On Sun, 19 Mar 2017 19:59:50 -0700, Jeff Liebermann
wrote:

On Mon, 20 Mar 2017 07:48:44 +0700, John B.
wrote:

I know nothing of U.S. mapping but I can assure you that using marine
charts there is a definite problem, in some cases, in comparing GPS
positions with existing charts. One of my friends favorite anchorages,
in the S. Philippines, is on dry land according to the current marine
chart of the area :-)

To the extent that some, perhaps many, charts include offset
information for use with GPS.

There are plenty of ways to screw up a map. The most common is to use
the wrong datum. WGS84 is the most common for GPS. However many
countries have their own. For example, the USGS maps are still NAD27
and are sloooooowly converting over to NAD83. At one time Google Maps
and Google Earth used the wrong datum for Santa Cruz CA and managed to
shift the entire city to the east by 200 ft. That wasn't fatal for
land navigation, but allegedly caused a problem when some fisherman
tried to navigate his way into Santa Cruz yacht harbor in the fog and
missed by 200ft.

Looks like the Philippines uses PRS92 datum.
http://georepository.com/datum_6683/Philippine-Reference-System-1992.html
There is also the Luzon Datum of 1911:
https://en.wikipedia.org/wiki/Luzon_Datum_of_1911
There are also various zones in the Philippines to be considered:
http://tool-online.com/index/systemes-coordonnees/philippines.html

My guess(tm) is that you'll find that the GPS is set for one datum
(probably WGS84), while the printed map is based on PRS92 or earlier.
The document mentioned in the above Wikipedia page footnotes, "Status
of the geodetic infrastructure of the Philippines" looks interesting
and will probably have come clues on conversions and errors. However,
the link doesn't work.

Looks like some tools are available:
https://www.google.com/search?q=convert+prs92+to+wgs84

Looks like the difference is -128 meters E-W and -67 meters NS.
http://georepository.com/transformation_15708/PRS92-to-WGS-84-1.html
That's plenty of room to put your friends anchorage on dry land.

There are other sources of error, but this is the most likely.


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

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.

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 :-)

--
Cheers,

John B.