On Oct 28, 8:45 am, Mark Borgerson wrote:
...The data was NOT
averaged over the 1-minute interval.
....
Maximum Position Error: 32.0 feet.
Average Position Error: 9.6 feet.
....

I've found that with my GP-31 (an older unit w/o WAAS) with 2 second
position averaging that in practice the GPS makes an amazingly
accurate anchor monitor. What happens to your data when you average
pairs of your 1 hz data?


1. With a modern GPS and a well-placed antenna, GPS precision
should not be a limiting factor for anchor alarms if you
are in an area with good WAAS corrections. ...

Really, you don't even need WAAS. I've lived at anchor for years on
my boat that holds all of my possessions in remote places with little
hope of outside help if I get into trouble. I take this seriously. I
don't think you can get better relative position data from anything
short of horizontal sextant angles and I wish you well getting them in
the dark.

2. Anchor alarm radius has to account for swinging
to the anchor in normal winds ...

The anchor alarm should be based on your position from the anchor not
from the position the boat takes after anchoring. I mentioned how I
do this with my GPS in my previous post.

3. Waking to the alarm with a wind or tide change is probably
preferable to setting the alarm radius so large it
will accomodate such changes.

Your alarm should not go off unless your anchor is moving.


4. GPS anchor alarms tell you nothing about the
position of other boats in the anchorage. ...

I guess I'm lucky since I don't usually have to anchor so close to my
neighbors that this is a problem. Of course, if swinging will put you
onto a coral head or some other hazard you need to restrict your
swinging. If you need to know if the wind or tide is shifting (say
you're anchored in a roadway) you should set the anchor alarm to the
boat's position rather than the anchor's and it will tell you when you
swing.

-- Tom.