[PUP] Lady Anne (W40) Hits Rock In Glacier Bay

[2elnav at netbistro.com]

Ron makes the point that "It is my belief that there is only one way 
available to us to fix
the position of an obstacle and then, only in reference to other immovable
charted feature - that's with radar."

I respectfully suggest  that doing so would place the radar equipped vessel 
directly on top of the rock.
Otherwise you need to play with offsets.  A technique  that introduces a 
whole new set of problems.
>From the photos, this rock looks to be  close to other visible shorelines. 
That means   more traditional methods are available to accurate positioning, 
such as horizontal bearings to other charted objects.   As to whether  these 
other objects are also  misplaced   due to datum conversion problems  is 
another issue. These can be done from a dinghy or small craft  able to float 
over the rock at various stages of tide.

On the subject of echart plotting.   Might I suggest  placing a circle 
around all potential hazards before laying down a route.
The radius of these circles would be  whatever margin of safety  the 
navigator is comfortable with.  ( 500 feet  -1000 feet)
Once all potential hazards have been so marked the actual course track plot 
can be done.  Potential  danger points  also include turning points in the 
plot.
If for any reason  a course must be plotted  in between  two  closely spaced 
danger circles, that is a good reason to note this as a location requiring 
special attention by look outs  and the navigator.  Or it suggests DON'T GO 
THERE - find another route

Admittedly I have not played with all the various chart plotter  programs 
presently available; but the ones I  worked with  before had  the ability to 
mark a point with a circle.


regards
Arild









----- Original Message ----- 
From: "Ron Rogers" <rcrogers6 at kennett.net>
To: <WillardBoatOwners at yahoogroups.com>
Cc: <passagemaking-under-power at lists.samurai.com>
Sent: Monday, June 08, 2009 12:57 PM
Subject: [PUP] Lady Anne (W40) Hits Rock In Glacier Bay


> First, for an excellent account of this incident, I refer all to <
> http://www.sailblogs.com/member/annehays/?xjMsgID=91112 > Anne's excellent
> narrative and critique of this regrettable incident. All is well and they
> are hauled and preparing for a repair.
>
>
>
> In her critique of the incident, Anne brings up points worthy of 
> discussion.
> First, she makes reference to the fact that the charted position of the 
> rock
> is wrong. It is my belief that there is only one way available to us to 
> fix
> the position of an obstacle and then, only in reference to other immovable
> charted feature - that's with radar. Insert all the necessary caveats. If
> you have the right software and GPS, you can locate a rock with absolute
> accuracy by having the software throw out all the "outliers" using the
> underlying GPS criteria and using the locus of the position over time.
> However, that is for the modern surveyor.
>
>
>
> The underlying survey is probably very old and likely inaccurate by 
> today's
> standards - even with an adjusted datum. So you cannot really say the rock
> is 138 feet from where it is supposed to be. Only a NOAA survey vessel or 
> a
> CG buoy tender likely has the right equipment and software for absolute
> accuracy. The school solution is to give charted obstacles and hazards a
> wide berth. Anne's point about Nobletec's plot line overwriting and
> obscuring the hazard is a good one, but would not obtain if the navigator
> had chosen an offset from the hazard greater than the known extreme error 
> of
> GPS. Had there been a hazard marker, their radar would have helped them
> avoid the rock.
>
>
>
> In sum, Willard's are very tough boats, it takes several things to 
> conspire
> to create a tragedy, and neither charts nor GPS are absolutely accurate 
> for
> practical purposes. Oh, and the USCG is wonderful.
>
>
>
> In case I'm coming across as a know-it-all; I wish to point out that with
> Selective Availability (SA) turned to zero in the first Gulf War, I 
> blasted
> between ledges at 7.5 knots under sail on the Western edge of Nantucket
> Sound. Had the ledge locations been off from their position on the charts, 
> I
> would have gotten to test the strength of my hull too.
>
>
>
> Ron Rogers
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