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Old 14-10-2010, 20:04   #196
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Re. GPS accuracy: Both the precision and the accuracy of a modern GPS unit, even a cheap one, are quite good when everything is working properly. When things are not working properly, there will probably be no reading- not a way-off one. (Selective availability has reportedly been removed from the hardware design on the newest GPS satellites, so no more signal scrambling.) The geographic registration of the charts themselves, of course, may be off (I remember an issue a few years ago where a surveyor using official 1:10,000 base maps picked off the co-ordinates of a site, and the office receiving the report insisted that the true position was several blocks away, having plugged the numbers into Google Earth). This sort of thing seems to happen quite a lot in less well charted areas.

Re. paper charts: I carry them and I use them as primaries, navigating mainly by visual bearings to landmarks. For canal, lake and river cruising, I haven't felt the need to rely on (ie. spend money on) an electronic version, although I do spend a fair bit of time messing around with OpenCPN. In an offshore environment, I would likely carry a plotter and a couple of spare GPS units. But I'm just not all that comfortable relying on anything whose functionality I can't duplicate if it breaks, so the gadgets- although useful- would, in my hands, be used only as aids and extra data sources, not as the only option.
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Old 14-10-2010, 20:16   #197
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Originally Posted by osirissail View Post
...

- - However, there is a bright side - recorded "Tracks" - If you set your nav system to record your track as you go from place to place you can very accurately "retrace" your "footpath" back again. Of course, this presumes you did not hit anything the first time. I collect and trade "track records with other cruisers and although the map presentation is sometimes off, the tracks are dead on. Unless the guy lied and did hit something and didn't tell you. Which is why you never trust anything no matter what the accuracy until you have done it personally yourself.
I would add the additional caveats:
  • You need to practice following tracks. After you have done it, compare the original track with your new track while zoomed in tightly. You may be surprised just how far you get off the track at times, especially during turns. Of the items I list here, this one is most likely going to be your biggest variable.
  • You need to know the accuracy that was being reported by the GPS at the time of recording the track. If less satellites were available, the accuracy may not be 6 ft. Might be 10, 15 or 20 or possibly even worse.
  • You need to watch the accuracy being reported by the GPS while following the track.
  • Add the possible errors from the first 3 steps. This is how far off you might be from the original path taken.
  • Then, you have to hope that when you laid down the track, you didn't come within a few feet of hitting something, because following the track, you might just be off by 1 more foot than that. This can be overcome if you have a 3-D depth finder you watch while laying down the original track.
Basically, following a track, you still need to be aware of the possible rate. If I was following someone else's track, I would assume the worst for their part, then add what I know, and not assume they were aware of how close they were coming to objects.

Now, all that above is assuming a narrow, obstacle ridden path. If it is a fairly wide channel and the track was done during good visibility, and you know there was plenty of buffer left, then following it in bad weather would be perfectly reasonable.

So, I'm not saying following someone else's track is not valuable. Just that you need to be aware the possible errors may be larger than you think.

-dan
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Old 14-10-2010, 21:20   #198
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One additional advantage of a library of past tracks is to use them to see how bad the offset is on that particular chart from real life. When the chart is up on the display and the superimposed past track is activated and happens to lie across a mountain or some other obviously non-navigable area - it is reasonable to be very wary of using the GPS derived "little boat" on that chart to determine your position. Better to use the old "mark 1, mod 1 eyeballs."
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Old 14-10-2010, 21:57   #199
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[QUOTE=osirissail;540548]- - Depending upon the year, make and model of the GPS you were using there are many reasons for large errors. The largest error was 104nm discovered by an around the world rally that left Indonesia and was sailing for the Red Sea. It was discovered after all the boat joined up at a common GPS position prior to the entrance that the nautical chart they were using was an old French chart. The lat/long's used off the chart for the meeting point were based on the original zero meridian being in Paris, not Greenwich.

I think you'll find the French have always used Paris as their Meridian...
Its like the US (and satellites)is the only country to have the Red and Green arse about face... Once you know that, everything's fine....
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Old 14-10-2010, 22:39   #200
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The first blue water sail I ever did was from Bermuda to Norfolk...with no power, we did the whole thing using a sextant and dead reckoning (and 3 position checks from passing freighters) we were spot on the whole way, granted we only had to sail "that away" (approx 300degree heading) until we hit North America, but it was comforting to know exactly where you were, especially when crossing the shipping channels with no running lights.
Yes there are all sorts of really kewl electronic features that can be integrated onto the laptop screen: charts+radar+GPS and all sorts of other goodies, but no matter how reliable these gadgets get they are still only as strong as their weakest link. I love all those techie gadgets, but electricity and salt water just don't mix and when you stop and think about it you are trusting your LIFE to millions of different things that could go wrong, any one of which would leave you totally clueless as to your position.
Not to mention there is something a lot more satisfying about a big paper chart. Yes I have a laptop with all the kewl software, connected to my GPS, radar, weather fax ,DSC, etc, but if that goes down I have several hand held GPS units and a sextant.
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Old 15-10-2010, 04:31   #201
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[QUOTE=boatman61;541067]
Quote:
Originally Posted by osirissail View Post
- - Depending upon the year, make and model of the GPS you were using there are many reasons for large errors. The largest error was 104nm discovered by an around the world rally that left Indonesia and was sailing for the Red Sea. It was discovered after all the boat joined up at a common GPS position prior to the entrance that the nautical chart they were using was an old French chart. The lat/long's used off the chart for the meeting point were based on the original zero meridian being in Paris, not Greenwich.

I think you'll find the French have always used Paris as their Meridian...
Its like the US (and satellites)is the only country to have the Red and Green arse about face... Once you know that, everything's fine....
This again - IMHO - is a chart implementation error not a gps error. If I read your post right all the boats arrived at the same GPS coordinates. This means accuracy and repeatability on, I presume, several models of gps was longitudinally and lattitudinally correct.

I may have mislead on the 300 ft comment. WAAS augmented gps can be accurate to 6 feet. The approaches, I think, are limited to ceilings of 300 feet. Meaning you gotta be able to see the runway to land. Of course aeronautical charts are very accurate.

Once again. The gps will put your position within 6ft on the planet surface - the errors are in the charts.

BTW - I defy anyone to get celestial navigational accuracy to within 6 feet and maybe even 6 miles...

In other words if the entrance is fogged in, I don't think anyone is marching in on ded reckoning from the most recent star fix. Although I know that people routinely navigate in under gps guidance.

I think we should do the poll and the reason I simply did it inposting is to thresh out the questions for accuracy - So the modification would be "while underway" as opposed to passage planning.

I certainly see the benefit of using a large scale chart to do long passage planning. But personally I would likely use something like Google earth to broadly pick out the waypoints and then put those waypoints in the chartplotter, PC or whatever to be used "underway" in with gps referencing.
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Old 15-10-2010, 05:20   #202
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This story baffles me... were all the boats using French charts... surely if they were not someone should have noticed the discrepancy before departure as a simple visual of the paper during personal planning would have shown up the 104 mile error...
or do Techies never double check and plan secondary ports/WP's....
Also I presume the 'Setter' of the Waypoint would have had his GPS set on the French Meridian... or is that not programable and the French have to compensate and allow 104 miles on their Charts/GPS's when using their French Charts....
or am I seriously stupid and missing something here...
Scary to think so many boaters take so much for granted.
It is a puzzlement....
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Old 15-10-2010, 06:04   #203
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Can GPS units be set to french datums? Interesting.
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Old 15-10-2010, 06:12   #204
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Quote:
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Can GPS units be set to french datums? Interesting.
Maybe thats why they're pushing for a European GPS System....
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Old 15-10-2010, 12:03   #205
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Quote:
Originally Posted by osirissail View Post
One additional advantage of a library of past tracks is to use them to see how bad the offset is on that particular chart from real life. When the chart is up on the display and the superimposed past track is activated and happens to lie across a mountain or some other obviously non-navigable area - it is reasonable to be very wary of using the GPS derived "little boat" on that chart to determine your position. Better to use the old "mark 1, mod 1 eyeballs."
Absolutely. That's a seemingly obvious use that had not occurred to me.

Thanks.

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Old 15-10-2010, 17:14   #206
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This story baffles me... . . . It is a puzzlement....
This was back in 1990 give or take a year. The ProNav 100 and Trimble were about the only units around and cost a fortune in 1990 dollars. The rally only a half dozen units spread out over 30 or so boats. So each "same size/speed" group traveled close enough together to be able to get together. Charts back then were even worse than today and of course with cruisers being "cheap" - no matter what the eon - few had any Indian Ocean charts more than the harbor charts they were going to use. The join up point was picked off the old French Chart as it showed a large 7,000 ft mountain on the shore which everybody could visually see if the GPS units stopped working. They didn't so everybody got together at the agreed Lat/Lon.
- - These original GPS units only displayed a string of digits for the Lat/Lon on a little tiny lcd screen. Nothing else. There was a menu option to change datums, but few folks knew how to do it - few, if any folks these days really read the "manual." Same back then. You turned it on and up popped a string of numbers showing your Lat/Lon - magic!
- - At the rendezvous point when the leader/cruisers got together on the "big boat" to try and puzzle out what went wrong - the mountain was missing! There was one cruiser back in the third row looking over the shoulders of the others who were all busy with their dividers and parallels trying to figure what went wrong. All he could see of the chart was the corner where the legend was located. He piped up and asked the "whizz-bangs" - "Hey, guys, when was this chart made?" And then asked "Was that before or after the convention that moved the world's prime meridian to Greenwich." There were a boatload of sheepish and embarrassed looks at their simple but significant mistake.
- - The story was published by a sailing/navigation magazine to illustrate the importance of not only reading the manual, but the legends of the charts, and applying the corrections necessary to bring the chart and the GPS into agreement by selecting the appropriate datum or by applying the offset manually.
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Old 15-10-2010, 17:26   #207
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I think another bit of advice would be to memorize one's route to an anchorage so you don't have to be running back and forth to one's nav station or be buried in a chart and not paying attention to the situation.
Well, there's another advantage of the paperless route. I have a chartplotter overlaid with radar at the nav station, and an identical setup serving as a repeater at the helm.

But I still keep paper chartkits aboard.
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