NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: 3-Star Fix - "Canned Survival Problem"
From: Greg R_
Date: 2008 Jun 13, 21:16 -0700
From: Greg R_
Date: 2008 Jun 13, 21:16 -0700
--- "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. 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