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Amgine · 12-06-2007, 07:14

Various charts are "normalized" (related to) different charting standards, called geodetic datum. GPS uses one specific datum (WGS84), and you need to reference your chart to find out if it is a different datum.

If the chart is not on the same datum, there are methods to calculate the difference; the chart and the GPS are probably *both* accurate, but they will show your position as consistently shifted relative to each other.

Amgine

**Ex-Calif** · 12-06-2007, 19:43

My (Garmin) gps will allow you to choose the datum reference in the setup. Don't they all do that?

You do need to make absolutely sure the datum on the charts and the datum on the GPS match.

casique8 · 27-10-2007, 08:37

if your in the ATLANTIC GO WEST IN A STRATE LINE COURSE YOUR BAUND TO HIT GROUND ASK CHRIST COLUBUS

Lancerbye · 27-10-2007, 10:22

I have only had GPS and a plotter for the last couple years. And I must admit it is pretty easy to plot and navigate a course when it is connected to the Autopilot. It can almost dock it for me which is the scary part. I fear I maybe getting lulled into a false sense of security. Before I would use compass, celestial, DR, radar and charts to get around. One felt a great accomplishment when you actually ended up where you had planned with minimal corrections. Now it is way too easy. Maybe after the big EM kick things will get interesting again. I admit to a certain streak of laziness and I will use the easy route. Also I like high tech gear. There is probably a cure for this but I don't think I'll like it, Tthe cure may be worse than the desease. LOL

Sandero · 27-10-2007, 17:12

As long as you maintain your navigation skills and have back charts and systems, GPS plotters are the cat's meow. But you be checking on them, cos that's you job as skipper.

If Columbus had GPS he would have used it.

Lancerbye · 27-10-2007, 18:35

If Columbus had a GPS and a plotter he would never have been credited for discovering the Americas. It wasn't lost but he was. He was looking for China. And being a man's man he didn't ask for directions. LOL

texwards · 08-11-2007, 06:57

GPS is great - but good ole' Murphy hangs around my boats too often. I wouldn't take any extended trip without my sextant and tables - no matter how many backup systems I had.

hellosailor · 08-11-2007, 18:14

Try this one, Tex. Place your sextant and your GPS on the portside berth, or the nav station. Now, roll the boat a full 360 once or twice. Or, take it out in a pounding storm and let each instrument hit the floor three times. Or, just hold it in your hand and fall down the companionway steps while holding it.

Tell me which one works better after that visit from Mr. Murphy, the tinkly bent sextant, or the GPS.

Mind you, I love the whole sextant concept as a mental exercise, but please, don't even think such a large, expensive, and fragile piece of machinery and optics is "robust" in any way.

**GordMay** · 09-11-2007, 03:38

Quote:

Originally Posted by hellosailor

Try this one, Tex. Place your sextant and your GPS on the portside berth, or the nav station. Now, roll the boat a full 360 once or twice. Or, take it out in a pounding storm and let each instrument hit the floor three times. Or, just hold it in your hand and fall down the companionway steps while holding it.

Tell me which one works better after that visit from Mr. Murphy, the tinkly bent sextant, or the GPS.

Mind you, I love the whole sextant concept as a mental exercise, but please, don't even think such a large, expensive, and fragile piece of machinery and optics is "robust" in any way.

Indubitably.

texwards · 09-11-2007, 06:51

ouch, that stings.... I'm sure you have way more experience then me, and in the scenario you describe I wouldn't be on deck even attempting a fix. I wasn't commenting on the robustness of the instrument just the fact that if I did not bring it, I would probably need it - attempting to support my vote in the survey.

The truth as to why I carry a sextant is probably similar to the reason that I keep my Pascal programming books - it is a skill I spent time and effort to acquire and am, therefore, reluctant to let it go.

rebel heart · 11-11-2007, 18:35

I keep my sextant in a box, not sitting out in the open. And in it's pelican box, it could certainly take a heck of a lot more blunt trauma than my gps could. Put the gps in a pelican, and everything's equal again. You could just as easily drop it in the drink as the cabin, so trying to imagine every scenario is kind of silly.

But it's more fundamental than that. Obviously a qualified mariner who is competent and skilled will use any tool to provide additional information by which they make good judgement calls. I don't really think there's any argument that an accurate position fix, whether delivered from a sextant, gps, or crystal ball, is any better or worse than anything else.

The bigger problem, in my mind, is that a mariner who knows how to use a sextant, 9 times of out 10, is a better mariner than someone who uses a chart plotter.

Again, I'm not saying that chart plotter people are bad, but if I had two captain's resumes in front of me, and one was very comfortable using a sextant, and the other wasn't, I would absolutely pick the sextant guy (or gal). And, I'd hope they'd be smart enough to use a chart plotter if it was available to them.

Sextant people, to me, 9 times out of 10, generally represent someone who understands meteorology better, and probably opts for more simplistic systems. If someone took the time to understand a sextant, with the availability of GPS, I would imagine they've taken the time to do many other skilled and difficult things, instead of opting for what's easiest.

Again, this is just my impression, and I haven't sailed every gallon of water out there.

rebel heart · 11-11-2007, 18:36

Oh yeah.. and what kind of fix are you going to take with a sextant in a storm anyway? Lol. You need to see past the clouds a bit.

hellosailor · 11-11-2007, 22:33

"Oh yeah.. and what kind of fix are you going to take with a sextant in a storm anyway? Lol." Haven't you ever experienced clear, or partly clear, skies while also experiencing a punishing sea state from a storm? Either rollers from a major storm a thousand miles away, or crossing wave sets that throw you back and forth? Or simply been thrown by a wake that seems to have come from nowhere? That's when a sextant can go tinkle-tinkle on the deck, just like anything else, including the GPS. But the sextant is a lot more vulnerable to damage, simply from being much larger and having fragile parts exposed all over. You can't just cup your hand around it, or let it fall on your instep so it doesn't take a hard hit on the deck. It's still going to be damaged. Pelican case? Nice, sure, but sooner or later you have to take it out of the case.[g]

Tex, don't mistake me for being anti-sextant. There are ways to sharpen the mind, entertain, and keep traditions, and sextant nav is all of that. But for simple, reliable, robust, practical navigation today? Nuh-uh, I'd say most sextants fall somewhere between chess and the dvd player: Entertaining and perhaps beneficial, but their time as primary navigation instruments has come--and largely gone.

I see new Bluetooth GPS units (no displays on them) advertising fifty channels, which I assume means they are scanning for the Russian and EU birds as well as the US system. Well under $100. Even the arguments that there is only one GPS constellation (the US) are falling.

**David M** · 12-11-2007, 11:55

Saying one way of getting a fix is bad because there are "better" ways of getting a fix is the wrong way of looking at it. There are distinct advantages and disadvantages to all methods of getting a fix...and the best way depends on your situation. To rely on one method is plain foolish.

The more methods you have of getting a fix, then the better off and the safer you will be. The bottom line is that the more methods you have of getting a fix, then the smarter that you are being.