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Go Back   SailNet Community > General Interest > General Discussion (sailing related) > Gps navigation advise
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Topic Review (Newest First)
12-14-2012 06:24 PM
smurphny
Re: Gps navigation advise

The age of expensive, bulky, quickly obsolete paper charts is obviously in decline but even if a paper chart is obtained from your chartplotter via a printer, it is still another source of information to weigh against what shows on the plotter screen. The more sources of navigation information the better. There is always an element of DR. Confirming what you are observing should have as many inputs as possible, not just looking at a digital image which COULD be wrong for one reason or another.
12-14-2012 01:52 PM
cb32863
Re: Gps navigation advise

Post Disclaimer: By posting the link below I am no way saying everyone should panic, run around in circles like their hair is on fire, scream like Chicken Little, go buy the next sextant they see and learn celestial nav., brow beat those that advocate paper is a necessary back up or those that say GPS is all you need. This is squarely informational for you to use as such.

How to bring down mission-critical GPS networks with $2,500 | Ars Technica
12-12-2012 08:58 PM
Omatako
Re: Gps navigation advise

The wonderful thing about all the new technology that has "replaced" paper charts is that one can now get a device that displays all the information on one screen.

AIS, radar, chart plotter depth sounder, everything one needs to exactly observe not only one's own position relative to all things stationary but also to observe one's position to and the identity of the more threatening of those things moving.

There are even some of us that believe the screen on this marvelous piece of equipment will never go black, leaving one without ANY form of positional data.

The paper charts I use cannot accidentally switch off. Sure I use GPS to find the spot on the chart where I think I'm at but if I'm able to hit something then I'm also able to see it using the good old Mark 1 Eyeball.

To each his own.
12-12-2012 05:22 PM
GeorgeB
Re: Gps navigation advise

It has been some time since my nav training so feel free to correct any inaccuracies I may proffer. Nav 101: Charts are flat, the earth is spheroid. Within a given chart, survey work done on the datum points give good relational accuracy which puts you pretty accurately on the chart given the bearings you take. Unfortunately, the earth is spheroid (not round or oval, but actually a little bit lumpy). GPS calculates where you are on the surface of the spheroid and not necessarily the cardinal coordinates for a given chart. If your chart has been updated to WGS 84, then it reflects this and you should have a pretty accurate fix on your (newer) paper or electronic chart. If the chart has not been updated and is showing something like WGS 72 or older, then the chart may or may not be that accurate depending upon how “lumpy” that particular patch of earth is. In the US (and most of Canada and Mexico) the WAAS system is used to ensure greater accuracy in the timing measurement. However, due to the placement and orbits of the WAAS satellites, places like the arctic, coastal Mexico (and potentially the US coastlines) may have slight variations in the timing signals and hence GPS accuracy.

GPS only “replaces” the celestial part of navigation and there are tons more to learn about navigation than turning on the Garmin and setting the autopilot. How many of us have chart #1 for example? Can you calculate set and drift? Heck, most people can’t even spell “ded reckoning” correctly. Sure, Columbus discovered the New World, but a skilled navigator will make landfall where he originally intended to go.
12-12-2012 04:10 PM
JonEisberg
Re: Gps navigation advise

Quote:
Originally Posted by boatpoker View Post
For those that believe in the infallibilty of GPS charts and charting. These two photos were taken from my helm within seconds of each other. I have dozens of such shots.
Yeah, I suspect anyone who's gone pretty much anywhere can offer such examples... Where was yours taken, btw?

When it happens in places known for rocks, and fog, it should really give one pause...

Damn, think of all those Newfies lying under those headstones, who might have been saved, if only they'd had the benefit of today's technology... (grin)



12-12-2012 03:55 PM
JonEisberg
Re: Gps navigation advise

Quote:
Originally Posted by travlineasy View Post

Fire up your GPS/Plotter and take a close look at the ICW behind Virginia's Barrier Islands from Cape Charles to Chincoteague. I think, John, that you'll find it interesting to say the least.

Good Luck,

Gary
Sure, I have no doubt that particular area as charted bears little resemblance to reality...

However, I doubt that waterway - with a project depth of 4 feet or so, and probably considerably less in spots, and the 35' clearance of the fixed bridges over Chincoteague Bay - is included in what most people think of when they hear "Atlantic Intracoastal Waterway"... (grin)
12-12-2012 03:40 PM
travlineasy
Re: Gps navigation advise

My primary navigational tool is, naturally, my eyes, followed by common sense, followed by my GPS/Plotter, and sometimes, I even look at the charts, which I have lots of onboard.

The area just south of Carolina Beach, where the ICW is about 100 feet wide, is where I noticed that there was a 50-foot descrepency. Also just south of Jacksonville Beach, for a distance of about 3 to 5 miles, the same 50-foot difference between the actual position and GPS location.

My Lowrance HDS7 uses Lowrance charts, and also Navionics - I prefer the Lowrance mode, which is incredibly accurate and detailed.

Fire up your GPS/Plotter and take a close look at the ICW behind Virginia's Barrier Islands from Cape Charles to Chincoteague. I think, John, that you'll find it interesting to say the least.

Good Luck,

Gary
12-12-2012 02:59 PM
JonEisberg
Re: Gps navigation advise

Quote:
Originally Posted by travlineasy View Post
John,

Nothing to do with MY setup - it's merely a function of the input data, which is wholely supplied by governmental agencies - not the private sector. To MY knowledge, there is no private sector surveying of the world's waterways being conducted - it's just cost prohibitive. Consequently, everyone using GPS, computer based charts, and paper charts, are by and large relying on information generated by USGS over the past century.

Granted, some of this is up to date, so to speak, early 2000 data, but there's an enormous amount of information that is older than many of the forum members. John, if you have the opportunity to traverse the ICW in South Carolina, Georgia, etc..., you will quickly discover what many travelers of this waterway have. Charts, regardless of where they're generated from, are by and large, quite inaccurate. It's not the fault of the electronic gear, it's the input data.

Cheers,

Gary
I understand what you're saying, and yes - I have had the opportunity to traverse the ICW thru SC, GA, etc... More than once, as a matter of fact (grin)...

I would disagree with you, in that in general, I find most of the charting to be incredibly accurate... Shoaling inlets are the obvious exception, of course, and anywhere else where the tidal ranges are larger, and the current runs strong... On Sunday morning, entering Port Royal Sound from outside from the north, I cut the corner by about 4 miles inside the sea buoy, shooting the narrow gap between the extensive shoals to the north of the very long entrance channel... As usual, I was astonished how accurate the charting of such a dynamic area remains, it was right on the money...

But again, I have to wonder what sort of e-charts or programming you're using, because in my experience, I've never seen any of them that have placed me on dry land anywhere along the ICW or its inlets (with the possible exception of a small, extremely changeable one such as Carolina Beach, perhaps) My C-Map for the waters inside Barnegat, for example, are slightly off - but anyone relying solely on e-charts in such a place, at such a scale, is really asking for trouble, anyway...

As gotd 25 suggests, that's when it's time to Look Out the Freakin' Window... (grin)
12-12-2012 02:43 PM
boatpoker
Re: Gps navigation advise

For those that believe in the infallibilty of GPS charts and charting. These two photos were taken from my helm within seconds of each other. I have dozens of such shots.
12-12-2012 02:30 PM
gtod25
Re: Gps navigation advise

Quote:
Originally Posted by MarkofSeaLife View Post
They all use electronics.

AIS has seen to that. It's illegal for them to operate without AIS so it means it is illegal for them to operate without electronic charts.
1. They all use electronic AND paper charts.
2. Re. Navies and Coastguard vessels; "illegal for them to operate without AIS", NO! Nor is it illegal for them to operate without navigation lights etc. You do realize that AIS can be displayed on a range of screens that do NOT have a charting capability.

As I said originally I could care less if you navigate your own boat using an ouija board. Experienced sailors (and you are one) need to be careful when dispensing advice to newbies. Stating that paper charts are "unsafe and obsolete" and that you "would never sail with anyone that uses them" implies that Electronic charts ARE "safe and up to date". This is patently NOT the case. ALL charts are just aids to navigation and should be read with caution.

As I tell students and anyone who will listen, remember the golden rule of navigation; L. T. F. W.









Lookout The Fu@&ing Window.
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