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    Re: Cannot dispense with the assumed position at sea
    From: Joel Jacobs
    Date: 2004 Feb 20, 09:25 -0500

    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{at}jimthompson.net
    > www.jimthompson.net
    > Outgoing mail scanned by Norton Antivirus
    > -----------------------------------------
    >
    > > -----Original Message-----
    > > From: Fred Hebard [mailto:Fred{at}acf.org]
    > > Sent: Thursday, February 19, 2004 11:43 PM
    > > To: jim2{at}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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