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» Online Classroom   »   » Public Discussion of Cel Nav   » Hawaii by Sextant: P2 FIX

   
Author Topic: Hawaii by Sextant: P2 FIX
r.orlik


 - posted March 30, 2016 11:17 PM      Profile for r.orlik           Edit/Delete Post 
I started sailing to Hawaii and after seven days of voyage I decied to plot all the positions on the real paper map of Pacific. Also I changed
universal plotting sheet to more accurate (page 3rd of UPS.pdf). What surprized me was that newly plotted running fix for July, 6th at 1527 was less accurate than expected. I started investigate and it reveals that on page 94th at row 8th there is wrong data for running fix at 1527.

The RFix in question is defined as follow:
(a) 07/06 1046, a-Lat 44 N, a-Long 131 18.8W, a=7.3T, Zn=104.9;
(b) 07/06 1527, a-Lat 44 N, a-Long 131 33.6W, a=12.0A, Zn=228.9.
Ship's course is 200, distance 34 NM.

According to my new plotting sheet it was 43 58 N, 131 12 W and answer given by the book was 44 05.0 N, 131 11.9 W. So, the error was about 7 nautical miles, more that I expacted using new UPS and more that I got using the older one.

I calculated interception of these two LOP using Maxima CAS. My solutions was 44 00.6N 131 12.4W, about 2.5 nautical miles of the plotted point.

I think that the actual number given in book s typographic error 44.050 instead of 44.005. I am not sure about longitude's value. This is inear approximation of the part of the circle but as GP of Sun is located at 22 N then we are at east 1320 nautical miles from this point and the curvature could be omitted.

Yours sincerely,
Rafal Orlik



From: Wroclaw, Poland
David Burch


 - posted April 01, 2016 03:02 PM      Profile for David Burch           Edit/Delete Post 
Notes on Problem 2.

The sight data are on page 28. six sights 2 in the morning 4 in the afternoon. if you do a running fix (by computation) with all 6 you get a fix at 1527 of 44 10.2, 131 5.8. however we know one in the afternoon is bad (15:22), so throw this one out and you get the 1527 fix of 44 5.0, 131 11.9, which is listed in the book and this has been checked and seems correct.

note correcting for the bad sight had a large improvement on the fix…. though this was an obvious bad one, compared to others we have to deal with later on in the voyage.

So that is the right answer to this sight session. The only way to improve that would be to do a fit slope method and try to get a slightly better value for the afternoon session. this would at most improve some tenths of a mile, i would guess.

Next it is important to recall how the book is laid out. we list all the sights and then we do computed solution using all of them, and these answers are in the computed solutions section and those values are summarized on page 94.

then of the sights taken we more or less randomly choose one from each session to work out in form and plotting simply to illustrate the process. the choice of sights is very unlikely to be optimized. We hope that readers will figure out the best sights on their own and use those values. Our goal was just to provide practice examples with filling the forms and doing the plotting.

we would hope readers can do better than we did. One way to evaluate your results is compare to the computed answer for just the two selected sights. That is, if you do it all by hand and evaluate properly these two answers should be close. you can always apply the fit slope method to a set of data to find a good sight in the session.

Comparing your plotted fix with the numerical fix from just those two is a check of your paperwork and plotting, but not a check on the fix. Ultimately you would compare your plotted fix to the computation of all sights.

The two sights that we chose for each set are marked with an * on page 28— again, keeping in mind these are really random selections, just glancing at the a-values or sometimes not even that.

then these two sights 1045 and 1527 are presented as filled out forms on page 38. But NOTE. the work form has an error in it, because the speeds are listed as 7.0 and for these sights the speeds were 7.3. this is indeed a typo (actually more a cut and paste error), but the logbook, and sight data and solutions and plots all have this correct at 7.3. we will add this to the errata.

The fix we get from our plots was 44 00.0, 131, 13.0 . One way to test your plotting work is to go back and do computed solutions of just the two sights selected. In this case you will get 44 01.1, 131 13.3. So it looks like our plot is off by about 1.1 mi it Lat and a few tenths in Lon. This is not exceptional agreement, but certainly acceptable.

It is acceptable because of the very nature of what we are doing. namely just showing the sight reduction as an example and showing what the plots look like so you can compare with your own work.

a few tenths difference here does not matter. More to the point, we know the right answer (44 5.0, 131 11.9) which we got from doing computations of all sights but the bad one. Notice in this example if you just chose two random sights we are now off by more than 5 miles.

this shows why it is important to take as many sights as you can in each session, ideally 4 or 5.

As far as i can tell, the only typos in this problem are the speed in the forms, as noted.

From: Starpath, Seattle, WA
r.orlik


 - posted April 04, 2016 03:54 AM      Profile for r.orlik           Edit/Delete Post 
Thanks for explanation. I wrongly assumed that these computed values correspond to sights that were presented in answers.
From: Wroclaw, Poland


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