Actually, a front coming through tonight is dropping the the temp dramatically. :)
As for the navigation thread, we were motoring a narrow section of the ICW before sunup this morning. Although Touché was in the middle of the "ditch", the GPS showed us about 150 yards north in the pine trees. Further in the ICW, the graphical depiction of the channel did not match the markers or our location. That is, the different colored channel was outside the markers. The markers appeared accurate. So, much of the inaccuracies we see are in the GIS data in the maps. I'd bet that if I'd plotted the location from the GPS position data on a paper chart, I'd have been in the channel and not in the woods. Gas wells in Mobile Bay were close to the charted positions. My auto steerer is always pretty much dead on because the waypoints I use are all observed. That is, they were set from actual boat position using the "mark" function of the GPS. Some were set several years ago. The fact that the GPS via the auto steerer brings me back to them accurately after several years is verification of the accuracy of the GPS system. Dennis C. Touché 35-1 #83 Mandeville, LA Sent from my iPhone > On Dec 6, 2014, at 8:58 PM, "Rick Brass" <rickbr...@earthlink.net> wrote: > > Hope you are having a great, and warm, weekend, Dennis. It is rainy, gray, > and the high was about 60 in NC today. Not a nice day for boating. > > We have all experienced the sort of GPS errors you mentioned at one time or > another. And because we all know that our GPS receiver can calculate out > position to an accuracy of 30 feet or so, we tend to think that the charts > are wrong. But that might not be the whole truth. > > I’d bet NOAA had pretty good GPS location numbers on the buoys you “hit”, and > is not far off on the position of the seawall. The 10 to 30 foot accuracy our > GPS reports is based on things like the number and position of the satellites > from which it is getting signals, allowing for things like the accuracy of > its internal clock, inaccuracy in the chart datum, and the radio waves that > carry the time signals from the satellites getting “bent” by the Earth’s > magnetic field. But there is another variable that the GPS can’t allow for. > > I remember reading, a few years ago, about the GPS system in one of the > science magazines aimed at geeks like me (Probably Scientific American or Air > and Space, but I can’t recall for sure). Seems the GPS system is a good > example of Einstein’s Theory of Relativity. Part of the theory says that when > you go faster, time slows down relative to time measured in a location that > is moving more slowly. > > The GPS satellites are traveling at something like 18000MPH faster than we > are on the Earth’s surface. So the atomic clocks on the satellites “tick” > just a wee bit more slowly than the clock on earth. There is a government > facility outside of Omaha where military personnel are tasked with adjusting > the clocks on the satellites, by a few microseconds or nanoseconds, several > times per day to maintain the accuracy of the time signals relative to the > earthbound time. As I recall, if the clocks were not adjusted for 24 hours, > the calculated position of a spot on Earth would be off by something like 5 > miles. > > That’s probably more than you wanted to know. But you can probably chalk up > all those buoys the chartplotter boat ran into to Albert. > > Oh, and another bit of Einstein trivia: He issued the original patents for > the recipe for Tolberone Chocolate, and the shape of the candy. Which is not > boating related, unless your Admiral likes really good chocolate. > Rick Brass > Washington, NC > > > > From: CnC-List [mailto:cnc-list-boun...@cnc-list.com] On Behalf Of Dennis C. > via CnC-List > Sent: Monday, December 01, 2014 8:48 AM > To: Della Barba, Joe; cnc-list@cnc-list.com > Subject: Re: Stus-List Navigation > > I was motoring up a harbor looking at a nice Raymarine system showing the > boat going through a sea wall 200 feet west of our actual position. > > Yesterday while motoring in the ICW channel in Santa Rosa Sound near Navarre, > FL, the chartplotter boat took out several of the buoys on the right side of > the channel. > > Dennis C. > Touché 35-1 #83 > Mandeville, LA > > Currently on the hook at > 30 23.054N 86 51.884W > > Sent from my iPhone > > > >
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