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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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