"Geoff Schultz" wrote in message
6...
"Paul" wrote in
:


"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

I really don't know much about AIS data and how often ships report, but do
other reports from that ship show it moving in a reasonable manner?

-- Geoff

Since this was a "ships passing in the afternoon" situation, I only had the
the one encounter with them. It looked like their reported longitude was
more or less correct -- their position was moving from west to east in a way
that matched what I was seeing with my eyes. It was just that the latitude
was consistently off by perhaps a minute.

Someone has mentioned (on the Panbo blog) that they saw a ship in San
Francisco reporting her position as being in the middle of Treasure Island
(a small island in the bay), so perhaps this problem isn't unprecedented. I
still have no idea what could be causing this error, other than an
un-reported GPS failure causing the position reporting to be running in
dead-reckoning mode. That would be two unrelated failures, or a failure and
a design flaw. In any case, I'm definitely going to keep using my eyeball
to verify what the AIS reports.

-Paul