Paper Charts and GPS

[GordMay]

Originally Posted by G4MJW

... I'm told there is a 1,000,000:1 chance or less in being struck by lightning just once in your lifetime! ...

Different sources give differing odds on various occurences.

From the National Weather Service:
NWS Lightning Safety Medical Information

According to Storm Data, a National Weather Service publication, over the last 30 years the U.S. has averaged 62 reported lightning fatalities per year. Due to under reporting, the figures are more realistically at least 70 deaths per year. Only about 10% of people who are struck by lightning are killed, leaving 90% with various degrees of disability.

Odds of being struck in your lifetime (Est. 80 years): 1 in 5000 (5,000:1)

According to “What the Odds Are” by Les Krantz,
The odds of your being injured by a lightning strike on any given day are only 1 in 250 million, but over the average lifetime are 1 in 9,100?
In contrast, the odds that the average citizen of Washington, D.C. will get "plugged, stabbed, poisoned, or bludgeoned to death" in the course of a year are only 1 in 1,681!

Once Thomas Jefferson had written the Declaration of Independence, John Adams helped him to edit and perfect it. The document was approved by the Continental Congress on July 4, 1776. Exactly fifty years later, on July 4, 1826, both Jefferson and Adams died.
What are the odds?

[svHyLyte]

This subject has previously been discussed at great length. See http://www.cruisersforum.com/forums/...ups-16325.html

[Amgine]

Unless the chart datum used by the GPS was developed using GPS, it's the same datum as will appear on the paper chart.

So, for example, portions of US ENCs for the region around Bellingham, WA date from 1926. This region of US waters has not been re-surveyed, the data have been manually converted to modern standards. The same is true, last I'd read, with the majority of US NOAA data because survey budgets can barely handle covering commercial routes. There's even at least one commercial company surveying US waters because it's viable.

Do you really think the west coast of Mexico has had more recent surveys?

I think everyone has heard the mantra GIGO - garbage in, garbage out. The GPS/plotter is exactly the same. Without good, relevant information it will certainly steer you wrong. Every summer when I take my pilgrimage to False Creek my GPS tracks me across the mainland, the garmin chart being well off the reality, and my eyes keep me in the channel under the bridge.

[David M]

Originally Posted by Amgine

Unless the chart datum used by the GPS was developed using GPS, it's the same datum as will appear on the paper chart.

So, for example, portions of US ENCs for the region around Bellingham, WA date from 1926. This region of US waters has not been re-surveyed, the data have been manually converted to modern standards. The same is true, last I'd read, with the majority of US NOAA data because survey budgets can barely handle covering commercial routes. There's even at least one commercial company surveying US waters because it's viable.

Do you really think the west coast of Mexico has had more recent surveys?

I think everyone has heard the mantra GIGO - garbage in, garbage out. The GPS/plotter is exactly the same. Without good, relevant information it will certainly steer you wrong. Every summer when I take my pilgrimage to False Creek my GPS tracks me across the mainland, the garmin chart being well off the reality, and my eyes keep me in the channel under the bridge.

I thought I made that clear in my previous post. Old surveys using old survey methods to create an electronic chart still does not make an electronic chart any more accurate than the original paper chart generated from the same survey.

Eventually the rest of the world will catch up to surveys done with GPS technology. Until then, you better know which charts were done using old survey methods and which were done using GPS. Its not that old survey methods used by the old survey team were inaccurate necessarily, its that they do have the potential of being inaccurate enough to put you aground.

[Amgine]

My apologies David M, but I didn't see that you pointed out what is on the GPS plotter is the paper chart, in many cases. For paper charts worked in old standards they are simply updated and stuck on the plotter without new surveys.

[Charlie]

Here is an example of problems with datum and Charts. This is taken from Gerrycruising.com. There is a two mile difference between the old charts and the new.


http://i543.photobucket.com/albums/g...orinMexico.jpg

[Captain Bill]

I use electronic charts almost exclusively in US waters. The primary reason for this is that I can update them weekly over the internet. Paper charts are pretty much out of date before you get them unless you use "print on demand" charts. I personally don't have enough money to buy new POD charts every week. I have two independent GPS systems on board and a computer hooked to a 22" flat screen running Coastal explorer. I have no fantasies about the accuracy of the electronic charts, they are the same as paper charts. In the Bahamas I use the Explorer paper charts for fine scale and electronic for large scale. I have noted that Bahamas charts are often off by at least a few hundred feet. While near Lee Stocking Island a few years ago in the channel on the north side the chartplotter had us on land. When we came around the island we found a trawler yacht hard aground because he trusted the chartplotter in the dark. I'm sure that the GPS knew our actual position on the planet, but the chart was wrong. If I would have been plotting the GPS position on the paper chart I would have had the same problem. Using paper or electronic charts is no excuse for turning off your brain. If your dumb enough to try running your boat aground just because a chart plotter says your on the water, there's little help we can give you.

[Amgine]

Charlie:

Looks like the charts are shifted about two miles NW-SE. It would be quite possible to determine the offset (I don't remember who described that above in this thread.) It would seem to be just another step in your regular navigation and piloting.

As Captain Bill put it, don't put all your eggs in any one basket. When you're near navigational hazards (land!), someone has to be on watch and eyeballing at twice your possible error (so approaching land at 5 kt, someone has to be looking at 10 nm from nearest approach on any chart; paralleling shore with offlying hazards may require someone on watch and looking 24/7.) Use multiple tools to determine where you are, and where hazards may be.

Navigators are paranoid. It's a job requirement.

[Lodesman]

Originally Posted by Captain Bill

I use the Explorer paper charts for fine scale and electronic for large scale.

Point of terminology - 1:5000 is large scale; 1:1,000,000 is small scale.

[GordMay]

Originally Posted by Amgine

Charlie:
Looks like the charts are shifted about two miles NW-SE. It would be quite possible to determine the offset (I don't remember who described that above in this thread.) It would seem to be just another step in your regular navigation and piloting...

Unfortunately the Chart "offsets" (ERRORS) are not consistent, hence cannot be reliable accounted for (calculated).

[sandy daugherty]

Satellite imagery is now contributing to NOAA's chart updates, which will resolve many of the discrepancies discussed above. But that does nothing for navigators outside the limits of NOAA coverage. If the paper charts are wrong, the GPS charts are wrong. They were taken from those erroneous charts. Unfortunately, the errors aren't consistant; you can't just shift everything 2 miles east and be safe. The data can be skewed, stretched, or distorted.
The suggestion that you can overcome these charting errors by contact navigation only applies if the original cartographer's data was obtained the same way, by hand bearing compass from a vessel, but that was rarely the case! Most observations were made from shore, with a transit and a small boat taking soundings, sent to the surveyor by flag signal! Sometimes the transit was out of position, other times there was a local magnetic interferance, and a lot of the time everyone was sick, drunk, or both!
Finally, the cartographer took all this data, reconciled it or collated it as best he could, throwing in a few personal opinions as humans are wont to do, and we're stuck with the results 100 years later.
I think the greatest wisdom lies on understanding all the sources of information available to you, including cruising guides and piloting information, and treating the mean with a healthy dose of skepticism.
As far as losing all your electronics in one cataclysmic flash, isolate your fall-backs, and don't forge on after a partial failure. Accidents happen when people sail on with accumulating minor problems, until they become overwhelming. On this I am an expert!

[RBEmerson]

Originally Posted by Captain Bill

I use electronic charts almost exclusively in US waters. The primary reason for this is that I can update them weekly over the internet. Paper charts are pretty much out of date before you get them unless you use "print on demand" charts. I personally don't have enough money to buy new POD charts every week. I have two independent GPS systems on board and a computer hooked to a 22" flat screen running Coastal explorer. I have no fantasies about the accuracy of the electronic charts, they are the same as paper charts. In the Bahamas I use the Explorer paper charts for fine scale and electronic for large scale. I have noted that Bahamas charts are often off by at least a few hundred feet. While near Lee Stocking Island a few years ago in the channel on the north side the chartplotter had us on land. When we came around the island we found a trawler yacht hard aground because he trusted the chartplotter in the dark. I'm sure that the GPS knew our actual position on the planet, but the chart was wrong. If I would have been plotting the GPS position on the paper chart I would have had the same problem. Using paper or electronic charts is no excuse for turning off your brain. If your dumb enough to try running your boat aground just because a chart plotter says your on the water, there's little help we can give you.

Two thoughts... first, you can always update US paper charts from Local Notices, now published on-line. However, unless you're a commercial operator, many of the changes aren't worth marking a chart up for.

Second, my sympathies to the trawler owner but I can't get too worked up about this. Sounds to me like someone totally ignored all the other clues, including the depth finder. Any radar on board? That might have helped, too. And, of course, no matter where you are, running into a strange harbor (or anchorage) at night isn't a good idea. Sorry, Mr. Trawler, but not that sorry.

[MarkJ]

I wonder if someone puts the boat on the rocks and kills a crew member and says he was using paper charts only (though he had a chartplotter on at the helm) that he is a stickler for the old ways and likes to go below to where the chart is, that he could be charged with negligence causing death?


We came into Sydney Harbour from the Pacific recently and its not easy for someone on first sight! Can someone ducking up and down the companionway transcribing GPS positions to paper and holding a hand bearing compass be anything more than a menace?


Asa hard copy backup to be used, fine, but a a primary nav scource? Its days must be dated?

Maybe to be allowed t use 5 miles+ offshore etc...


Mark

[Charlie]

Originally Posted by MarkJ

We came into Sydney Harbour from the Pacific recently and its not easy for someone on first sight! Can someone ducking up and down the companionway transcribing GPS positions to paper and holding a hand bearing compass be anything more than a menace?



Mark

This is one of those situations where you need to plan ahead. The plotter gives you instant feed back. that's why it is easier. Coming in from the Pacific you would need to plot a course to get you to witin visible range of the lighthouses. Going up and down thru the companiaonway would not be practical but knowing the bearing to the next buoy pre-written on the chart works. For Example Going into Port Jackson I would sail till I could line up the light house on Grotto Point with the lighthouse behind it near Chinaman's Beach and then steer a course of 294 and account for any current that might be coming down the coast.
I don't mean to imply that it is easy but it has been done for a long time w/o a plotter. I find it much easier to study charts on paper rather than electronic form.

[Ex-Calif]

Originally Posted by GordMay

You're right - you don't get it.

In the Bahamas and Caribbean, some of the paper charts are extremely old and contain errors of 500 feet or more.
In other areas of the world, the errors may be several miles.
Sometimes charts are inconsistent with each other, and the same latitude-longitude plot may locate your boat in two different locations on two overlapping paper charts.

Gord - I agree. But no matter how you fix your position the error will be the same. If a chart is wrong it is wrong.

If you are navigating by star shots or gps or by visual fixes, you are just as on the rocks with any method.

Originally Posted by Ex-Calif

Being able to shoot landmarks with a compass and triangulating a position is pretty easy and is a good skill to have for coastal cruising.

And now, "The rest of the quote..."

I would never, ever, ever try to enter a coastal area in the dark or fog that I had not been to before under electronic navigation alone.

It's crazy.

If everything was accurate we could eliminate channel markers.