NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: AP terminology
From: Jeremy C
Date: 2009 Nov 15, 15:19 EST
From: Jeremy C
Date: 2009 Nov 15, 15:19 EST
LOP's are not always straight lines. I don't know where you may have
gotten this from.
LOP from a visual bearing is a straight line
LOP from a radar range is an arc of a circle
LOP from a LORAN station is a part of a hyperbola
LOP from a radio DF bearing is an arc of a circle
LOP from celestial coordinates are a small arc of a circle (usually drawn
as straight lines on the scales we typically use. Unless you are doing
high-altitude sights when we revert back to arcs of circles.)
For me, in a practical sense, I have two basic tools to draw LOP's on a
chart or plotting sheet. I have a straight edge and a compass. I use
the compass for Radar and high altitude celestial bodies, and triangles for
everything else (including loran). Generally most of my LOP's (with the
exception of the two mentioned previously) are DRAWN as straight lines,
even if they aren't in the truest sense of the word.
Jeremy
In a message dated 11/15/2009 4:36:16 A.M. Eastern Standard Time,
geoffreykolbe---.com writes:
Frank, Peter H and Peter F.
The confusion comes from a loose (I hesitate to say incorrect) use of
terminology. Peter Fogg is right, I think, that because the tables
generally used for sight reduction require an Assumed Position, this
term has degenerated into a generic term for the starting position
when using the St Hilaire method - but that does not make it the
"correct" term.
Frank has a point, that we bring the baggage of our backgrounds with
us when getting to grips with new concepts and problems and this can
cause confusion. Peter Hakel tells us that his background is as a
"theoretical and computational physicist (in the area of radiative
properties of plasmas)". As I did my Ph.D. in the radiative
properties of (atoms in) plasmas, there is no excuse there ;-).
My problem was the term 'LOP'. To me, an LOP (or position line as we
call it over here) is a straight line. We must remember that the St.
Hilaire method is essentially a graphical method where a fix is
generated by drawing lines on a chart. The LOP's drawn on the chart
are lines which are tangential to a circle of position, centered on
the Geographical Position of the celestial object of
interest. Looking at Bowditch, there is some fuzziness in the
definition of a line of position. "Circular lines of position" are
synonymous with "circles of position" and lines of position, it
seems, can be circles. But when discussing the St Hilaire method,
Bowditch (article 1703 in my 1984 edition) describes the LOP as a
straight line drawn to be tangential to the circle of position.
In his post [NavList 10683], Peter Hakel finally tells us that...
"The LOP's are circles", so now we know where he is coming from and
his single parameter to describe a circle of position makes more
sense. But since we are talking about LOPs in the context of the St
Hilaire method, where LOPs are straight lines, I could not see what
John Karl was getting at when he talked about "calculating the LOP
directly" and I suspect he, too, was talking about the circle of
position, not the LOP as (usually) drawn on a chart.
Geoffrey
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