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Go Back   SailNet Community > General Interest Forums > Seamanship & Navigation
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Seamanship & Navigation Forum devoted to seamanship and navigation topics, including paper and electronic charting tools.


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  #21  
Old 02-17-2010
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Quote:
Originally Posted by Omatako View Post
...[GPS] should never be depended on in proximities closer than 1 mile. Many charts are not that accurate.
There are areas in the world with inaccurate or often reduced precision U.S. mapping - however that is not the case for the coastal U.S. Here, chart and GPS correspondence is very close on the coasts with a few known (reduced precision) areas. Unfortunately, the Bahamas is one of these.

I take your point to, "open the eyes and look", but you should know that anyone's ability to locate their position on a paper chart, using coastal piloting techniques will be significantly less accurate than WAAS GPS, except perhaps, again in reduced precision chart areas (non U.S.).

Last edited by wwilson; 02-17-2010 at 09:10 PM.
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  #22  
Old 02-22-2010
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Quote:
Originally Posted by wwilson View Post
. . . . . but you should know that anyone's ability to locate their position on a paper chart, using coastal piloting techniques will be significantly less accurate than WAAS GPS, except perhaps, again in reduced precision chart areas (non U.S.).
A lot of things are significantly less accurate than WAAS augmented GPS.

AFAIK WAAS is only available in the US and there's a heck of a big world out there.
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  #23  
Old 02-22-2010
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Just a note of thought: The pre-GPS Navigators use celestial until they sighted land and then used Coast Piloting... Yes! I've done this many times and never been confused as to my position.
GPS is a good tool for navigation as long as you back it up with other means. Now that Loran has been killed by the present administration. And the RDF Stations have been turned off years ago. That leaves you with only two options.
1. When far out to sea you use Celestial.
2. When Running along a coast, the ICW, or Rivers, bays and Sounds you use Coast piloting.
Remember Your GPS is only good until the batteries die.
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  #24  
Old 02-22-2010
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Originally Posted by Omatako View Post
AFAIK WAAS is only available in the US and there's a heck of a big world out there.
Omatako you make a valid point though, I thought the European & Japanese systems were up and running and compatible.

MSAS for aviation use was commissioned on September 27, 2007
The official start of EGNOS/SBAS operations was announced by the European Commission on 1 October 2009

My real point however was less a tribute to GPS, than a reminder that there is a whole lot of error in taking compass bearings then transferring them to a paper chart even if the coastal pilot ignores the inherent error that map/chart makers estimate and provide.

A + or - 2˚ compass sighting of an object 1nm away is pretty good on flat water and unobtainable on a pitching deck. That alone can produce 47 meters of error!

A 0.5mm wide pencil line on a 1:10K scale chart adds another 5 meters of error, @ 1:20K 10 meters of error, @ 1:40k=20meters of error, @ 1:80k=40 meters of error.

NOAA has specified the accuracy for its nautical charts in terms of the accuracy with which features are plotted on the chart from their original surveyed position. The plotting positional accuracy of most features is approximately 1mm at chart scale. To put this into perspective, at 1:20,000 scale 1mm is 20 meters of error and at 1:80,000 scale 1mm is 80 meters of error.

You are an advocate of coastal piloting techniques - me too, but it doesn't produce "ground truth" only another estimate with a pretty good error chain of its own!
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Old 02-22-2010
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Actually when running along the coast you can use cell phone triangulation as a backup for GPS. Sort of like a short-range LORAN.
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Old 02-22-2010
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Also one minor detail: The position you plot is a history of where the boat has been. Remember your boat is moving and those hard won positions is to affirm that you are on your track line and when to tack before you run aground.
So whether you get that fix via compass bearings, radar ranges or the GPS (and insuring that position's depth it says that you are at is in agreement with your Fathometer.) Use them wisely.
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Old 02-23-2010
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Quote:
Originally Posted by wwilson View Post
A + or - 2˚ compass sighting of an object 1nm away is pretty good on flat water and unobtainable on a pitching deck. That alone can produce 47 meters of error!
I guess the point that I would make is that if you have something that is 47 metres away, you should have no interest left in the chart - only in whatever is 47 metres away!

There is an old adage that I have always applied to navigation - "You are NEVER where you think you are". As far as my navigation goes, if there is something near me that represents a danger to me, nothing beats a look out the window.
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Old 02-23-2010
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Omo: that's fair... but if the danger is invisible, then all you have is the line you drew on your chart. I understand this is frequently an issue in piloting.
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Old 02-23-2010
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Quote:
Originally Posted by Omatako View Post
I guess the point that I would make is that if you have something that is 47 metres away, you should have no interest left in the chart - only in whatever is 47 metres away!
"Something" is not 47 meters away. It is a nautical mile away - with an uncertainty of 47 meters. And that may be for each LOB of the three used to make the fix.

I thought the reasoning that brought us this far down the thread was: Don't trust GPS in a coastal waters - use coastal piloting techniques. [#20]

I will cease and desist now, but reiterate one last time that GPS, with or without WAAS, is more accurate than coastal piloting can be. (for all the reasons stated above) No one said anything about not looking out the port holes! If GPS fails, then coastal piloting is a perfectly reasonable alternative, and always a useful check of a working GPS fix.

Last edited by wwilson; 02-23-2010 at 10:51 AM.
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Old 02-23-2010
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Quote:
Originally Posted by wwilson View Post
"I will cease and desist now, but reiterate one last time that GPS, with or without WAAS, is more accurate than coastal piloting can be. (for all the reasons stated above) No one said anything about not looking out the port holes! If GPS fails, then coastal piloting is a perfectly reasonable alternative, and always a useful check of a working GPS fix.
Sorry WWilson, But I have found that my skills at coastal piloting is and always will be more accurate then GPS. Many times I have had the GPS had me more then a hundred yards on the beach and not in the center of the Bayou that I am transiting. So your above statment leaves much to be desired. I believed my eyes and Coastal plotting and not the GPS.
The GPS depends on Good Antenna placement, Good Antenna connections, good batteries, proper setup and A strong Second means of navigation, such as being SKILLED in Coastal navigation. And to become skilled in CP is Practice, Practice, and Practice.
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