"Jim" wrote in message
. ..
Paul wrote:
Has anyone seen a ship position as reported by AIS being off by over one
nautical mile?

snip
Any thoughts?

Thanks,
Paul

This is really off the wall but is there a possibility that the datum
being used was that far off? Isn't the LAT/LON calculated for the chart
datum?
Jim

I don't think so, since the chart datum isn't an issue unless the positions
are being shown on a chart (whereas my display is similar to a radar
screen). Even so, my boat's position relative to the ship's position should
be displayed correctly regardless of any datum discrepancies. What I've got
is a situation where a ship that is physically to the north of me is
transmitting a latitude that is to the south of me.

I'm not completely ruling out cockpit error or bad code on my part, but I
can't find it and the raw data seems to exonerate me. If anyone wants to
help figure it out, here is the minimum NMEA data capture that shows the
situation. The first line is my position, and the second is the AIS message
from the ship:

$M2RMC,225040,A,4038.518,N,15149.375,W,5.6,072.5,0 40806,014.5,E,D*6
!AIVDM,1,1,,A,15@HsT001wE8wopG@0K5:3=N0@L6,0*6

The "$M2" has been substituted to indicate which multiplexer port the data
came from.
My position is 40.642433 deg (N), -151.820917 deg (W)
The reported ship position is 40.630167 deg (N), -151.822000 deg (W)
The range and bearing to the ship are 0.7NM, 183.8 deg true

Trust me, the ship was actually to the north. I have photos!

-Paul