Time to Buy a Sextant ?

[barnakiel]

BTW For all those who are interested in Harrison story I can recommend the book by Dava Sobel 'Longitude', there is also a movie based on it (but I do recommend the book).

b.

[barnakiel]

Originally Posted by Hiracer

You still have addition and subtraction, even with sight tables.

;-)))

I was not aware the case was this hopeless ...

On the second thought, even Columbus could add and subtract ...

;-)
b.

[bastonjock]

Originally Posted by Hiracer

I have had the chartplotter locating the boat on dry land while cruising in Alaska.

And closer to home in nearby Canadian waters they have had the boat about 300 yards farther south than actual location.

Them chartplotters are just an invitation to disaster if you don't regularly second guess them. These people who transfer their video skills to navigation scare me.

I have a powerboater friend who is into all things electronic. He has multiple very expensive chartplotters. That they might incorrectly situate his boat relative to land or rocks has literally never occurred to him, and he will not listen to me because I'm perceived as a Luddite.

And I swear he may runover some kayakers some day.

I use my chartplotter for guidance,last year i was on the coast heading for the Humber and my CP showed that i was at the "gap" in an area known as the Binks,my eyes told me different,i was 50-75 meters away from the gap,i dont know if it is down to poor electronic charts or a poor GPS signal,unfortunately one of the navigational problems on the coastal area where i sail is shifting sand banks

[Hiracer]

Originally Posted by bastonjock

I use my chartplotter for guidance,last year i was on the coast heading for the Humber and my CP showed that i was at the "gap" in an area known as the Binks,my eyes told me different,i was 50-75 meters away from the gap,i dont know if it is down to poor electronic charts or a poor GPS signal,unfortunately one of the navigational problems on the coastal area where i sail is shifting sand banks

I've heard the the story about people who have the coordinates for channel buoys. They plug those coordinates into their autopilot and then have the unfortunate experience of actually hitting the bouy. Where's the inaccuracy when you need it??

[CarinaPDX]

I am amazed at how everyone assumes the error is in the electronics, and not the charts. Most of the world's charts are at least partially based on pre-GPS data, and in many cases they do indeed date back to the sextant/chronometer days. Many boats have been lost because they trusted the GPS to keep them out of trouble and ignored other errors. Many charts are using pre-WGS-84 references, and in some cases they note corrections to Lat-Lon to align them with the new reference; unfortunately this doesn't correct errors in coastline shape, etc. (This is true whether it is done manually or by changing the map datum in the GPS.)The US (NOAA) makes available all US charts for free online, including both vector and raster, updated every few days. It has been a Herculean effort to convert the old paper charts to the modern vector format, aided by outside contractors over a period of years. But most of the vector charts are merely digitizations of the old data and contain the old errors. It will be decades before the US, and the rest of the world, have vector charts derived from precision GPS-based raw data stored in databases and referenced to a single map datum; for now most of the raw data is on old vellum and paper records. As I said before, the prudent navigator NEVER relies on a single instrument, and that includes the charts. Seeing your boat in its slip on the chartplotter just gives a very false sense of confidence - the marina is new data.

Edit: on second thought, sometimes it is the electronics. The GPS is perfectly capable of being 100m off at times. For grins, leave the GPS on for a day or two in the slip with tracking turned on. Most positions will cluster within a few meters but note the large outliers.

[barnakiel]

Originally Posted by CarinaPDX

I am amazed at how everyone assumes the error is in the electronics, and not the charts.

Oh, no - actually, there is a split with great many, if not vast majority, of forum members agreeing strongly with your point!

Originally Posted by CarinaPDX

Many boats have been lost because they trusted the GPS to keep them out of trouble and ignored other errors.

Many more were lost before the GPS era though. I bet introduction of the GPS did reduce the number of navigation-related accidents.

Originally Posted by CarinaPDX

It will be decades before the US, and the rest of the world, have vector charts derived from precision GPS-based raw data

I think it will be ages. I cannot see any job being done towards this end (unless you mean geo-referencig of the nearest gas station and the KFC booth there).

And for grins, with the waning economy of the US I bet it will be the Chinese who will get the job done.

I say God bless America, for the GPS.

Cheers,
b.

[dacust]

Originally Posted by CarinaPDX

I am amazed at how everyone assumes the error is in the electronics, and not the charts.

Originally Posted by barnakiel

Oh, no - actually, there is a split with great many, if not vast majority, of forum members agreeing strongly with your point!

barnakiel - I agree.

CarinaPDX - well said. The entire post.

Here is my take on Chart Errors. It discusses paper vs. electronic and raster vs. vector.

-dan

[AK_sailor]

Originally Posted by bastonjock

......last year i was on the coast heading for the Humber and my CP showed that i was at the "gap" in an area known as the Binks,my eyes told me different,i was 50-75 meters away from the gap.......

Your GPS was 50-75 meters off??? Harumph! No doubt the sextant users could have done better than that!

This thread is both entertaining and informative!

[ConradG]

So what have we learned? I think the most advantageous thing we can take away is that to rely on any one form of getting information is folly, even the charts we relate that information gathered to can't be relied on. We need to use what we can the best we can and realize it is all their in an effort to help us do what we want to do but ultimately we must take responsibility for our actions. The more tools we can use, so long as they are used correctly, the better our situational awareness will be!

[s/v Beth]

It's about time for us to all surround the fire and sing cumbaya...
But before we we do, there should be a progression of things we trust. How about using your eyes first the compass second and occasionally looking at the chartplotter and/or chart. Sextant is for (with me anyway) when I have no other visual clues that I trust. Or when I am really lost. Am I going to learn to use it? You bet.
FYI- I found one under a bunch of old parts in a new to me boat

[bastonjock]

Originally Posted by barnakiel

BTW For all those who are interested in Harrison story I can recommend the book by Dava Sobel 'Longitude', there is also a movie based on it (but I do recommend the book).

b.

Looks like i missed the first part,but the second part of the movie is on Brit TV tommorrow

[sec906]

Originally Posted by s/v Beth

It's about time for us to all surround the fire and sing cumbaya...

I would, but I'm not seeing the fire on my chart plotter.

[Pete7]

Yes, good book, my Dad is reading it now. Topical thread, I have just picked up a 20 year old Davis Mk 15. The box even came with the cardboard rapping around the outside, so don't think it has ever been used. Just waiting for some decent weather in the UK and I will take the fishing rods down to the beach and have a play.

Whilst a nice brass or ali one would be nice, if a plastic one will work it means more funds for other projects on the yacht and the Memsahib won't complain.

Pete

[dacust]

Originally Posted by sec906

I would, but I'm not seeing the fire on my chart plotter.

See, here's another example of someone not knowing what they are doing.

Obviously you are using outdated charts. Otherwise you'd see the fire.

If you were using a sextant you could find it easily. Just hold it in your hand and move it around. When it gets warm, you are near the fire.

-dan

[btrayfors]

Brief summary of this thread follows:

1. Sextants are old technology, but in the right hands might still be useful....occasionally.

2. GPS is incredibly accurate, usually, but not always.

3. Charts are usually pretty good, but not as good as GPS.

4. GPS-only affectionados are brain dead.

5. The Mark One eyeball is a primary navigation tool, but is not infallible and may be increasingly underutilized.

6. Using only one source of navigational data isn't a good idea. Even for "experienced" navigators.

7. Sextants are (choose one or more):

• fun
• outdated
• expensive
• cheap (plastic)
• not too accurate
• too much trouble to learn
• great fun to play with
• well proven as navigational tools
• cool.

Those who have a curious mind, the explorer's spirit, a sense of history, and don't take themselves or modern technology too seriously will likely embrace celestial navigation.

Those who don't, won't.

Brain-dead gadget freaks obviously won't. They're also likely to interface their GPS to their autopilot -- and every other electronic instrument aboard, including the AIS and the TV and the Palm Pilot -- whence they can just choose a couple of waypoints and do nothing more until they hit their destination. Or, literally, some intermediate obstacle.

The sea is the sea....same as it's always been for thousands of years. Just waiting to teach another lesson for those who didn't get it the first few hundred times.

Is there any more to say?

:-)

Bill