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I have a couple of questions.

1. I'm receiving only one side of the dialog, and would like to see what
"Frank" is saying. Where can I find his comments?

2. Why, at least for those in this country, is there a need to construe
definitions that are different from those found in the standard U.S.
published navigation texts of which most list members are aware.

3. I have sold mine, but I recall the tables such as HO 214, 229, and 249
also had sections with definitions. The Nautical Almanac had definitions.

I realize many changes have taken place, but are none of these sources
satisfactory?
Should we ignore Bowditch as another example?


Joel Jacobs


----- Original Message -----
From: "Jim Thompson" 
To: 
Sent: Friday, February 20, 2004 8:02 AM
Subject: Re: Cannot dispense with the assumed position at sea


> Fred, I still think we are convergent:
>
> One can use a precise DR position in the Ageton-Bayless table, Reed tables
> and computer programs or calculators to do the same thing as a
whole-degree
> AP does in HO 229: determine an Hc and Zn to plot the LOP.  Those 3
methods
> can accept the precise longitude to determine meridian angle from a
precise
> LHA, and the precise latitude in calculating Hc and Zn, as in:
>
> Hc = arcsin [cost x cosD x cosL) + (sinD* x sinL)]
> Z = arccos [(sinD* - sinL) x (sinHc / (cosHc) x cosL)]
> where
> t = meridian angle, precise decimal DMS.
> D=declination of the body, precise decimal DMS.
> L=DR latitude, precise decimal DMS.
> *Note the sign (+ or -): negative if L and D are contrary in name (N or
S).
>
> Which means that one can use a precise DR position as an "AP" in the sense
> that you mean by using whole-degree AP's as an entering argument for HO
229,
> except that one cannot use HO 229 for a precise DMS DR position, and so
> would have to use one of the alternative methods that can.
>
> If a navigator puts an EP box around the point on the celestial LOP
> perpendicular to the position used to create the LOP, then that EP has
more
> significance if the "assumed position" is part of the DR plot.  Of course
> the workaround using a whole-degree AP would be to subsequently drop a
> perpendicular to the DR position, I think achieving the same end except
with
> extra plotting steps if a whole-degree AP is used as an intermediary.
>
> With respect to semantics, I think my understanding of AP is that I see it
> as a general term for the position for which Hc and Zn are determined.
Thus
> in my mind any position used for that purpose is an "AP" (Henning uses
> "initial position" or IP).  Owing to the whole-degree history of the
> entering argument AP, it seems to have traditionally acquired a more
> specific meaning.
>
> Jim Thompson
> jim2@jimthompson.net
> www.jimthompson.net
> Outgoing mail scanned by Norton Antivirus
> -----------------------------------------
>
> > -----Original Message-----
> > From: Fred Hebard [mailto:Fred@acf.org]
> > Sent: Thursday, February 19, 2004 11:43 PM
> > To: jim2@jimthompson.net
> > Subject: Re: Cannot dispense with the assumed position at sea
> >
> > Jim,
> >
> > This isn't semantics.  As Doug said, they mean different things.  An AP
> > is used for sight reduction tables such as H.O. 229.  One enters these
> > tables at a whole degree of latitude, such as 36* N, rather than a
> > fractional value, such as 36*16.5'N.   You _can't_ enter the tables
> > from other than a whole degree of latitude.  Likewise, the longitude is
> > chosen to give an LHA in whole degrees; again, one cannot enter the
> > tables from a fractional LHA.  One then plots the azimuths and
> > distances from that AP.  It also makes locating the latitude of the AP
> > a bit more convenient.
> >
> > The EP and DR are places where you actually reckon you are, so they are
> > almost never at whole degrees.  In contrast, the AP is not a place
> > where you reckon you are, but the closest to where you reckon you are
> > in whole degrees of latitude and fractional degrees of longitude that,
> > combined with the GHA of a body, give an LHA in whole degrees.
>

   

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