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


Thread: Lines of Position Reply to Thread
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Topic Review (Newest First)
05-27-2007 11:26 PM
sailaway21 tenuki,
Being a pessimist, as well as a contrarian, I suspect just the opposite is true. GPS and chartplotter assisted grounding, due to over-confidence or over-reliance, is a very real issue. Training in the proper use of electronic navigation equipment is essential and under-appreciated. The term "radar assisted collision" is not just a punch line to a bad joke; it happens. The small boat navigator with nothing but the Mk I, Mod 0, precision eyeball is unlikely to leave, or enter, port when the fog is dense preferring to wait for safer conditions. The GPS reliant navigator is probably more likely to say, "so what if we can't see anything, we can tell where we are right here on the black box".
05-26-2007 03:28 AM
tenuki I suspect GPS and chartplotters have overall reduced pilot error despite introducing the problem you are rightly pointing out. I imagine that there are a large number of people who now have access to a greater range of sailing experience due to GPS, they simply would have grounded before leaving the harbor otherwise.
05-26-2007 01:34 AM
sailaway21 Actually Cam it does not tell you what your margin of error might be. GDOP, geometric dilution of precision relates solely to the angular differences of the various LOPs. That is what your unit is showing you. It is not showing you such things as the effect of transient noise on each individual LOP. Anotherwords, the computer is doing the thinking and evaluating for you. Similar factors are what make your shallow water alarm go off consistently in 500 fathoms of water and yet not utter a peep as you gring your way ashore.(g) Similar technology is what makes your automatic acquisition radar track minute squalls but manages to overlook a sailboat 5 miles away. Again, I am not trying to be critical of GPS, but rather the manufacturing process that leads to, "they don't need that" mind-sets.

Perhaps an illustration would make the point better. "Transit" was generally employed with a gyrocompass input and a manually entered speed input, what with very few ships having pitot logs. Theoretically, if the speed entered was off a significant amount the receiver should not be able to acheive a "fix". Nevertheless, the receiver would merrily pump out positions, some ten miles off, as if all was well in a carefree world. Advances have certainly been made, and the receiver's estimate of GDOP would reveal the potential for large errors, if monitored. But the unit still does not give the operator the type of information he needs to either accept or reject individual LOPs. "You are here" logic is the quite predictable result.
05-25-2007 12:02 PM
Valiente In most pilotage situations, I don't bother with the GPS. I use a pelorus and "60 D Street". I use it to compare my DR to my lat/lon a couple of times a day, but I find that when I can see the coast, it's most useful for ETAs, compass headings and cross-track estimates.

I don't assume it correct, but do assume it's "correct enough". Offshore, I have the sextant, after all, and truly gross errors are going to become apparent pretty quickly, particularly if you've been charting your course on paper. Besides, hourly fixes gets me off the deck to reapply sunscreen and to rehydrate!

I also agree that radar makes a nice rangefinder and that a few classes in how to use it can make a big difference in how it's used (self-evident, one might think, but I've seen it used incorrectly on more than one occasion). You can, after all, avoid another boat easier than you can avoid a penisula with a strong current wrapping around it.
05-25-2007 11:56 AM
camaraderie Aww...c'mon guys...all this "in theory" is nice...but my little GPS tells me exactly where I am and tells me what my margin of error might be.
Would I navigate without it...NO...Would I like additional input from radar...sure. In my recent trip up the Bay...I used both and NEEDED both but my eyeballs would have been absolutely useless. In ALL the last 10 years or so of using a GPS I have never once had cause to doubt the accuracy of my GPS within a few hundred feet. Most often I am within 10-30 feet of my waypoints.
As far as Loran goes...I used that too a long time ago and it was a good tool in its' day (and still excellent for REPLICATION of previous waypoints...i.e. fishermen!) but then the abacus was a good tool in its' day as well. (G)
05-25-2007 02:49 AM
sailingdog
Quote:
Originally Posted by camaraderie
Dawg....let's keep chartplotters and GPS separate. Getting a lat/long from a GPS in coastal situations is an accurate means of establishing a position within a few hundred feet at worst. Not in anyway dismissing the "eyeball" but in VERY many coastal conditions (i.e. it is dark outside and the eyeball has no way of establishing a true position or fog or haze etc.) the GPS is by far the best nav tool available.
Actually, in that situation, RADAR would be far better IMHO, since it works with what is actually there, not what might or might not be there, depending on how good the signals from the satellites are and whether the SA feature is turned on or off.
05-25-2007 01:06 AM
sailaway21 Not surprisingly, my friend Camaraderie, has missed the main point of my post.

That point being, you have no way of knowing whether your position is accurate to within the few hundred feet you mention, or the six hundred feet I mentioned, or worse. For the GPS "fixed" this is not an attack upon GPS, it's actually an attack upon the receivers, and the dumbing down of those receivers for convenience sake. I own a GPS and find it to be a fine navigational tool. IMHO, relying on GPS alone, in a coastal navigation situation, ie..."piloting" is foolish. Darkness alone should be no barrier to navigating coastal, with the plethora of lights along the US coast. Fog, and perhaps haze, would behoove one to haul further offshore, but then you'd not be piloting anymore, and would be using GPS at it's strongest suit-offshore navigation. If I have somehow misconstrued cam's post I am sure illumination, if not conflagration, will soon follow.(g)
05-24-2007 08:36 PM
camaraderie Dawg....let's keep chartplotters and GPS separate. Getting a lat/long from a GPS in coastal situations is an accurate means of establishing a position within a few hundred feet at worst. Not in anyway dismissing the "eyeball" but in VERY many coastal conditions (i.e. it is dark outside and the eyeball has no way of establishing a true position or fog or haze etc.) the GPS is by far the best nav tool available.
05-24-2007 02:14 PM
sailingdog Of course, those errors being transient, can be easily corrected for or avoided. If you're using GPS to navigate while near shore, you're making a grave mistake. Coastal pilotage skills should take priority over an icon on a screen if you are within Mark I eyeball range of land.
05-24-2007 01:54 PM
EpicAdventure These errors probably don't make difference for typical boat navigation. Although, it is interesting to think about them as far as precision landing approach aids for aircraft.

The good thing is that the FAA is quite the stickler about making sure the probability of these types of errors affect the safe operation of a aircraft is very low.

I have heard that a two frequency version will be made available for commercial purposes just because the FAA is so concerned. (Currently, two frequency GPS is restricted to military use.)
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