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Re: Poor St. Hilaire
From: Gary LaPook
Date: 2007 Oct 29, 01:40 -0700

```Gary writes:

You asked a while back about the use of the word "assumed" and whether
that implied some inaccuracy. What you are actually saying is: "I know I
am not there, but assuming, for the sake of the computation that, if I
were there, then  this is the altitude I would have measured." The
assumed latitude and longitude are exact but are only used to feed in
the values need to compute an altitude for comparison with the actual
measured altitude. Using the Sumner method one also assumed a latitude
for the computation because you need this value to plug into the
computation. There was no advantage to using your D.R. latitude. Even if
you used your D.R.  its latitude was actually assumed for the
computation and  the other latitude was some convenient latitude most
likely one of the lines printed on the chart.

In response to your "A" statement, because the distance is too great to
plot on the chart at a sufficiently large scale to be useful (except for
very high altitude shots for which you actually plot the circle of
position around the GP) you must calculate where to draw a segment of
the circle of position on the chart near where you think you are and the
AP gives you exact values to feed into the computation for St. Hilaire
or an exact latitude to use for Sumner.

gl

John Karl wrote:

>I sure can understand why some of you are wondering what all the St.
>Hilaire fuss is about.  I'm new to the List and 'am finding out how
>hard it is to carry on technical discussions in this format - as some
>of you have mentioned.  A major, major, part of the problem is
>definitions, and understanding what each other means - as others of
>you  have said.  After starting this topic, now with 36 posts under
>got my point across.  I'm embarrassed, and apologize for not being
>able to communicate better.
>
>I was speaking from the viewpoint of a modern author introducing the
>St. Hilaire method and the reader trying to understand.  I'm talking
>about how the method is introduced in any number of commonly used
>books: books by Blewitt, Bowditch (2000 edition), Cunliffe, Dutton
>(2004 edition), Howell, Letcher, Meyrier, Moody, Schlereth, Turner,
>and Toghill, to name a few.  These authors all are discussing using HO
>229, HO 249, or a calculator.  They are all talking about finding a
>single LOP - not a fix, not an iteration of fixes.  They're not taking
>about older log-trig methods or the Sumner method.
>
>My statement was that all of these authors either don't attempt to
>explain the REASON for the assumed position, or they explain it
>incorrectly.  For example,
>
>A. Some say that an assumption is necessary because the distance
>between the sun's GP and the ship is too great to plot.  While that's
>true (for most sights and charts), it's not the reason for the assumed
>position.
>
>B.  Some say the AP is necessary because there's insufficient
>information to plot the LOP.  That's false.  We know everything
>necessary to plot the LOP
>
>C.  Others say the AP is necessary because we don't know how to plot
>the exact LOP.  That's also false.  They are several ways to plot the
>LOP exactly.
>
>I mused about WHY so many authors either don't explain the reason for
>the AP, or they have it incorrect - I think that the term "assumed
>position" has misled these folks.  But of course, that's only my
>guess.
>
>I wonder what List members think the reason is for their
>misunderstanding, particularly when they're supposed to be teaching
>others!  No one has addressed this - yet.
>
>John
>
>
>>
>
>
>

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