I suspect that you are correct, Dennis.
If you look at the legend of a paper chart, it will tell you when the cartographic data was recorded. The last section of the ICW in North Carolina was dug in the mid 30’s, and much of the survey data was collected between the 1930s and the 1950s – when accuracy was far below the plus or minus 10 feet you can get with modern GPS. There are sections around here where a boat in mid-channel will show to be 100 feet or more off to one side and on dry land. Segments that are regularly dredged or where markers have been moved in the past few years will have much more accurate cartographic data. The age and accuracy of the survey data is one of the reasons why NOAA no longer prints the “Magenta Line” on current ICW charts. Another potential inaccuracy, somewhat spotty, is an actual proof that Einstein was correct in general relativity. The geosynchronous GPS constellation is has a velocity somewhat greater than that of the point below it on the Earth’s surface. General Relativity predicts that time moves more slowly as velocity increases. Consequently the clocks on the satellites get reset by a few milliseconds every couple of hours to match the official atomic clock. There was an article in a science magazine about the GPS system a few years ago, and the writer mentioned that on the day he visited the GPS system control center in Colorado there was a communication issue between the atomic clock and the control center for several hours – with the result that the GPS in the reporter’s car directed him to a spot more than ½ mile away from the GPS control location. That’s why a prudent mariner uses multiple sources of navigation information – and why you need to keep your head out of the boat. Rick Brass Washington, NC From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Dennis C. via CnC-List Sent: Tuesday, December 18, 2018 1:42 PM To: CnClist <cnc-list@cnc-list.com> Cc: Dennis C. <capt...@gmail.com> Subject: Re: Stus-List Electronic navigation - chartplotters The positions were accurate during the rest of the 178 nm trip except in that short section of the ICW. My guess is the cartography was inaccurate there. Dennis C. On Tue, Dec 18, 2018 at 12:34 PM Jerome Tauber via CnC-List <cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com> > wrote: Is your GPS set to output positions using the corrrect Geodetic Datum? Jerry -----Original Message----- From: Dennis C. via CnC-List <cnc-list@cnc-list.com <mailto:cnc-list@cnc-list.com> > To: CnClist <CnC-List@cnc-list.com <mailto:CnC-List@cnc-list.com> > Cc: Dennis C. <capt...@gmail.com <mailto:capt...@gmail.com> > Sent: Tue, Dec 18, 2018 11:22 am Subject: Stus-List Electronic navigation - chartplotters Over the weekend I brought Touche' back home to Mandeville for the winter. Nice trip but a bit cold for my Southern blood. Here's something I observed. For those who rely exclusively on charplotters and electronic navigation, here's something you need to look at. Go to Google Earth and enter the following in the search window: 30°18'2.24"N 87°37'34.10"W That will take you to a marshy island in Portage Creek, a portion of the ICW near Orange Beach, AL. That's where the GPS chartplotter showed the boat symbol representing Touche' was when Touche' was actually in the middle of the channel adjacent to island. If I had been using the boat symbol for navigation and had been steering to keep the symbol mid-channel, Touche' would have been in the trees on the south bank of the ICW. The GPS continued to show Touche' in the trees on the north bank during the entire transit of this section of the ICW. Having navigated these waters dozens of times, I'm very familiar with them but a boater transiting here the first time in heavy fog............................ Once out of the man-made ICW the GPS returned to being fairly accurate. Just saying, once again, how important it is to use your eyeballs and not rely on electronics exclusively for navigation. Maybe it doesn't seem like a big deal when you're in a channel that is well marked but there are lots of areas with no marks, no land and seriously shallow shoals or rocks. Dennis C. Touche' 35-1 #83 Mandeville, LA _______________________________________________ Thanks everyone for supporting this list with your contributions. Each and every one is greatly appreciated. If you want to support the list - use PayPal to send contribution -- https://www.paypal.me/stumurray _______________________________________________ Thanks everyone for supporting this list with your contributions. Each and every one is greatly appreciated. If you want to support the list - use PayPal to send contribution -- https://www.paypal.me/stumurray
_______________________________________________ Thanks everyone for supporting this list with your contributions. Each and every one is greatly appreciated. If you want to support the list - use PayPal to send contribution -- https://www.paypal.me/stumurray
