|Topic Review (Newest First)|
|02-16-2011 11:41 AM|
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.
|02-16-2011 10:33 AM|
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.
|05-19-2010 01:32 PM|
Originally Posted by wwilson View Post
|05-19-2010 10:39 AM|
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.
|05-19-2010 06:08 AM|
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.
|04-17-2010 06:59 PM|
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.
|04-17-2010 04:09 PM|
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!
|04-17-2010 02:35 PM|
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 :-)
|04-17-2010 01:59 PM|
Originally Posted by LinekinBayCD View Post
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.
|04-13-2010 08:24 AM|
|norsearayder||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..|
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