelled to explain some aspect of my stupidity to me. No need, as I repaired that keel the following spring - the thought occurred!
Townsend Gut has 3 "glacial erractics" that are unmarked. One of the people who lives along the Gut has sent numerous letters to the coast guard trying to make them rechart the area but has not gotten anywhere. Apparently several boats hit them each year. They are just outside of the 1' contour on the mainland side, boothbay end. I don't know exactly where they are but you should hug the other shore in this area. Sorry I couldn't give a better idea.
I also know that there are some chart issues in Hurricane sound. There is at least one sandbar on the western side near a few marked rocks that is uncharted. I will try to pull out the chart when I next get a chance and be more exact.
Thanks for the heads up on Townsen Gut. I go through there all the time. Just launched a week ago Monday. Will likely be through there Memorial Day weekend.
In process of development I get to compare charts with satellite imagery and it is hit or miss. It is evident that errors enter NOAA ENC data set from time to time due to errors in scanning or setting types of objects from the raster charts.
The most common error I find is location shift. If it happens, it usually will be uniform for a specific chart, and most of the time will be relatively small (under 20 ft), though some charts are shifted by as much as 70ft or more. Of course, since real precision of GPS units is often not much better, it should not be a problem for navigation if navigator does not assume to be exactly where the little "icon on a screen" says he is.
Here is a random screen shot to illustrate the point.
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