You phrased the question badly. If everything is displaced along with the
rock it is rather straight forward to correct if you know of the error. If
a rock is mischarted or uncharted you are correct. I think mischarted
rocks are very rare. Mostly placed badly with everything else or missed
completely.

On the west coast we often deal with rapidly rising land masses...often at
angles of more than 60 degrees with reference to the horizontal. You better
be going real slow if you expect to get a depth sounder alert.

This is actually a place where the GPS operator is at a disadvantage. A
piloting operation is relative to local landmarks while GPS is
absolute...so if the GPS guy is not aware of the error he has a problem in
comparison to a piloting operator.

On the other side of the equation in is reasonably easy for a gps operator
to maintain someting close to optimal clearance of hard stuff. It is a
complex and difficult task for a pilotage operator to do the same.

Jim Donohue

"Dave" wrote in message
...
On Fri, 21 Jan 2005 08:15:10 -0800, "Jim Donohue"

said:

In many places you use the numbers from last time. Many of the numbers
are
published in guides or privately. You can also set up the course to
minimize exposure.

In general the errors are area wide. You work out the correction from
known
objects. You use radar and the bottom to assure yourself you did it
correctly. Go slow when in doubt.

Not responsive to the question. The question was:

OK, how is that GPS going to help you avoid hitting that rock that's
shown
in the wrong place on the chart?

The responsive answer would be that a GPS is as likely to send you into
that
uncharted rock as any other method, but there are other ways to avoid the
rock that's somewhere other than where the chart shows it, like a depth
sounder, and plotting a course well away from known rocks.