I have just been examining my charts for Baja and Mexico.
Some are US Department of Mapping stuff, some are from the Baja to Panama
Chart Booklet.

The charts some of which date back 20 years have datums:

Local
Local Astro
NAD 1927.
WGS 72.
WGS 84.

The most recent printings generally state WGS 72 or 84 on them.

I think that the only satisfactory solution is to use the offset feature in
most GPS's to relocate your origin point such that the chart you are using
lines up with reality.
And then reset it when you move charts.
My post of the User Datum feature in the Garmin's describes how to do that.
Other machines have similar methods.

At 03:03 PM 8/23/02 -0700, you wrote:

Capt. Mike Maurice
Near Portland Oregon.

Howdy All,

When installing a map cartridge into a GPS chartplotter, does the
chartplotter automatically switch the datum it is using to match up with the
chart you are viewing?  I could find no evidence of this feature on either
the C-Map, Garmin, Interphase or Furuno (now there is a worthless website)
websites.

Most of the most recent NOAA charts appear to use
the WGS84 datum.  I was looking at the C-Map NT+ web information and
discovered their
Mexico charts use a mix of NOAA and CMP (?) series charts.  I'd be willing
to bet that NOAA and CMP(?) use different datums even though they are on the
same cartridge.

I searched the Mexican hydrographic office website and found no reference to
"CMP" as a
hydrographic survey organization nor did I find any reference to the datum
used on
the CMP (or any Baja Peninsula) series charts (unfortunately my Spanish
stinks).  C-Map and CMP seem
suspiciously similar.

I've been playing with different datums in my handheld and find that many of
them move me much more than a few seconds.

Arild, Captain Michael Maurice and Keith have been more than kind in giving
me guidance on this subject.

For sure, there are documented problems with digitizing and quality control
by the chart cartridge companies.  Taking that into consideration, I'm
starting to think that some of the problems with chartplotters showing a

datum of the

chart.  If the plotter doesn't switch the datum automatically and is using a
chart with something other than the WGS84 datum, that may be a source for
part of the inaccuracy.  If we could somehow learn the datum used for the
Baja charts we might be able to put the boat back in the drink, where it
belongs.

You folks that cruise the Caribbean are on your own.  I'm having enough
trouble with Spanish websites, let alone those written in the Queen's
English.

Regards to all (TGIF),

Frank & Claudette Weismantel
Elverta, CA
Boatless for a little while longer

REPLY
When a paper chart is digitized as either a raster or vector format the process
includes applying a block shift so that  the default WGS 84 in th eGPS is correct for
the displayed charrt.

Do not get confused by the facct a raster chart may show NAD 27 in the part
showing the printed title block.
As displayed on the screen it is corrected by a block shift to WGS 84.
Same thing for vector charts in C-Map and Navionics etc.
While a paper chart for the same area may still be drawn to an older datum the
elecrtonic version is corrected.

REPLY
Not so, see above note.  C-Map or Navionics or Furuno has made the shift if the
Mexican  government is still using some other datum.

If the Baja charts have a discrepancy, the problem may be that  the actual datum of
that particular  area is not known in suffficient  detail to  apply a block shift correctly.

Arild