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  #11  
Old March 20th 17, 11:33 AM posted to rec.bicycles.tech
John B.[_3_]
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On Sun, 19 Mar 2017 23:59:36 -0700, Jeff Liebermann
wrote:

On Mon, 20 Mar 2017 12:25:07 +0700, John B.
wrote:

The marine GPS' that I've used were all WGS84.


Yep. That's the usual default setting.

Generally speaking those who sail outside the U.S., are using British
Admiralty charts, or copies there of. I used to buy Thai charts from
the Thai Navy and they were based on Admiralty charts. I don't
remember but I think that they were not WGS84.


NOAA nautical maps use NAD83 (which is very close to WGS84).
USGS uses NAD27 but is slowly moving to NAD83.
Google Earth uses WGS84.
Geocaching uses WGS84.

Here's what the military thinks of "civilian" charts:
http://msi.nga.mil/MSISiteContent/StaticFiles/Files/NautChrts_GPS_index.htm
"Isolated datums, such as those used to position many islands
in the Pacific Ocean, can be in error by a half mile or more
(see figure). The datum shift to WGS 84 can be quite large,
depending on the area of the world and the local datum in use."

See the chart of Farallon De Pajaros Island, which requires a 1/2
nautical mile shift for the map to agree with GPS.
http://msi.nga.mil/MSISiteContent/StaticFiles/Files/island.jpg

But datum aside, I had a copy of a chart of an island in the S.
Pacific and the notes stated, it was based on surveys made by the HMS
something or another, in 1790-something. I always thought that if I
ever got onto the S. Pacific that I would approach those islands in
the daylight with great care :-)


Possibly Captain James Cook, who went through the south pacific
between 1768 to 1771.
https://en.wikipedia.org/wiki/European_and_American_voyages_of_scientific_explor ation

:-) No, it was later than that :-)

I have a nice Tamaya sextant, out of date HO 229/249 tables, and some
charts. It's not very practical these days, but it does help one
understand how such things work.


Way back when, I went to considerable effort to learn proper
navigation. Sextant, HO tables and lessons from the lead navigator in
a B-52 squadron. What sort of took the shine off the effort was when I
did the usual three shot position and got a "cocked hat"that was about
a mile and a half on each side. When I told the Major about it he
commented that I was doing real good. I replied that I didn't think
that a triangle that was a mile and a half on each side wasn't very
accurate he assured me that it was "pretty good for celestial
navigation.... which is why we don't use that for the B-52's" :-)
--
Cheers,

John B.

 




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