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.