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--- "Greg R."  wrote:

> Hmmm... just had an idea - I'm going to go back and "massage" all 3
> sights with Navigator to make them overlap perfectly (i.e. zero
> intercepts) and then plot those LOPs - maybe that'll help point to
> where the error in my rough plot is coming from.

It dawned on me that "zero Hs error" is probably a better way to say
that than "zero intercepts" (I use Navigator to check the accuracy of
my own sights, and using a known GPS position as the Assumed Position
the intercept from that position becomes the error for that sight -
hence in this case "zero intercept" = zero sighting error).

And since the horizon wasn't clear the evening that I did the shots for
the exercise, I also introduced some "dither" into the Hs values (via
Navigator) to better reflect reality (and also to generate a cocked-hat
for the fix vs. 3 perfectly crossing LOPs).

So let me re-phrase that to say I'm going to go back and massage the
earlier sights to reflect zero sighting errors so that (hopefully...)
all 3 of them will cross at just one point - which can also serve as a
plotting accuracy exercise.

--
GregR



 
> Hi Jeremy:
> 
> This exercise turned out to be a little more "challenging" than I'd
> thought, since the declination of one of the sights (Vega) is out of
> range for using 249 vol. 2 (but in a "survival" situation, I guess
> you
> take what you can get). So I used 249 vol. 1 for that one (but
> ignored
> the precession/nutation correction since it was almost parallel to
> the
> LOP itself). 
> 
> I also started to solve it with 229, then realized I didn't know how
> to
> work the interpolation tables (though I figured that out last night -
> it's a lot more involved than the 249 method, so I'll have a go at
> doing it that way later on).
> 
> > Thanks for the nice exercise Greg.
> 
> No problem - we couldn't really expect you to do the work for all of
> these "real-world" exercises. Just be glad I didn't include the horde
> of hungry marauding mosquitoes in the NavList post... ;-)
> 
> > I literally had to dust off the ship's Vol III of HO 229 and 
> > deflower a Plotting sheet 925 to work this one out.
> 
> Hopefully it wasn't too painful for either of them... ;-)
> 
> > Since I was bereft of electronic gadgets, I did this with a
> plotting 
> > sheet, 2 triangles, a pair of dividers, 2 books, a pencil, and
> small 
> > piece of scratch paper (wouldn't have reams of paper in the
> Lifeboat). 
> 
> Glad *somebody* actually noticed that part of the exercise (though I
> did say the navigator managed to grab all of the navigation tools
> before abandoning ship, but your method is also valid).
> 
> >  I have attached a picture in to this message with the plot and the
> 
> > tools.
> 
> OK, extra points for the visual aids...  ;-)
> 
> > My Lat is a bit lower (plotting or math error?).
> 
> Yeah, I reduced the 3 sights yesterday and did a quick plot before
> having to stop and leave for work. Mine came out that way too (i.e. a
> few miles south of the GPS and computer positions), so I'm going to
> re-plot it when I have the time to be more methodical about it and
> try
> to figure out what's causing that error (pardon the
> chicken-scratches,
> I did say it was "rough"...  ;-)):
> 
>
http://www.geocities.com/gregr_navlist/exercises/canned_survival/rough_plot_1.jpg
> 
> (Central parallel = 34� N, central meridian = 119� W) 
> 
> And here's a shot of the intrepid navigator doing the "survival"
> sights
> (at least, when not fighting off that horde of marauding mosquitos...
> 
> ;-)):
> 
>
http://www.geocities.com/gregr_navlist/exercises/canned_survival/interpid_navigator.jpg
> 
> > I used an assumed position method and HO 229
> 
> I used 34� N and longitudes based on getting whole numbers for the
> LHAs.
> 
> > I had to assume we were drifting and no current (didn't advance or 
> > retard the lines).
> 
> Your assumption would be correct, especially since the exercise
> narrative said: "S/he now finds him/herself washed up on an unknown
> shore"...  ;-)
> 
> > I had not done a full HO 229 paper reduction of a star in many
> years,
> > and I had to think a second to remember how to use the
> interpolation 
> > pages on the inside covers for the declination interpolation.  
> 
> Ditto that - see above.
> 
> > I usually whip those off with the calculator.  
> 
> Substitute "computer" for "calculator" and ditto that too - sure is
> easy to get lazy with all this modern technology...  ;-)
> 
> > Still, I got pretty close to the computer solutions with Lat 34deg 
> > 11.9' N and Longitude 119deg 16.0'W.
> 
> My rough plot is a couple of miles even further south of yours - I
> came
> up with 34� 09' N / 119� 16.8'W.
> 
> > As to how you would get an Eastern sight on the west coast, you 
> > would have 2 options in general.  The first would be a back sight. 
> 
> > This would be particularly difficult with a regular sextant at such
> 
> > a low altitude.  
> 
> Agreed - the arcs on both my Astra and Davis (and I think most of the
> modern-day sextants) read to 120�, which would rule out a backsight
> with an Hs of 24� (I'd need a range of ~160� if I did the math
> right). Not to mention that there was a fog/haze bank hanging on the
> horizon that evening which would have made that sight even more
> problematic.
> 
> > The other option would be to use a bubble sight tube or other 
> > artificial horizon.  
> 
> Agreed, but not seeing a need for it out of the ocean, the intrepid
> navigator had left those at home in the box with the other extra
> navigation gear.  :-)
> 
> > If you were across a bay, you could also use a dip short of the 
> > horizon table.  That's all I can think of at the moment.
> 
> How about across a parking lot?...  ;-) 
> 
> Actually, your guesses are pretty good (and definitely thinking like
> a
> good navigator), but across the street from this particular part of
> the
> beach is a collection of shops (Ventura Harbor Village) - and as luck
> would have it, when I brought down the Vega sight to where the
> horizon
> would normally be it landed squarely in the doorway of one of them. 
> 
> I took a guess at what my eye height would have been in that doorway
> and used that for my "horizon line" (surprisingly enough, it turned
> out
> to be within 15' of what it actually should have been), and I later
> "massaged" it with the Navigator program to get an Hs that would have
> been more realistic (the "Canned" part of the subject line was
> supposed
> to be a clue that those weren't all "normal" sights...  ;-)). 
> 
> Wonder if a dip short correction would have made that "artificial
> sight" more accurate?...  ;-)
> 
> Hmmm... just had an idea - I'm going to go back and "massage" all 3
> sights with Navigator to make them overlap perfectly (i.e. zero
> intercepts) and then plot those LOPs - maybe that'll help point to
> where the error in my rough plot is coming from.
> 
> --
> GregR
> 
> 
> 
> --- Anabasis75@aol.com wrote:
> 
> > Thanks for the nice exercise Greg.  I literally had to dust off the
> 
> > ship's 
> > Vol III of HO 229 and deflower a Plotting sheet 925 to work this
> one 
> > out.
> >  
> > Since I was bereft of electronic gadgets, I did this with a
> plotting
> > sheet,  
> > 2 triangles, a pair of dividers, 2 books, a pencil, and small piece
> > of scratch 
> >  paper (wouldn't have reams of paper in the Lifeboat).  I have
> > attached a  
> > picture in to this message with the plot and the tools.
> >  
> > My Lat is a bit lower (plotting or math error?).  I used an assumed
> 
> > position 
> > method and HO 229.  I had to assume we were drifting and no 
> current
> > (didn't 
> > advance or retard the lines).  I had not done a full HO 229  paper
> > reduction 
> > of a star in many years, and I had to think a second to remember 
> how
> > to use 
> > the interpolation pages on the inside covers for the declination 
> > interpolation. 
> >  I usually whip those off with the calculator.  Still,  I got
> pretty
> > close to 
> > the computer solutions with Lat 34deg 11.9' N and  Longitude 119deg
> > 16.0'W.
> >  
> > As to how you would get an Eastern sight on the west coast, you
> would
> > have  2 
> > options in general.  The first would be a back sight.  This would
> be 
> > 
> > particularly difficult with a regular sextant at such a low
> altitude.
> >  The  other 
> > option would be to use a bubble sight tube or other artificial 
> > horizon.  If you 
> > were across a bay, you could also use a dip short of  the horizon
> > table.  
> > That's all I can think of at the moment.
> >  
> > Jeremy
> >  
> >  
> > 
> > 
> > 
> > **************Vote for your city's best dining and nightlife.
> City's
> > Best 
> > 2008.      (http://citysbest.aol.com?ncid=aolacg00050000000102)
> > 
> > > 
> > 
> 
> 
> > 
> 


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