Jack Dale wrote:


My lack of faith in GPS was reinforced when the chartplotter showed
my boat on land while safely anchored stern-to in Princess Bay on
Wallace Island.

On the other hand, I did navigate through the rocks in Race Passage in
last year's Swiftsure using GPS. I had a paper chart in front of me
while I did it.

Jack



This is the problem/situation that many are noting, especially those
using chart plotters.
During most piloting exercises where we're underway, many minor
discrepancies between the chart plotter position and actual will not be
readily apparent as they are relatively small and due to the fact you
are normally giving a "safe berth" to most points you are passing, of
little consequence.
However, once you are anchored or moored or even working around a tight
docking situation, these discrepancies DO become readily apparent.
In most cases, I'm dealing with chart plotters on different vessels (all
gyro stabilized) that are using same/different/similar electronic
packages and unknown chart data (some charts I know to be older versions).
Depending on the vessel, I've noted errors of from @10' - 100' of a
variable nature (sometimes between trips, sometimes between vessels).
in this particular port. The most obvious being when alongside the dock.
Personally, when piloting, naturally my first choice is eyeball, but if
I have a GPS readout handy to where I'm standing I use it to confirm
speed and get a backup to my sense of set and drift, and where I have a
chart plotter to look at, I glance at it for a "birdseye" view, though I
put more weight on the "birdseye" view from the radar where accuracy is
concerned, as long as the particular radar picture is clear.
Naturally, what I'm discussing is for a particular port. Each port and
set-up will vary/differ .... my main point is that you should use
everything at hand, be aware of possible drawbacks to each and make
maximum use of the positives.

otn