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Re: Problems with AstronavPC
From: Jim Thompson
Date: 2004 Feb 17, 06:53 -0400
From: Jim Thompson
Date: 2004 Feb 17, 06:53 -0400
-----Original Message-----
From: Navigation Mailing List [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Gary HarkinsThe closer your AP to the real position (or DR if you are using Ageton's or the Calculator method) the more accurate your plot will be. Remember that the "real" plot should be an arc with a radius of the co-altitude. If your AP is hundreds of miles off, your plot will be less accurate because the "practical" plot is a straight line. In fact when you are doing high altitude sights or when your AP is far removed from your DR, there is a method to recalculate your plot to allow for this curvature.
Thanks Gary. I'm still not clear on
whether we could, in some circumstances, dispense with the AP. Here is an example of a circumstance where
a non-AP technique might be useful. I received an email from a
non-navigator amateur historian (recall that I am a very amateur
navigator). He wanted to find the coordinates of an historical event that
occurred on land in the 19th century. He had the altitude of a body and
the time. I approximated the position by systematically entering
alternative data into an online CN calculator, but that was pretty crude.
In the end it turns out that his altitude/time information were too imprecise to
be useful anyway.
I understand that the LOP we plot actually
is a tiny segment of a huge COP (Celestial Line of Position = "CLOP", if one
wants a new acronym), but if the computer was able to return (or we
were able to calculate):
1. The lat/long coordinates
(EP) for the intercept of the azimuth with the LOP (COP
or CLOP), and
2. The azimuth of the GP,
then all we would need to do on an
ordinary plotting sheet is:
1. Put a dot at the intercept,
2. Place square box around it (EP symbol),
3. Draw a short segment of the azimuth from
the intercept (EP) toward the GP,
4. Draw the LOP as a straight line
perpendicular to the azimuth.
I realize that this generally is not good
practice, because we should be keeping a tight DR plot to complement the CN
sights, but is this not a realistic alternative to using an AP?
George Huxtable might have partially
answered my question in this thread:
> -----Original Message-----
> From: Navigation Mailing List
> [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of George Huxtable
>
> Aubrey O'Callaghan said, referring to section 11, "Position from intercept
> and azimuth", in the Nautical Almanac"-
> >That being said, I remember plugging this formula into a
> spreadsheet (on a
> >Psion 3A - I was at anchorage by some Caribbean island at the time)
> >I then placed my initial position nowhere near where I was. After about 3
> >iterations it converged to my actual position. I was quite surprised as I
> >had thought that one's initial guess should be approximately where one is
> >(at least within a few degrees). I even tried different hemispheres as
> >starting position.
>
> There's a bit of extra information about that matter, given in section 7.4
> of the AstronavPC booklet, but not in the Almanac, as follows-
> From: Navigation Mailing List
> [mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of George Huxtable
>
> Aubrey O'Callaghan said, referring to section 11, "Position from intercept
> and azimuth", in the Nautical Almanac"-
> >That being said, I remember plugging this formula into a
> spreadsheet (on a
> >Psion 3A - I was at anchorage by some Caribbean island at the time)
> >I then placed my initial position nowhere near where I was. After about 3
> >iterations it converged to my actual position. I was quite surprised as I
> >had thought that one's initial guess should be approximately where one is
> >(at least within a few degrees). I even tried different hemispheres as
> >starting position.
>
> There's a bit of extra information about that matter, given in section 7.4
> of the AstronavPC booklet, but not in the Almanac, as follows-
>
> "It is possible, but not advisable to start the iterations with a position
> that is in a different hemisphere. Provided L(I) is kept in the range
> -180deg to +180deg and B(I) in the range -90deg to +90deg the solution in
> most cases will begin to converge after a few iterations."
>
> That seems pretty all-embracing, doesn't it?
> "It is possible, but not advisable to start the iterations with a position
> that is in a different hemisphere. Provided L(I) is kept in the range
> -180deg to +180deg and B(I) in the range -90deg to +90deg the solution in
> most cases will begin to converge after a few iterations."
>
> That seems pretty all-embracing, doesn't it?
Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
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