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Go Back   SailNet Community > Skills and Seamanship > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #121  
Old 08-20-2006
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Quote:
Originally Posted by Tea-Rex
maybe it is because english isn't my first language, but there's something i don't understand in your post.

why is it a problem when GPS is off half a mile, yet no problem at all when your celestial fix is off severall miles?
or why is signal loss for an hour or so a problem when you only need a fix every so often to sail savely to your destination with "traditional methods"?
When a celestial fix is off by several miles, I am talking about being miles and miles out to sea, where pinpoint precision isn't necessary. Once you are close enough to see land, then you aren't generally going to be using celestial navigation.

The reason the half-mile error, or the loss of GPS signal, even for a few minutes can be so critical, is that many sailors, who use GPS, come to think that the little boat icon on the screen actually represents where their boat is—IT DOESN'T. Many sailors who become dependent on a GPS think that the GPS tells them exactly where they are—again, IT DOESN'T.

I've seen sailors who tried to navigate by GPS and hit something because they didn't bother to look around with the Mk 1 eyeball...which is still, and should be, the primary navigation tool.

Also, the displays of many GPS chartplotters is often cluttered and displays less information than the paper nautical chart of the same region. This can lead the sailor to mis-reading the chart, and cause navigation errors. These errors could be caught, if the sailor would pull his eyes off the GPS screen and take a good look around.

Just a few days ago, I was on a sail into a small harbor for lunch. This harbor is unmarked, and I doubt the GPS information for it is all that accurate, as it has little or no commercial or military value. There is a very large rock, in what is supposed to be the channel into this small harbor that is about three feet under the surface at low tide.

This rock is not marked on any chart of the area that I have seen. If I had been entering this harbor via a GPS, it is very likely that the boat I was in at the time, which draws 7' of water, would have hit the rock and been damaged.

I'm not saying that GPS isn't a good tool, and an exceptionally useful one at that... but it is just that...one of a series of tools for navigation. It is not a magical solution to solve all of your navigation problems. It does have its shortcomings, and discrepancies between the information on the GPS systems and the real world can be fairly serious and dangerous. It is also dependent on electricity, and in a saltwater environment, electrical systems and electronics, no matter how well maintained or made, have a strong possibility of failing.

Most serious accidents aren't caused by a single thing, but by a cascade of events, each of which would be relatively minor to deal with on its own, but when combined, become disasterous.
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  #122  
Old 09-11-2006
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Well. my take on this is: There are far to many people out off shore with boats that don't belong there, they have minimal or no cruising skills other than mixing drinks and burning animal carcasses over propane things. They are a hazard; therefor encourage them to not bother with celestial navigation, and to rely only on electrons. When you want to help clean the place of idjits, it's best to fill there broom closet...mother nature and Murphy will take of them.
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  #123  
Old 09-11-2006
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Aye...Didereaux...and let's ban those sight reduction tables too...too many idiots out there can't figure out the logs, relying on other peoples calculations. I certainly feel safer knowing that there are people out there who don't know where they are within 30 miles! I roast animal carcasses in the hopes that they will smell them and think they are close to land!
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  #124  
Old 10-01-2006
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I'd like to agree with and amplify on Sailingdogs posts. When sailing south this spring from the Turks and Caicos to Hispaniola we made a morning landfall at Luperon in the Dominican Republic. Using eyeball navigation we entered the harbor at dawn with no issues. Later when I reviewed our course as plotted on the Garmin 2010 chartplotter, the course sailed crossed over a good deal of land on the way to our anchorage once we had entered the harbor. I truly love this Garmin chartplotter and have used it a lot, and have bought a backup to it just in case, however there are places in the Caribbean where the chart on it isn't to be relieved upon. Eyeballs are better when at close quarters to land. Later as explained by Bruce Van Sant, "...until the US Marines decide to invade Hispaniola, don't expect the chart inaccuracies to be corrected." A similar event also occurred on the East coast of Puerto Rico. Trust your eyeballs.
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  #125  
Old 10-02-2006
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I'd like to emphasize one point:

The closer you are to land, the more accurate your navigation needs to be.


If you are 150 miles from the nearest rock/atoll/island and have 500+ feet beneath your keel, then being off by 2-5 miles, which is about the accuracy of celestial navigation from a small boat, isn't much of an issue. If you're trying to make a harbor entrance, between two rock jetties that are 150' apart, being off by 20' is going to be an issue.

Generally, if you can see land, you're not in a celestial navigation situation, but a coastal pilotage situation—where traditional seamanship and pilotage skills come into play. Fancy electronic navigation aids, like GPS, are just that—navigation aids.

You really need to use the Mark I eyeball, traditional pilotage skills, and common sense, once you're that close to shore. Most charts aren't all that accurate, especially once you're away from large ports of commercial or military importance. Many remote areas haven't been surveyed in decades.

If you think that GPS is the panacea to navigation, I'd like to point you to this article. Quoting from the article:

Quote:
Navigation, power and communications systems that rely on GPS satellite navigation will be disrupted by violent solar activity in 2011, research shows.

A study reveals Global Positioning System receivers to be unexpectedly vulnerable to bursts of radio noise produced by solar flares, created by explosions in the Sun's atmosphere.

When solar activity peaks in 2011 and 2012, it could cause widespread disruption to aircraft navigation and emergency location systems that rely heavily on satellite navigation data.
I plan to still be sailing in 2011 and 2012, and will be keeping my sextant and nav almanac handy, just in case.
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You know what the first rule of sailing is? ...Love. You can learn all the math in the 'verse, but you take
a boat to the sea you don't love, she'll shake you off just as sure as the turning of the worlds. Love keeps
her going when she oughta fall down, tells you she's hurting 'fore she keens. Makes her a home.

—Cpt. Mal Reynolds, Serenity (edited)

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  #126  
Old 10-19-2006
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I really don't understand why some people oppose having a sextant so much;
take both!

I have been a pilot for years now. If there's one thing I've learned it's that you can almost always rely on the 'old school' way of doing things.

A lot of people aren't taking into account some very real things that can happen to technology. Have you ever stepped on your gps? I once broke an electronic flight calculator by shutting the aircraft door on it. The display was smashed and useless (No big loss, but had that been a GPS I'd be without electronic navigation).

I've been rendered 'out-of-date' when the aircraft I was flying hadn't had a GPS database update in years. Locations of tall structures, towers, powerlines, etc could have been far out of date - that's dangerous.

You can at least always count on your magnetic compass, a timer and a chart to help you navigate for when more convenient items such as a GPS are not available, for WHATEVER reason, including stepping on it or dropping it overboard (or out the window of an airplane - haven't done it yet but I'm sure it's happened..)

Having a good sextant, CN charts as well as sea charts is essential for being a well rounded, responsible and safe navigator.

And I do find it a little hard to believe that some people would consider themselves real sailors if they can't even navigate by one of the most ancient methods of sea navigation known to man. If the Greeks could do it in 400BC, you can do it in 2006AD.
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  #127  
Old 10-20-2006
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Quote:
Originally Posted by paradoxbox

You can at least always count on your magnetic compass, a timer and a chart to help you navigate for when more convenient items such as a GPS are not available, for WHATEVER reason, including stepping on it or dropping it overboard (or out the window of an airplane - haven't done it yet but I'm sure it's happened..)
I totally agree, a clock and a compass coupled with a good map/chart recon can't be beat.

Jerry
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  #128  
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Escape must be either young or foolish or both. I have been surveying on both land and water for 30 years and enjoy the benefits of modern science, but to put all you eggs in one basket and rely on one type of system when in dangerous situations is not good planning and I would certainly not enjoy the trip with such meager planning. Things break, burnout, burnup and get lost not to mention that the GPS system is left on for us private users at the Federal Governments pleasure. During the recent wars that our country has undertaken overseas the GPS constellation went through many changes that many did not know. However as previously mentioned if you are within site of land and you have 3 gps units thats not too bad but in an emergency it is mighty comforting to reach into a bag and find some additional help. And besides some of us like to mantain the link of how we got to be where we are. Lighten up Escape we traditionalists are not all that bad and besides when your power fails and we find you drifting in the see out of your mind we will throw you a line .
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  #129  
Old 10-20-2006
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I read Escapade's post over and over again. There is no sign of him being either young or foolish or both. In fact, everyone here seems to be educated, mature and successful in their career. We may have different way of doing things or different level of sailing objectives.

Cheers.
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  #130  
Old 10-20-2006
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Ditto rockDawg...I am neither young or foolish (except if you ask my wife about the latter) and I would rather have 2 cars in my garage rather than a car and a horse...EVEN THOUGH IT IS NICE TO KNOW HOW TO RIDE A HORSE!
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