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

[2elnav at netbistro.com]

Good point Mike.
ECharts are derived from  paper charts  which are  created to be used at a 
certain scale.
Unfortunately with eCharts most users forget  this basic  element of chart 
creation.  The ability to zoom in simply aggrevates the  situation.

Ther are rules  governing  how much detail to include for a given scale. 
All hydrographic offices follow similar rules  in guiding  them  in creating 
a chart for a given area.

When we  went out  to do revisory surveys  the hydrographer in Charge  told 
us what scale we shoudl be working to .  Our field note work was always done 
to four times the detail level  as what th efinished weork woudl be 
displayed at.   So a finished chart  at  40,000 : 1  woudl be surveyed to  a 
10,000 : 1 scale.   That told us  when to include or ignore some details. 
If we didn't  we would be cluttering up  our field work notes  needlessly 
and create a nightmare  for the  cartographer  in the office to sort 
through.
Arild




----- Original Message ----- 
From: "Mike Maurice" <mikem at yachtsdelivered.com>
To: "PUP" <passagemaking-under-power at lists.samurai.com>
Sent: Monday, June 08, 2009 2:06 PM
Subject: Re: [PUP] Lady Anne (W40) Hits Rock In Glacier Bay


> Jeff Siegel's Active Captain and charts shows the position the boat's GPS 
> shows where it struck and the location where the rock is as almost 
> indistinguishable. The chart scale is 80K to one.
>
> In other words, 2 waypoints; one each at the rock and the boats position 
> overlap such that at 100 percent zoom you can barely tell them to be two 
> separate waypoints.
>
>
> The difference is about 100 feet north and 30 feet west.
>
> The only way to get the dots to separate is to over zoom.
>
> The scale of the chart is simply to small to support such chart plotting, 
> whether using or not using the GPS.
>
> It would take a chart scale of about 10K or less to support such close 
> proximity approach and using the GPS.
>
> In other words, the chart is not of such detail to be used to fix the 
> rocks position within about 100 feet. And even though we now have the 
> exact position accurate to within??? 10 feet or so, the chart won't 
> support plotting it to that accuracy.
>
>
> Mike
>
> _________________________
> Capt. Mike Maurice
> Tigard, Oregon (Near Portland).
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