NavList:

Er, "advance all the LOP's by 4'" because of the 4.0' difference between on and off arc?  Unless I've totally missed something, that just doesn't work.  All you can adjust is the Hs reading to change from a reading 2.0' the "wrong" way to 2.0' the "right way" (that is, add 2.0' to or subtract 2.0' from, as appropriate, the Hs value).  The LOP's, and the heading to the LOP from the AP in particular, cannot be moved to just accommodate the Ic error in in the Hs data. Put another way, 2.0' of error in altitude reading is not 2.0' of error in a fix's position.  Try reducing the sights with the "wrong" and "right" Ic's and you should see why moving the LOP's alone simply isn't right.  If it happens that the LOP's do move by 4.0', it's only coincidence. 

Rick Emerson
S/V One With The Wind

P.S.  I'd be much obliged if someone would send me, >off-list, please<, the original excise data. 

Federico Rossi wrote:

I agree with Jeremy that knowing the exact nature of the index error in practical navigation is crucial.

I’ve also tried to advance all the LOPs by 4’ (considering thus a -2’.0 index error instead of + 2’.0) and found that, apart from getting a tighter group of LOPs, the fix (intersection of bisectors between opposite LOPs) is almost the same, in fact bisectors tend to eliminate systematic errors like this one and are more robust from a statistical point of view than single LOPs.

Federico.

 

Da: NavList@fer3.com [mailto:NavList@fer3.com] Per conto di Anabasis75@aol.com
Inviato: venerdì 15 agosto 2008 17.12
A: NavList@fer3.com
Oggetto: [NavList 6139] Re: Ocean Yachtmaster Exercises

 

Jim wrote:

 

" I teach my students to ignore index error, since what we
use is the index correction. They are opposite, and that can be
confusing."

 

Jim,

I agree with you on this for the paperwork reductions with given information, but when checking the sextant for alignment, you had better know how to read the micrometer drum to know if the error is on or off, and having read it; knowing whether the correction is to be added or subtracted. I think I may be missing something, but you can't just drop the whole concept of on and off the arc errors in practical navigation.

Jeremy

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