Len Krauss wrote:
No, Dennis, nothing embarrassing found. Was just waiting for more
input from everyone.

I was beginning to wonder if magnetic variation could possibly have
something to do with the position discrepancy. My GPS is set to
automatically adjust for it. I've downloaded for my Capn software the
latest magnetic variation data table. I concluded, however, that
variation would only influence magnetic heading displayed, and not
lat/lon. In other words, it has nothing to do with the problem. Does
that sound right?

Yes.

The only other thing I can think of is that chart datum and GPS datum
need to be in agreement. Chart is NAD83 and with GPS set to NAD83,
position error unchanged. Same with GPS set to WGS84. Datum is the
only kind of "calibration" possible, as far as I know.

Len

As far as I am aware, Garmins download all data from the satellites in
WGS84, and setting any other datum in the set-up program only changes the
numbers of Lat./Long. in the display.

If this is correct, the sentences being passed to your charting program are
in WGS84, and the chart datum is set to NAD83. However, since you maintain
that at that location, the two datums are virtually the same, there should
be little, if any discrepancy in the displayed position.

I am not familiar with the Cap'n software, but in Ozi, you have the facility
to change the datum of the displayed chart, i.e. the Lat/Lon overlay, and to
change it back again at will. Asssuming you change it to a datum which the
chart was not calibrated to, and there is a position difference between the
two datums, all that happens is that the positions of buoys and other marks
changes relative to the basic graphic. I usually place these to 2 decimals
only, since Reeds and other almanacs only gives the positions to 2 decimal
places (in UK).

Since the Lat/Lon shows up immediately I move the mouse cursor over a buoy,
if the third decimal place is anything other than a zero, I know that
something has changed, and usually this is the datum of the map/chart.

The many facilities for self-calibration etc. in Ozi enable the experienced
user to work out if something is not just as it should be, but with other
software it may not be possible to dig deep into the program and suss things
out. Datums, calibration, and projections are the usual culprits, plus of
course an assurance that the basic graphic is dead accurate (not always the
case, particularly with NASA satellite photographic downloads).

My guess is that the problem lies with the basic graphic, but with so many
potential variables, it's one hell of a job finding out. You could of course
take a precise reading on a buoy on the chart, and compare this to the
position shown in your almanac, just as a check.

Do you have another chart of the same area? If not, tell me precisely where
you are and I'll send you one, even if you are at the South Pole! How's that
for an offer?

BTW, for the record, I too use a GPS48, but have recently acquired a GPSMAP
60CS. Let me assure you that whatever else is wrong, it ain't your GPS! My 2
instruments are usually within .002 of longitude of each other, which I
make to be about 7 feet! (at 56 deg N.!)

Dennis.