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    Re: Cannot dispense with the assumed position at sea
    From: Jim Thompson
    Date: 2004 Feb 20, 11:51 -0400

    > -----Original Message-----
    > From: Navigation Mailing List
    > [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Noyce, Bill
    > My attitude is that an AP is simply a convenient place from which
    > to compute an Hc, and plot an intercept.  If using certain tables,
    > there are constraints on how it's chosen; if you're plotting on an
    > actual chart there may be other constraints you'd like to follow
    > as well.
    > But I disagree with the following, if I understand it correctly:
    > > A celestial EP will only be as good as the AP. The more one has to
    > > guess at
    > > the AP, then the less confident one can be about estimating where the
    > > vessel is on the celestial LOP.
    > One interpretation is that you reduce a single sight, draw a LOP,
    > and then drop a perpendicular to it from some previously-chosen
    > position (perhaps called your AP?).  In that case, I agree that,
    > while the LOP might be pretty good, your position along it has all
    > the uncertainty of the previous position.
    That was all I was trying to convey, Bill.  You then went on an added more
    depth to the discussion:
    > On the other hand, if
    > your observations and sight reductions are good, then your true
    > position is much more likely to be somewhere near the LOP than far
    > from it.
    > Another interpretation would be that a round of sights leading to
    > a celestial FIX is "only as good as" the AP(s) chosen for reducing
    > them.  This I strongly disagree with.  If your celestial fix comes
    > out very far from your AP, then use the new fix (or positions near
    > it) as a new AP, and do the calculations over again, getting new
    > Hc's, Z's, and intercepts.  Really this is just correcting for the
    > fact that we use straight lines to approximate circles of position
    > -- if we plotted the circles, or otherwise corrected our LOP's to
    > better approximate circles, this wouldn't be needed.  But simply
    > doing the whole thing over is "simpler" because it uses familiar
    > operations, and also serves as a useful check.
    Agree completely.  I was only discussing the case of a single celestial LOP.
    As soon as more celestial LOPs are added, especially if taken within minutes
    of each other, then the better the position estimate becomes.  In fact it
    then becomes a "fix", not an EP, is that not so?
    > The only place this can fall down is if you make just two
    > observations, and at least one of them is very high, or the
    > azimuths are nearly the same or nearly 180 degrees apart.  Then
    > the two circles of position may intersect in two points on the
    > globe that are fairly near each other.  A badly-chosen AP can
    > lead you to the wrong one of those intersections.  A third
    > observation at a quite different azimuth will resolve the
    > ambiguity.
    Good point.

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