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Old 17-05-2017, 09:44   #1
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If Pilot Charts are "Not to be Used for Navigation" Then What is?

I have yet to read much more on celestial navigation than the samples of what's available on Google Play's book selections. Apparently a hard copy on the subject is best Anyway I was looking at pilot charts online and they say they "are not to be used for navigation"; okay, well, what is to be used for navigation, then? I know at least some people use pilot charts to mark their progress across the oceans but is there something else I'm missing? I'm guessing the accuracy of GPS way out in the big blue is laughable but then that probably isn't all that critical for small sailboats. I might not have a GPS with me anyway since I'll have to have paper charts anyway and I'm not likely to want to fork out thousands on a chart plotter when there is so much more I could buy that would be more important.
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Old 17-05-2017, 10:16   #2
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Re: If Pilot Charts are "Not to be Used for Navigation" Then What is?

Marking your progress and navigating are different things.
Pilot charts cover very large areas. Many details are not shown because of the scale. Small islands, reefs, etc, aren't shown. What were curved longitude lines become straight. Also a Mecator projection greatly stretches the horizontal distances at the top and bottom. Using a chart covering a small area reduces the stretch errors. You might read up on how charts are made and what A Mecator projection is and great circle charts. When I plot star shots, I use a plotting chart, blank except for latitude and longitude lines and comes in particular latitude ranges. The resulting fix is transferred to a conventional chart. (Old school way)
GPS is just as accurate in the middle of the ocean as near shore or on land because it is satellite based. Older systems like Loran A & C and others used towers on land to transmit. The ground signal only traveled about 500 miles. After that sky waves were used that were less accurate, but good enough for open ocean navigation.
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Old 17-05-2017, 10:26   #3
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Re: If Pilot Charts are "Not to be Used for Navigation" Then What is?

"Pilot Charts" as issued by the NGA, depict averages in prevailing winds and currents, air and sea temperatures, wave heights, ice limits, visibility, barometric pressure, and weather conditions at different times of the year.
The charts are intended to aid the navigator in selecting the fastest and safest routes with regards to the expected weather and ocean conditions. The charts are not intended to be used for navigation.

Navigational Charts such as those issued by NOAA in the United States come in a variety of formats, and are intended as a direct aid to navigation. The info including lights, buoys, depths, etc are updated on a regular basis and published in local "notice to mariners".

Hope that helps...
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Old 20-05-2017, 18:11   #4
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Re: If Pilot Charts are "Not to be Used for Navigation" Then What is?

Quote:
Originally Posted by Lepke View Post
Marking your progress and navigating are different things.
Pilot charts cover very large areas. Many details are not shown because of the scale. Small islands, reefs, etc, aren't shown. What were curved longitude lines become straight. Also a Mecator projection greatly stretches the horizontal distances at the top and bottom. Using a chart covering a small area reduces the stretch errors. You might read up on how charts are made and what A Mecator projection is and great circle charts. When I plot star shots, I use a plotting chart, blank except for latitude and longitude lines and comes in particular latitude ranges. The resulting fix is transferred to a conventional chart. (Old school way)
GPS is just as accurate in the middle of the ocean as near shore or on land because it is satellite based. Older systems like Loran A & C and others used towers on land to transmit. The ground signal only traveled about 500 miles. After that sky waves were used that were less accurate, but good enough for open ocean navigation.
I am fairly familiar with coastal charts and the electronic versions thereof.

I am just asking this because I want to know what I would need to cross from the Canaries to the Caribbean. Not entirely sure but I'm guessing the only real obstacles to vessels with drafts under 5 feet would be areas with unfavourable weather. Never heard anyone mention anything about reefs or islands on that route.

My GPS being inaccurate comment was related to the discrepancy between satellite fixed positions and the charts used by the GPS. If I'm not mistaken many areas are not surveyed very well nor very recently and many use different methods to give positions etc. I'm assuming this is especially true mid-ocean as there's probably not much need for pinpoint accuracy most places.

What charts would you use for a crossing like I am contemplating?
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Old 30-05-2017, 03:48   #5
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Re: If Pilot Charts are "Not to be Used for Navigation" Then What is?

...it's actually nice to see the daily progress on the oceanic chart!
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Old 30-05-2017, 04:36   #6
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Re: If Pilot Charts are "Not to be Used for Navigation" Then What is?

There are, or used to be, a series of large-scale paper charts specifically for ocean passages. You could mark your passage on them without all the distractions of wind roses and other clutter that the pilot charts sport. With the right one of these, and one or two charts of your destination, you could be all set for an Atlantic crossing. Personally, I'd have at least some chart coverage of the whole eastern caribbean and the coast of S. America, just in case I decided to make landfall elsewhere than originally planned (this has happened to me more than once--weather, circumstance, and whimsy being what they are, it's good to keep options open.)
There also exist some beautiful great circle sailing charts--I have one of the entire Atlantic Ocean--that are meant for passage planning and on which you could plot your position, and which record all significant rocks and obstructions.
The reason for the disclaimer on the pilot charts is because their primary purpose is not course plotting, but weather information. If someone were to pile up on, say, St. Paul's rocks there in mid-atlantic, and try and grouse that they couldn't see them on the chart because of the clutter of wind roses and what-all, the publishers could say: "We told you not to try and navigate with these, fool!" and not get sued.
It's the same as a screwdriver. They're not meant for opening paint cans, but everybody uses them for that. If you know what you're doing you can open cans all day without damaging the lid. But it's still not the tool's intended use. Same with pilot charts. Personally, I'd hate to mark up my nice pilot atlas with pencil marks, since I intend to use it again and again (I also make my paper chart markings as discreet as possible to avoid confusion on future passages.)
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