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Subject: Re: TWL: WAAS capable receivers
From: Arild Jensen (elnav@XXX.XXX)
Date: Tue Mar 19 2002 - 14:39:21 EST
Zeke wrote:
> There is a thing called repeatable accuracy which if observed over a
> long span of time will give one a confidence in his system, whatever
> it is. If a piling appears repeatedly on screen in the same place it
> is to the eye, then you can be pretty confident it will be the same in
> the dark of night or fog.
>>> snip<<<
If I can repeatedly be within 2 meters of actuality, I will
> rely on that if it's all I have. And I will be going very slow.
>
> Zeke Anderson
REPLY
Zeke.
You are mixing up several concepts. Robert Bryett has the right
idea. He said:
If your charts are not accurate to the same limits, the
accuracy with which you can fix your position on some
theoretical globe is rather academic.
In addition, if the original chart which you have a raster
copy of is scaled as 1:40,000 then you are kidding
yourself if you think you can discriminate a distance of
10 meters on that chart.
My point is that the boat icon on most raster charts is so
large that its scaled size amounts to a vessel several
hundred feet long.
If you think that you can bring your vessel close to a nav
aid or point of land but not hit it as depicted on the chart
you will have a rude awakening.
I posted a lengthy description of charts and scaling a
couple of years ago. Check the archives.
The entire process of reducing actual survey data in the
field to a pictorial representation on a computer screen
or even a printed paper chart is full of approximations
and educated assumptions not to mention calculated
omissions of selected data.
To see what I mean find an area where you have a
general chart plus a small scale detailed chart of a
select portion. Compare the details of the same points.
Look at shorelines, depth contours, and depiction of any
man made structures such as docks pilings or isolated
hazards like rocks etc.
After you have accounted for the inherent problems of
depicting the real world on flat paper and correcting for
datum changes and scale you now have to add the
inherent errors of the chart display system and the
electronic positioning system.
If you think that you can see the difference in positioning
a vessel within a 100 foot circle as displayed on a
typical coastal chart drawn at a scale of 1:20,000 or
1:40,000 then you are kidding yourself.
I have worked with commercial systems where you can
see such differences. I know what is required to
configure such displays, install the positioning
equipment and what is required to survey the actual
world in order to depict it accurately to the required
details and accuracy.
The average consumer system using the normal raster
charts running on a PC laptop simply isn't capable of
doing what you expect.
If you think you can see the difference between WAAS
and regular GPS at normal full availability and accuracy.
you are kidding yourself.
BTW
I got my navigation practice in an area which is even
less forgiving that the ICW.
Try keel clearances of one or two feet, channel widths of
50 - 100 feet and course changes every few hundred
feet as you move along.
The bottom is hard granite and the boundaries of the
safe channel are lined with sharp jagged rocks that
would rip open the hull in a second.
So yes I think I know what you are looking for.
Cheers
Arild
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