Larry,
I can't see that this is overly significant. Please note that the color swing is from +200 NT to -200 NT..(1 NT = 1T x 10 -9)
The Earth's magnetic field typically ranges between 3 and 5 T x 10 -5, at least according to Wikipedia. .I use a magnetic compass
as a backup only. The cost of satellite compasses today is very affordable. and some also have dead reckoning and a fluxgate built
in incase of signal loss. This beats the hell out of the donut you create when the autopilot is slewed to a fluxgate and you sail
over a sunken power cable. This can be very exciting.
Steve

"Larry" wrote in message ...
"Steve Lusardi" wrote in
:

It's wonderful, but how do you use it?
Steve

"Larry" wrote in message
...
http://www.geomag.us/models/WDMAM/WDMAM_NGDC_V1.1.pdf

No WONDER the damned compass won't compensate!



--
"iPad is to computing what Etch-A-Sketch is to art!"

Larry



I'm not sure. It's calibrated in nanoteslas + and - so there seems to
be pushing and pulling the card at various magnetic levels that keep
shifting, it seems. Zooming in on the Charleston-Savannah area, there
are several strong magnetic anomalies of BOTH directions.



This chart may also cause the compass to spin in both directions at
times....a very worrying condition:
http://www.kitco.com/images/live/gold.gif


--
"iPad is to computing what Etch-A-Sketch is to art!"

Larry