Traditional or digital navigation - which is safer?

[Oceansailor]

Originally Posted by El Pinguino

Q?

What is more important?

Your Lat/Long or your relationship with terra??

No doubt relationship to terra:-)

[jtsailjt]

Originally Posted by Oceansailor

You are correct that the paper charts are as much out of alignment as the digital charts.

But when you navigate in the traditional way you estimate your position in relation to land, lighthouses etc. The alignment is not so important. As soon as you do not do visual navigation the alignment error will be a problem.

When you do electronic navigation your position will be placed on the wrong place on the chart as the chart is not aligned.

When I navigate with digital charts I still use visual cues such as landmarks and lighthouses, etc. just as if I were using a paper chart and I think that’s a pretty common practice amongst the people I sail with. I learned ‘BG’ (before gps) and I found the accuracy of LORAN to be unreliable and mine just gave me LORAN coordinates as opposed to being a plotter type, so I can’t help myself from using everything available to me. Yes, that was traditional back when all we had was paper charts and even before then, and I hope it remains a common practice no matter how technologically advanced our nav systems become. It’s amazing how much useful info you see when you remember to ‘look out the window’! That’s one reason I like digital charts, they give me at least my approximate location with no effort or time with my head down plotting points on a chart, so I have more time available for what you call ‘traditional’ navigation.

But even if digital charts are being used in an area where the chart is not aligned properly, it’s usually still about as accurate as celestial nav on a good day. The mistake some digital navigators make is assuming that they’ll always have that 10’ or 20’ accuracy that’s usually available using their chart plotter so they attempt to do things almost nobody would have dreamt of 25 years ago such as entering unfamiliar harbors on a moonless night or trying to thread their way through the 100’ wide break in the reef with the sun in their eyes, etc. and are surprised when they hit the reef or find themselves on the wrong side of the channel markers. In other words they forget that once in a great while the ‘magic’ doesn’t work quite as advertised. But when using paper charts you intuitively understand that the width of your sharpened pencil lead is probably more than 50’ so you’re less likely to attempt to blindly thread your way through a 100’ wide opening based solely on your superior plotting/DR abilities. But I don’t blame overconfidence in and misuse of digital charts on the technology itself. It still comes down to operator error when you hit the ledge.

[chrisr]

i must confess i have not read this thread right to the beginning, but obviously the correct response is to use all means available

however a side issue (that may already have been raised here, and certainly has been raised elsewhere) is the problem of getting an accurate GPS fix and plotting such on a chart that is less accurate ie you know exactly where you are, but the reef you are trying to dodge is not where you think it is.

in the old days, a celestial position on a small boat accurate to within a couple of miles was normal or better, so it didn't matter if the reef was half a mile out.

nowadays, we know exactly where WE are so can be tempted to cut the corner...forgetting that what we are dodging might not be where we think it is.

personally i never come within 5' of an obstacle, unless in waters i know well - no matter what form of navigation is being used.

cheers,

[Pizzazz]

Originally Posted by chrisr

...but obviously the correct response is to use all means available

I fail to see the obvious part in the correct response. One can measure depth using a lead line or a depth sensor. Both are available on a typical boat. Should we use all means available to measure depth? Take depth readings with a lead line once every couple of hours and confirm that the depth sounder is calibrated correctly?

Too much information is just as bad as too little information.

[Bycrick]

My point was always, as others have said, use all of the available information all the time. That doesn’t imply that you should use an electronic depth sounder while you’re throwing your lead line.

One of my most scary entries was into Esmeralda, Ecuador. I had a Raymarine plotter with CMap. I also had all the paper charts available, but the one for Esmeraldas wasn’t available. There was no large scale chart data for Esmeraldas. I anchored in the large ship anchorage within spitting distance of a marker buoy charted in 30’. When I finally got the port captain on the radio, he gave me a course to steer to find the entry channel. I went around the buoy and found 2’ of water, the hard way. Followed the port captain’s course. Two depth sounders went from 30’ to 10’ to No Bottom. At that point, I was WORRIED! Fortunately, I was advised to just follow a fishing boat in. I did, past the breakwaters, past the sand bars, dry land apparently on either side and in front. No Bottom on the depth sounders. The Navy sends out a boat. Follow me, he signals. The electronic depth went from No Bottom to 12’ in a couple of boat lengths. I finally got tied up and had a good stiff rum. When I finally talked the Navy out of a current chart, well used but better than I had, I discovered a very, very narrow 400’ deep channel all the way into the entrance to the fishing boat harbor. That made it obvious why neither of the depth sounders could find bottom: the depth of the water combined with the signals bouncing almost horizontally when they did hit bottom. They chart was wonderful, a 2002 edition. It was also not WGS84 datum, but (I think) Venezuelan 1957 and quite a bit off in position.

I love electronic navigation, but I think that it’s important that you understand its limitations and potential problems. The same is true for traditional navigation. Just because I have an electric screwdriver doesn’t mean that I threw all the other away.

[Benz]

Originally Posted by jtsailjt

If you're cruising in an area where the charts are misplotted, doesn't that apply equally to paper charts as it does to digital charts? The chartplotter knows your latitude and longitude and height above sea level and it knows the latitude and longitude and height of the rocks. Unless somebody fouled up or didn't have the technology to plot the rocks accurately during the survey (long ago), then your chartplotter will show where the rocks are just as your paper chart will. You might have to steer a slightly different course to get there than you first thought but that's true for paper charts as well if the variation has changed significantly enough to be noticeable. In most sailing conditions other than flat calm, I can only steer plus or minus about 5 degrees anyway so I'm always correcting to get my course line pointed to where I want to go. It's MUCH easier to stay updated on my constantly changing position, and to know what might be an appropriate correction using up to the split second info available via digital means than it is to take time out to try to take a celestial shot, do the math to figure out where that puts me, then compare where I thought I'd be at the time of the shot, guess how accurately I've been steering my intended course and also guess whether my new shot or my DR plotted position is probably more accurate, and then figure out what course to steer in order to get myself back on course and then to know when I am back on course.

BTW, you say you sail more often where charts are not well aligned than you do where they are well aligned and I'm wondering where you've been doing most of your recent sailing that would cause you such problems? You must now be a VERY long ways from Bristol RI in a place where there is cheap enough Internet access to fairly regularly post on this forum, but in a place where the charts, or at least digital charts aren't accurate enough to safely navigate by. And you say it's like this more often than not. Where are these places? How far off are they and how much worse accuracy does that yield than when you determine your position from a celestial shot aboard a moving sailboat and then use DR until you have an opportunity to take another celestial shot and plot that position? Also, on a 31 foot sailboat with 4 people, how do you find room to carry all the detailed paper charts to navigate to all these remote spots so you have accurate information (more detailed and more accurate info) than on a chartplotter?

What you're missing is that the surveys were done before the geoid was fixed, so the lat/long of the rocks is off. However, if you're piloting along a shore with an ancient chart, and taking bearings off of landmarks, you don't give a hoot about lat/long; you care more about range and bearing. Try and fix your position by GPS, and you might hit the rocks; fix it by bearings, you'll be all right.
I'm actually not far from Bristol just now, but currently land-based and with a good internet connection. When I do cruise, I'm often terribly far from internet, and haven't the machinery to overlay Earth images over radar shots on a screen. My last significant cruise was around Newfoundland and the Cote du Nord; even the most recent Canadian government charts admit that islands are not where charted, an leave vast blank spaces labelled: "not surveyed to modern standards." To enter those spaces we had to scrounge an old Admiralty chart, surveyed originally by James Cook, and pilot around with that. Worked perfectly, but we never plotted a GPS position on it.
Other places we cruised where charts and geoid were misaligned: Pacific coast of Mexico and Central America; San Blas archipelago; Yucatan Peninsula. I'd say I've cruised as many miles with misaligned charts as with perfectly aligned ones.
As for cruising a 31-foot boat with 4 other people--sure, it was tight, and tighter as the children increased in size and eating capacity, but by the time we moved (temporarily, I trust) ashore, we had onboard paper charts from SF to Panama, the entire Caribbean, US East coast, Nova Scotia, Newfoundland and St-Lawrence River, as well as western England, Ireland, and some assorted Spain, Madeira, and Med charts. Turns out paper is pretty flat, and you can get a lot of charts in a small locker if you stow with care. If it were just me and the good lady onboard, we could easily pack complete charts for an RTW with side trips.
To sum up, then, and answer your broader question: on an inaccurate chart, a GPS position will be just as good or better than a sextant position, in every situation where I'd be using a sextant. But when land is in sight, and all the clutter of reefs and rocks and shoals approaches, I normally would put away the sextant and would pilot using other means. I would even use the GPS if I was inclined to, but would treat the position with suspicion if I though the chart and geoid were misaligned.

[Bycrick]

To show how little is known about the things people talk about, the magnetic model distributed by the governments of the US and the UK is not simply a database of position and variation. It’s a series of software implementations for various operating systems and a series of coefficients which are computed to find the variation at a given position.
It sounds like the tide prediction programs. Certainly some software developer might choose to run the model to create a simple database of position vs variation which is installed in their product, but then you just have more potential unknowns in the process.

[Bycrick]

With regards as to how good is the chart data. I do most of my sailing now in Las Perlas, Panama. The only current marine paper chart/chart book is based on surveys of that area from the 1970s or earlier. The last series of real marine charts for the area is from 1976. As an aside, I’d love to find anybody that still has a set of charts 21605, 21606, 21607, 21608 and 21609. They’ve been out of print for decades, but are the best charts of the area that I’ve seen. Panama prints a set of topographic maps with some soundings, but they’re all in UTM grid not lat/Lon. There are some cruisers guides to the area, but would you trust your boat to them? With more out-of-the-way places like Islas Ladrones, I’ve been unable to find charts. Doing it the old way is sometimes the only way.

[jtsailjt]

Originally Posted by Benz

What you're missing is that the surveys were done before the geoid was fixed, so the lat/long of the rocks is off. However, if you're piloting along a shore with an ancient chart, and taking bearings off of landmarks, you don't give a hoot about lat/long; you care more about range and bearing. Try and fix your position by GPS, and you might hit the rocks; fix it by bearings, you'll be all right.

.

To sum up, then, and answer your broader question: on an inaccurate chart, a GPS position will be just as good or better than a sextant position, in every situation where I'd be using a sextant. But when land is in sight, and all the clutter of reefs and rocks and shoals approaches, I normally would put away the sextant and would pilot using other means. I would even use the GPS if I was inclined to, but would treat the position with suspicion if I though the chart and geoid were misaligned.

I’m actually not missing that the rocks are plotted in the wrong place, I just realize that it’s the same misplotted chart whether it’s displayed on paper or digitally on a screen. You are assuming (and we all know where that leads) that those of us who prefer our charts to be displayed digitally aren’t ‘looking out the window’ just as you say you do and instead are just blindly driving the little boat on our chart plotter without paying any attention to the fact that the reefs aren’t located as depicted in areas where we know the chart and geoid are misaligned. Even in areas where they aren’t misaligned, I’m always double checking to confirm that what I see on the screen matches what I see around me by visual cues and/or radar or by bottom contours. Whether you prefer paper charts or digital ones, it’s always wise to be skeptical about any single source of info and you seem to get that but I wonder why you seem to assume that those of us who prefer digital charts don’t.

[Paul L]

Originally Posted by Bycrick

With regards as to how good is the chart data. I do most of my sailing now in Las Perlas, Panama. The only current marine paper chart/chart book is based on surveys of that area from the 1970s or earlier. The last series of real marine charts for the area is from 1976. As an aside, I’d love to find anybody that still has a set of charts 21605, 21606, 21607, 21608 and 21609. They’ve been out of print for decades, but are the best charts of the area that I’ve seen. Panama prints a set of topographic maps with some soundings, but they’re all in UTM grid not lat/Lon. There are some cruisers guides to the area, but would you trust your boat to them? With more out-of-the-way places like Islas Ladrones, I’ve been unable to find charts. Doing it the old way is sometimes the only way.

Those charts are still available ondemand printing
https://frugalnavigator.com/products/21605

[Demonius]

For me it's going to be digital with multiple redundancy. I'll still know how to work with paper but unless there is a catastrophe, I won't be pulling out a paper chart. If the catastrophe is that bad that my three GPS systems fail then I reckon I'm probably in more trouble than a paper chart is going to save.

[conachair]

Originally Posted by Bycrick

With more out-of-the-way places like Islas Ladrones, I’ve been unable to find charts. Doing it the old way is sometimes the only way.

Available online through sasplanet.
Navionic on left, cm93 right.


Google sat image not great resolution for here but yandex plenty good enough.
Datums seem close enough.

yandex sat on left, cmap right.


Sasplanet to mbtiles is such a fantastic resource, though copyright must be a little vague...

[Cpt Pat]

Originally Posted by requiem

From the safety side I don't think there's a good argument that the old-fashioned methods are safer than using real-time satellite positioning data. The amount of error introduced, both spatial and temporal, by traditional methods is huge in comparison.

That may not matter until you find land someplace unexpected. But, as others said, the underlying concepts must be understood and there's a reason for phrases like "all available means". Getting caught up in paper vs. electronic is almost a red herring in that regard.

As an aside, I was reading a bit about the "automation dependency" problem in aviation recently; someone suggested that a wise approach to addressing the brain switching off might be to design software that works with the pilot rather than for them.

A GPS (GNSS) driven chartplotter will tell you exactly where you are. The paper chart will show you the hazards. Those two don't always agree. There can be surprisingly large disagreements between GPS and the chart.

I'm a commercial pilot. I learned to fly with only analog instruments (50 years ago). In today's aircraft, a large part of the pilot workload in a new aircraft is just learning the "user interface" of all the digital electronics. I recently asked a 787 captain what it's like to fly that aircraft. He said: "I don't know. But ever since I transitioned to it, I've learned how to type 100 words per minute." Here's a good airline pilot training video that describes the problem:

https://www.youtube.com/watch?v=5ESJH1NLMLs

.

It was made in 1997. The problem still hasn't been solved.

Pilots are forgetting how to fly. They are becoming "systems managers" and "redundant components." The same could happen to sailors.

For a really harrowing example, see: https://www.vanityfair.com/news/busi...ight-447-crash. That crew made mistakes that would shame a 20 hour flight student.

[Bycrick]

Thanks for the feedback. The 21605 chart is current. The other are a set of 4 small scale charts of only Las Perlas. They’ve been out of print since 1976. But I will contact frugalsailor direct and see. As for the Ladrones, the charts you mentioned are better than nothing, but not much. The last time I visited, I told my crew to drop the anchor when the depth sounder indicated 40’. He did and I backed down to set the anchor. When we dived to cool off, we found a white sand bottom at 40’, but the anchor was on top of a white-sand-covered pinnacle that rose to within about 10’ of the surface. It wasn’t on any of the charts, including CM93 and wouldn’t have been visible on satellite photos.

That doesn’t keep me from visiting the islands, it just means that I’m not using the electronic toys to tell me what’s happening.

[requiem]

Originally Posted by Cpt Pat

Pilots are forgetting how to fly. They are becoming "systems managers" and "redundant components." The same could happen to sailors.

For a really harrowing example, see: https://www.vanityfair.com/news/busi...ight-447-crash. That crew made mistakes that would shame a 20 hour flight student.

Yes, those are some of the articles I was reading! Safety goes up in aggregate, but skill goes down, leaving a chance for normally-preventable catastrophic errors. I've noticed similar in rock climbing; basic skills are lost, but things like assisted braking belay devices cover up much of those errors.