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Old 18-05-2008, 19:52   #31
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Don,

Wow! That's a huge difference...much more than one would suspect, given the thousands of vessels traveling to Bermuda in recent years.

Could you please give us:

1. The date of the chart 26030 you were using.
2. The datum shown on the chart (WGS84, NAD83, etc.).
3. Other charts you compared (their numbers, dates, datum, and correspondence to your radar position)?

This would be a big help.

Many thanks,

Bill
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Old 18-05-2008, 19:54   #32
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Originally Posted by ssullivan View Post
Can you really? I almost never disagree with DefJef, but in this occasion I have to.

Do you have a leadline?

Do you have something to throw overboard and count off the seconds as it passes from bow to stern to estimate your speed through the water?

If so, can you do this single-handed while leaving the helm because autopilots don't work without electricity?

If you can't do the above, you can't plot your positions and use dead reckoning, therefore, I submit that you can't navigate without electricity on your boat.

So... by extension, saying electronic charts aren't reliable due to electrical issues isn't a good argument - since the average cruising boat can't be navigated without electricity in this day and age anyway.

I don't disagree with you sean that most boats need electricity, but i still think that's a mistake and I still carry the items you mention (and a wind vane ) and yes I can do just fine without electricity.
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Old 18-05-2008, 21:43   #33
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Originally Posted by ssullivan View Post
People often make the statement that electronic navigation will fail you because it requires electricity to work. Then again, most modern yachts (unfortunately) can't be run without electricity for any length of time anyway:

*Refrigerators storing food
*Starter motors
*Electric windlasses
*Lights to read your paper chart by
*12VDC Solenoid to open your propane connection to eat
*Depth sounder
*Knot log and wind
*Compass light
etc...etc...
We consider refrigeration to be a luxury, and for luxury items. We can live just fine without it and make sure the boat is stocked with this in mind. It's not hard, people have been doing it for thousands of years.

We don't need our engine except to pull into some marinas, so not having a starter motor is not a big deal, since we don't stay in marinas. Not having an engine might change your plans, but you cartainly shouldn't be stuck. What ARE those big poles sticking up out of your boat for, anyway?

Our windlass is manual (does yours not have a manual backup???)

We have an oil lamp (and a supply of fuel) for the cabin and as another as a back up anchor light.

The propane solenoid can always be bypassed in an emergency.

We have a lead line.

We have a mechanical "propeller log" in case of instrument failure, and know how to use a chiplog. And are you really suggesting that an electronic wind instrument is essential to navigating the boat????

Compass light, hmmmm.... might have to squint. But if you are sailing with a windvane, you aren't checking the compass but occasionally anyway. and the compass mounted in the cabin can be read by light of the oil lamp.

We have a manual pumps for both the bilge and for potable water.

Anyone who is planning a long ocean passage NEEDS to be sure they can get to a civilized destination without electrical power. What would you do if you left Mexico for the South Pacific, a 3 week passage, and a week out you lost your power? Call for help? You would have no alternative if you had only electronic charts, and no way to anchor, and no way to steer and no way to....

I'm not saying that sailing without electrical power is FUN. But if your boat CAN'T, maybe you should keep your trips no more than one full battery away from a harbor with full repair capabilities!

And, there really is only one way to know if you can live without electricity, and that is to turn it OFF for a week and see. You'll be surprised what you find.

People say I worry too much, but the things I worry about, never happen.

Bill
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Old 18-05-2008, 23:54   #34
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Quote:
Originally Posted by donradcliffe View Post
Here is a case in point:

Today I came into Bermuda for the first time. Using SeaClear with chart 26030, it showed my position as 13 miles south of Gibbs Hill on the south side of the island. The only radar return I had was 21 miles out. Further investigation and comparison with other charts showed that the coordinates of Bermuda on chart 26030 were 8 miles out of position!
That is quite a huge error Don but just confirms the kind of example Calder gave in his article:

“And sometimes there are outright mistakes. On a recent cruise in the West Indies from St. Lucia to Bequia, using latest-generation electronic charts in a chart plotter, we found fixes in St. Lucia were pretty accurate;

but those along the coast of St. Vincent were consistently off by a half mile in terms of their longitude; and those in Bequia were once again pretty accurate. The track of our chartered boat took us over the hills of St. Vincent and through a church and fort!

My guess is that when the chartmaker converted the chart datum of the source paper chart into WGS 84, a simple mistake threw everything off by half a mile.

This kind of thing is a bit shocking.

We get the same thing here in the Philipines
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Old 19-05-2008, 02:20   #35
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Wouldn't some of the original surveying date from many years back? - pre GPS, especially in the less commercial areas?......ie that the Survey team were never recording data within 3 foot accuracy.

But Bermuda being 8 miles away is a bit of surprise to me. Was the GPS / Chartplotter using the same datum as the Chart?
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Old 19-05-2008, 03:10   #36
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Is this a problem with electronic charts per se or with just the non official ones?

For official charts if there is an error on the electronic chart I would have thought that the error would also be on the paper ones as the data comes from the same hydrographic database and is subject to the same high level of quality control.

Obviously, if it is an official raster chart then the same error definitely exists on the paper version but just less likely to cause a problem if navigation is by bearings off charted features rather than celestial (which would need a big chart error to have a problem) or GPS, as long as it is just a whole chart positional shift.

So maybe the problem is just with the poor quality of the second level electronic chart producers charts (which, by the way, cannot be used for navigation by SOLAS vessels - and I would not use them myself either, but that means a PC has to be used, not a pleasure type plotter) and not of the official ones?

Regarding Bermuda, for which another poster reports a chart error, a client of mine found that a chart (can't remember which one without hunting back through my files) from one of the better known non official electronic chart producers (I won't name them because this was around 3 years ago) had a significant error in that it was correct at one end of the group and considerably out at the other - but from my memory that was around 1/2 nm, not miles.

I did some work for him so that he could approach the chart producer who it turned out seemed to be aware of the problem and said it would be fixed in the next revision of the electronic chart but could not say when that would be - so that sounded to me like a production fault rather than a hydrographic data fault. Anyways it must have been fixed because some time later while I was there doing some other work the chart was correct.

In unfamiliar waters I am always very wary even in well charted waters, whether paper or electronic charts as neither can be guaranteed to be complete. In waters I regularly cruise in even here in NZ where the official charts are of high quality there is quite a long reef extending out from an island and which dries and covers with the tide - it is not shown on the chart as anything but a short shallowing of the depth contours. The first time I entered the waters near it the reef was covered but, fortunately, there were some shags sitting on the outer rock, feet under water, else I would have gone straight over the top of it.
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Old 19-05-2008, 04:50   #37
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interestingly I transfered some co-ordinates from google to my chart plotter of an
un-surveyed harbour entrance on the west coast (NZ) to use as a means of lining up the channel into the harbour and found that the 2nd waypoint put me 1/2 a mile up a hill side, definitely not where I wanted to be. Time to re-evaluate and identify land marks from the google pic rather than trust the co-ordinates.
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Old 19-05-2008, 05:01   #38
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What is pretty bizarre is that GPS is used for doing land surveying now and you would have thought that with its accuracy that by now the planet would have been completely surveyed with GPS and the data made it's way into BOTH new paper and digital charts.

There is no reason for charts to be inaccurate by the amount people are reporting. A whacky GPS received could conceivably report itself in the wrong position, but by now charts should be pretty precise.

When one thinks about how many people are effected by this you would have thought this would be a high priority for governments. Obviously they don't care.
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Old 19-05-2008, 05:20   #39
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I think these are just clerical mistakes that have not been fixed yet.

That is my whole point in all of this….. to remind the newbies that large mistakes are still out there and not to blindly follow their chart plotter
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Old 19-05-2008, 05:23   #40
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Much of the modern-day charting of the Lesser Antilles in the Caribbean use the Admiralty charts developed in the time of Lord Nelson and Admiral Rodney as source data.

Why haven't they been updated through hydrographic surveys? Well, the folks who live here don't use the charts anyway. They navigate by eyeball and long experience, in daylight or at night. Why spend the money to hire a foreign survey firm to update charts that they don't need, when so much else of a critical nature goes begging for public funds? Like schools, water supplies, roads, etc.
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Old 19-05-2008, 05:35   #41
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You are *very* well prepared. Excellent work.

The point I am making is not about my own boat, it's about the majority of boats on this forum and in the world.

They cannot be navigated without electricity anyway, so saying electronic charts are any worse than paper just because they use electricity is kind of um... misguided.

If you are without electricity, you are stuck when it comes to navigating unless you are as prepared as GreatKetch, who is properly prepared for the situation.

(I am partially prepared for the situation)

Lastly, we all may be talking about different things when we speak of "electronic charts."

I am speaking of NOAA issued raster (BSB) and ENC charts, downloaded directly from NOAA last week. I would venture to guess my electronic charts are a long sight more accurate than anyone's paper charts, as I have the newest charts available, including new wrecks, etc... You don't have these new wrecks and other information if you didn't buy your paper charts in the past couple months. Talk about a hazard to navigation! Outdated information is the biggest hazard there is.

Anyway, I do think we're talking about different things when it comes to "navigation with electronic charts." Some are talking about taking an old paper chart surveyed by Magellan himself and sticking it into the software, then expecting it to be somehow more accurate than it was when created.

Obviously this can't be true.

Any error in the chart will be an error on the screen as well.

I'm also getting the feeling some of the people "against" electronic navigation are people who don't use all their senses... as in every possible input to navigation:

*Eyes
*Depth
*Markers
*Color of water
*Ripples on water
*Sounds
*Chart
*GPS
*Standard Compass
*Speed/Time
*Landmarks on horizon
*Running fixes
*Knowing where you are 24/7, etc...

If you aren't using all this input, and just sitting there steering a boat based on what you see on a screen with only that single input to navigation, well... duh! Of course you'll crash. No different than sitting there looking at a paper chart with your head up your #ss and not using the other inputs.

The bottom line is there is no difference at all between electronic navigation and paper navigation, with the exception that you can react more quickly and plot out routes more quickly with a computer than you can by hand. Also, finding charts is faster.

And... the point of my original post was to say that the argument against electronic charts due to them needing electricity is kind of mis-guided, since most boats (GreatKetch excluded) are not set up to navigate without electricity anyway.


Quote:
Originally Posted by GreatKetch View Post
We consider refrigeration to be a luxury, and for luxury items. We can live just fine without it and make sure the boat is stocked with this in mind. It's not hard, people have been doing it for thousands of years.

We don't need our engine except to pull into some marinas, so not having a starter motor is not a big deal, since we don't stay in marinas. Not having an engine might change your plans, but you cartainly shouldn't be stuck. What ARE those big poles sticking up out of your boat for, anyway?

Our windlass is manual (does yours not have a manual backup???)

We have an oil lamp (and a supply of fuel) for the cabin and as another as a back up anchor light.

The propane solenoid can always be bypassed in an emergency.

We have a lead line.

We have a mechanical "propeller log" in case of instrument failure, and know how to use a chiplog. And are you really suggesting that an electronic wind instrument is essential to navigating the boat????

Compass light, hmmmm.... might have to squint. But if you are sailing with a windvane, you aren't checking the compass but occasionally anyway. and the compass mounted in the cabin can be read by light of the oil lamp.

We have a manual pumps for both the bilge and for potable water.

Anyone who is planning a long ocean passage NEEDS to be sure they can get to a civilized destination without electrical power. What would you do if you left Mexico for the South Pacific, a 3 week passage, and a week out you lost your power? Call for help? You would have no alternative if you had only electronic charts, and no way to anchor, and no way to steer and no way to....

I'm not saying that sailing without electrical power is FUN. But if your boat CAN'T, maybe you should keep your trips no more than one full battery away from a harbor with full repair capabilities!

And, there really is only one way to know if you can live without electricity, and that is to turn it OFF for a week and see. You'll be surprised what you find.

People say I worry too much, but the things I worry about, never happen.

Bill
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Old 19-05-2008, 05:37   #42
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Ahh... and sorry, Pelagic. Your post makes sense for newbies who have their nose down in the computer (or paper chart) and aren't using their senses to navigate. There is a little thread drift going on here. Your post was a good one.
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Old 19-05-2008, 06:03   #43
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Thanks Sean, I was wondering where you were going with all of that, but since you have been stuck up the creek without a paddle, I'm happy to make allowances LOL
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Old 19-05-2008, 06:35   #44
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After a night's sleep and a cup of coffee, I took another look at the SeaClear II chart in which Bermuda was off by 8 miles. The chart is DMA 26030, "Bermuda Operating Areas", and is a BSB Chart modified 9/5/1997. The Datum printed on the chart is "Bermuda 1957 Adjusted Astro Datum", but a datum note on the chart indicates the corrections to WGS72 are less than 0.1 minute.

On a hunch, I moved the cursor over to the scale on the side of the chart, and found that when I put the cursor on 32 degrees 10 minutes north, SeaClear indcated 32 degrees 2.126 minutes. The error was consistent throughout the range of the chart and there was no error in longitude. The conclusion is that BSB screwed up by 8 minutes of latitude when they digitized the chart.

I'm not sure where I got the chart--it may have been from the NOAA website, but the Admiral wants to use the wifi, so that will have to wait till later. The lesson is that errors exist, and you should use all the navigational inputs available and not just depend on your chartplotter.
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Old 19-05-2008, 06:38   #45
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Quote:
Regarding the scaling of the chart the electronic charting system (ECS) I use allows viewing the chart at "Compilation Scale" which looks to me to be the same as the same paper chart (although I have never bothered to hold a a paper chart beside it to check for sure). Despite what I took to be the inference in the article that the charts peculate at this, the charts are perfectly sharp when so displayed (in fact it is the default display).
FWIW, all US charts are created from digital systems. They have not produced charts by paper in more than 20 years. The paper charts you buy are maintained in a vector format that is proprietary to Map tech (the contractors to NOAA). They use this database format to create the paper and ENC charts from the same data.

The concept that paper charts are at a resolution of 1000 dpi forgets the fact the shrink and stretch of the paper is greater than that. What is exactly accurate is the concept of over zooming an electronic chart. Much of the map bse is created from aerial photos. The highest zoom level possible from an aerial photo is 10 times the negative scale based on a 9 inch by 9 inch negative. The film itself is about 1000 DPI and after a 10X enlargement there is no accuracy left that you don't already see. Even the ENC charts have the nominal scale to them that you should be able to tell how far they can be zoomed with reliability. It's stated as the scale of the old fashion paper map it represents. What you will see is that the ENC charts look better when over zoomed where the BSB charts will begin to look fuzzy. The true accuracy lies at the zoom level where the BSB chart still look sharp. The scale accuracy is not any better than what the printed map would show if it was a perfectly new copy.

If you find coordinates in the Coast Pilot they are in fact as accurate as they are stated to be as these are single points located by GPS. Much of the photo work is ratioed and rectified using GPS. That means the control points are dead on. The problem is the map won't tell you where the control points are so you can't know what parts of the map are most accurate. You have to assume the inaccuracies are spread evenly and take any location as a pretty darn good guess and not an absolute position of a single point. The relationships between points should always be accurate but your location on the map may be in some degree of error.

Google Earth photos are stretched to fit together and not ratioed (adjust for the altitude) and rectified (adjusted for the tilt). The edges of the photos would not line up if they were so they make them pretty by "rubber sheeting" the edges so they match. Who could know except a sailor trying to navigate by one? I like to examine things on Google Earth but I would never pick coordinates off them - ever. Guessing might be safer.
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