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    Re: Hello (+ a few questions)
    From: John Brenneise
    Date: 1999 Feb 08, 14:09 EST

    As for question two, where the dateline (or any other parallel or
    meridian) falls is
    a matter of which map datum (or model of the earth's shape), e.g. WGS
    84, that the
    atlas or chart is based upon.  It is not surprising that two atlases
    would disagree,
    unless of course, they both were based upon the same datum.
    > ----------
    > From: 	Thomas Schmidt[SMTP:schmidt{at}XXX.XXX]
    > Reply To: 	Thomas Schmidt
    > Sent: 	Monday, February 08, 1999 6:08 AM
    > To: 	Navigation mailing list
    > Subject: 	Re: [Nml] Hello (+ a few questions)
    >
    > R.H. van Gent wrote:
    > ...
    > > 1) I am trying to locate an obscure reference cited in Bowditch's
    > > _American Practical Navigator: An Epitome of Navigation_ (p. 423,
    > 1958
    > > edition; vol. II, p. 546, 1977/81 edition) to a study performed by
    > the
    > > Carnegie Institute of Washington on the application of the standard
    > > formula for the correction for dip:
    > ...
    >
    >
    > Dear Mr. van Gent,
    >
    > I have taken the liberty of forwarding your question on HASTRO-L to
    > this
    > list last Friday, and I received this reply which I'll simply repeat
    > here
    > since you've joined us now:
    >
    >
    >
    > Richard Langley wrote:
    >
    > > For a good discussion of astronomical refraction, including
    > horizontal
    > > refraction, see Chapter 15 of Jean Meeus's Astronomical Algorithms.
    > He gives
    > > as one reference a paper by Bennett: "The Calculation of
    > Astronomical
    > > Refraction in Marine Navigation" in the Journal of the (U.K.)
    > Institute of
    > > Navigation, Vol. 35, 1982, pp. 255-259.
    > > -- Richard Langley
    > >    Professor of Geodesy and Precision Navigation
    > >
    > > On Fri, 5 Feb 1999, Tom McHugh wrote:
    > >
    > > >R.H. van Gent wrote: (and others as well have stated substantially
    > the
    > > >same thing regarding dip)
    > > >>
    > > >> The small angle between both horizons is known as the 'dip', and
    > can be
    > > >> approximated by the following relation found in almost any
    > astronomical
    > > >> or navigational handbook:
    > > >>
    > > >>   dip (minutes of arc) = 0.97 sqrt(h[ft])
    > > >>
    > > >> with 'h' denoting the height of the observer's eye above sea
    > level in
    > > >> feet.
    > > >
    > > >I think, that for beginners on the list, to avoid confusion, it
    > > >would be well to state clearly that the above dip formula refers to
    > > >the sea level as being one's local horizon of reference. and
    > relates to
    > > >one's
    > > >vertical elevation of eyepoint above sea level. It must be pointed
    > out
    > > >that this formula will not be correct if one is on a horizontal
    > plane
    > > >at some considerable distance above sea level where the local
    > horizon is
    > > >also
    > > >well above sea level. Put another way, someone living in Denver or
    > > >other high plateau regions would have to calculate dip based upon
    > > >height above the local horizontal plane, not referred to sea level.
    > > >I am of course, referring to that portion of dip which is related
    > to
    > > >atmospheric refraction. Naturally, the eye level position above the
    > > >horizontal
    > > >will be the same.
    > > >
    > > >Even at "sea level" there would be differences in dip, as it has
    > been
    > > >determined that there are areas of the ocean's surface which are
    > below
    > > >mean sea level because of mass concentrations within the earth's
    > crust
    > > >or mantle.
    > > >
    > > >Tom McHugh
    > > >
    > > >tbmchugh{at}XXX.XXX
    > > >
    > >
    > >
    >
    > >
    > ======================================================================
    > =========
    > >  Richard B. Langley                            E-mail: lang{at}XXX.XXX
    > >  Geodetic Research Laboratory                  Web:
    > http://www.unb.ca/GGE/
    > >  Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506
    > 453-5142
    > >  University of New Brunswick                   Fax:      +1 506
    > 453-4943
    > >  Fredericton, N.B., Canada  E3B 5A3
    > >      Fredericton?  Where's that?  See:
    > http://www.city.fredericton.nb.ca/
    > >
    > ======================================================================
    > =========
    > >
    >
    > --
    >   -------------------------------------------------------------------
    >   Thomas Schmidt                    e-mail  : schmidt{at}XXX.XXX
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