Cruisers Forum
 


Join CruisersForum Today

Reply
 
Thread Tools Rate Thread Display Modes
Old 18-02-2010, 16:45   #76
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 17,314
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 is offline   Reply With Quote
Old 18-02-2010, 16:50   #77
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 17,314
Quote:
Originally Posted by Hiracer View Post
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.
__________________

__________________
barnakiel is offline   Reply With Quote
Old 18-02-2010, 17:13   #78
Registered User
 
bastonjock's Avatar

Join Date: Nov 2008
Location: Lincolnshire UK
Boat: Mac 26x
Posts: 169
Quote:
Originally Posted by Hiracer View Post
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
__________________
bastonjock is offline   Reply With Quote
Old 18-02-2010, 18:01   #79
Registered User

Join Date: Oct 2006
Location: The boat lives at Fidalgo Island, PNW
Boat: 36' custom steel
Posts: 992
Quote:
Originally Posted by bastonjock View Post
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??
__________________
John, sailing a custom 36' double-headed steel sloop--a 2001 derivation of a 1976 Ted Brewer design.
Hiracer is offline   Reply With Quote
Old 18-02-2010, 18:32   #80
Registered User
 
CarinaPDX's Avatar

Join Date: Jan 2010
Location: Portland, Oregon, USA
Boat: 31' Cape George Cutter
Posts: 1,674
It's the charts, not the GPS, that is the problem

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.
__________________
CarinaPDX is offline   Reply With Quote
Old 19-02-2010, 08:39   #81
Senior Cruiser

Cruisers Forum Supporter

Join Date: Aug 2009
Location: between the devil and the deep blue sea
Boat: a sailing boat
Posts: 17,314
Quote:
Originally Posted by CarinaPDX View Post
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!

Quote:
Originally Posted by CarinaPDX View Post
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.

Quote:
Originally Posted by CarinaPDX View Post
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.
__________________
barnakiel is offline   Reply With Quote
Old 19-02-2010, 09:32   #82
Obsfucator, Second Class
 
dacust's Avatar

Cruisers Forum Supporter

Join Date: Feb 2008
Location: Southeast USA.
Boat: 1982 Sea Ray SRV360
Posts: 1,743
Quote:
Originally Posted by CarinaPDX View Post
I am amazed at how everyone assumes the error is in the electronics, and not the charts.
Quote:
Originally Posted by barnakiel View Post
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
dacust is offline   Reply With Quote
Old 19-02-2010, 09:38   #83
Registered User

Join Date: Nov 2008
Location: Anchorage, Alaska
Boat: Newport 28
Posts: 136
Harumph!

Quote:
Originally Posted by bastonjock View Post
......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!
__________________
Alaska: We're here, because we're not all there!
AK_sailor is offline   Reply With Quote
Old 19-02-2010, 09:48   #84
Registered User

Join Date: Jan 2010
Location: Wells, Vt
Boat: 42ft Colvin Gazelle - TLA HLA
Posts: 504
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!
__________________
ConradG is offline   Reply With Quote
Old 19-02-2010, 10:48   #85
Senior Cruiser
 
s/v Beth's Avatar

Cruisers Forum Supporter

Join Date: Oct 2008
Location: Coos Bay, Oregon
Boat: Valiant 40 (1975)
Posts: 4,066
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
__________________
s/v Beth is offline   Reply With Quote
Old 19-02-2010, 10:58   #86
Registered User
 
bastonjock's Avatar

Join Date: Nov 2008
Location: Lincolnshire UK
Boat: Mac 26x
Posts: 169
Quote:
Originally Posted by barnakiel View Post
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
__________________
bastonjock is offline   Reply With Quote
Old 19-02-2010, 11:11   #87
Registered User

Join Date: Jan 2009
Boat: Charter
Posts: 171
Quote:
Originally Posted by s/v Beth View Post
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.
__________________
sec906 is offline   Reply With Quote
Old 19-02-2010, 11:12   #88
Moderator
 
Pete7's Avatar

Cruisers Forum Supporter

Join Date: Aug 2009
Location: Solent, England
Boat: Moody 31
Posts: 8,554
Images: 14
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
__________________
Pete7 is online now   Reply With Quote
Old 19-02-2010, 11:25   #89
Obsfucator, Second Class
 
dacust's Avatar

Cruisers Forum Supporter

Join Date: Feb 2008
Location: Southeast USA.
Boat: 1982 Sea Ray SRV360
Posts: 1,743
Quote:
Originally Posted by sec906 View Post
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
dacust is offline   Reply With Quote
Old 19-02-2010, 11:49   #90
Senior Cruiser

Cruisers Forum Supporter

Join Date: Jul 2006
Location: Chesapeake Region and Maine
Boat: 42' Bob Perry sloop
Posts: 4,038
Images: 4
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
__________________

__________________
btrayfors is offline   Reply With Quote
Reply

Tags
sextant

Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Australians: Maxsea Time Zero and Furuno Hardware - Save money and Buy from the USA! TrevC Navigation 21 25-06-2011 02:43
Trade: Ocean-front Maui Condo Time for Caribbean Cat Time jbilotta Meets & Greets 5 29-07-2009 02:04
Best Time to Buy a Used Charter Boat? SailingYoungs Atlantic & the Caribbean 2 12-05-2009 08:28
Time to buy Macopa Dollars & Cents 3 09-05-2008 13:25



Copyright 2002- Social Knowledge, LLC All Rights Reserved.

All times are GMT -7. The time now is 08:31.


Google+
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.
Social Knowledge Networks
Powered by vBulletin® Version 3.8.8 Beta 1
Copyright ©2000 - 2017, vBulletin Solutions, Inc.

ShowCase vBulletin Plugins by Drive Thru Online, Inc.