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 » Online Classroom   »   » Public Discussion of Cel Nav   » Hawaii by Sextant

Author Topic: Hawaii by Sextant
 climber posted March 19, 2016 06:52 AM                   I know this isn't part of rage nav course but since completing the course I have just started on the problems in Hawaii by Sextant and I don't know where to get questions answered.In the Analysis section I have been trying to understand the Automatic advancement of LOPs beginning on page 98. I cannot duplicate the a-value corrections depicted on page 99 using the equation in A1. First of all, I cannot calculate the correct value for the correction. Secondly I'm not sure whether to add or subtract the correction.In Figure A1 I am assuming that the the point at "D=S x (T2 - T1) is your DR position and that D is the distance for which you are correcting the sight. You are then advancing LOP T1 to this point using the parallel ruler. The a-value is then corrected using the distance between LOP T1 and the advanced LOP. From Figure A1 it is obvious that you would add the correction to the a-value. Is there a rule for this if you don't have it plotted? Also, the equation in Figure A1 is "a = a +D* cos(C - Zn)" while the equation in the narrative is " D * cos( Zn - C). I cannot reproduce the corrected a-values in Figure A2 using this process.Can you clarify the procedure for me? I don't know if you can answer this type of question or not - just thought I'd try.Thank you From: apalachicola
 climber posted March 19, 2016 07:34 AM                   I figured out part of my problem. On an iPad you have to convert degrees to radians to obtain the proper value of the Cosine of an angle - my old calculator did not require that. My bad. I can now duplicate the correction values.I still haven't found a rule for adding or subtracting the correction. Can you help there?Also, what are the axes for the plot in Figure A3? I haven't been able to figure that out although the rest of the section makes sense, so far.By the way, if I can't ask these questions here is there somewhere I can ask them?Thank you. From: apalachicola
 David Burch posted March 19, 2016 01:17 PM                   This is the perfect place to ask questions about that book! From: Starpath, Seattle, WA
 climber posted March 21, 2016 11:10 AM                   Thanks. I have a few questions.First, when advancing LOPs as in the Analysis section of the book, how do you know whether. To add or subtract the correction to the a-value?Second, what are the units for the axes in figure A3?Third, is there any way to determine which sight is the best to use in problem 3 without going through the entire sight reduction for each observation if you DON'T use a calculator? I'm trying to do as much as possible manually to be sure I understand things.Thank you. From: apalachicola
 climber posted March 21, 2016 01:11 PM                   A few more questions: these are for Problem 3.1. I plotted the time vs Ho points and drew a "best-fit" line through the points. I then picked what I eyeballed to be the points closest to the line. I did the sight reduction for four of the morning sights and two of the afternoon sights. I then picked the sights with the smallest a-values.I don't know if this a valid approach or not. I did not correct for distance traveled.Is this approach at all valid?2. For the :08:32:28 sight on 7 July the min. were 34.5 and the d-value was 36.8. That puts the tens at 30 and the units at 6.8. Entering the interpolation table at 34.5 for the 30's value I get 17.3, not the 19.8 in the form at the top of page 39. I get the same value for the units as the one on that form.Am I reading the table incorrectly?3. Using the 14:00:05 sight, my DR position is close to the one in box 1 of the form at the bottom of page 39. Your DR position is 41 degrees 36' N. The a-Lat used in the book is 41 degrees N. Since the minutes are 36', which is greater than 30', why wouldn't the a-Lat be 42 degrees N?Thanks for the help. From: apalachicola
 Capt Steve Miller posted March 21, 2016 01:12 PM                   Hello Climber,I am looking into all your questions and will reply soon. From: Starpath
 climber posted March 25, 2016 01:01 PM                   I have one question and one comment on problem 4 in Hawaii by Sextant.1. In the worksheet on page 40, for the 13 07 23 sun sight you use an Hs of 70 degrees 38.2'. On page 13 the Hs for that sight is 70 degrees 54.0'. There is no 38.2' value. Is the 38.2' an average? Were all three sights advanced to 13 07 23? I've tried that and still can't get the 32.8' value.Can you clarify this for me?2. In the same worksheet for the 13 07 23 sight you come up with an a-Lon of 132 degrees 35.6'. However, in the plot on page 80 the AP2 is plotted as 131 degrees 35.6'. It appears that there is an error in the worksheet in box 3 where the a-lon should be entered as 131 degrees, not 132. Thank you. From: apalachicola
 David Burch posted March 25, 2016 01:35 PM                   Can you tell me what printing of the book you have? my copy shows 54'. Please check the copy right page, so see the date...thanksPS. if you do not have the latest we will see that you get one, as we very much appreciate your careful working of the problems. From: Starpath, Seattle, WA
 climber posted March 26, 2016 09:25 AM                   My book is dated 2014. No need for me to get a new one. I used 54' and got really close to the book answer. I didn't get the book from Starpath but from another company when I ordered some other materials you don't carry.If 54' is the value to use I'm good. Thanks for clearing that up. From: apalachicola
 David Burch posted March 26, 2016 09:28 AM                   There are several typos in the books that were found and fixed in the 2nd printing. Very shortly we will have a PC ebook of that printing and i can send you a s/n for that to check similar things if they might show up. we were actually planning to work on that on Monday. From: Starpath, Seattle, WA
 David Burch posted March 26, 2016 11:19 AM                   I will address these and be right back. From: Starpath, Seattle, WA
 climber posted March 26, 2016 11:52 AM                   Sorry to bother you with another question but I'm really having a hard time with the selection of the best sight.I've worked the sight reduction for the first and last sun sights in problem 1 to try and duplicate Figure A5. I assume that the the Hc values of 26 degrees 54.5' on the right side of the figure are typos. I get 27 degrees 04.2' for the 15 41 22 (GMT) sight and 28 degrees 27.1' for the 15 47 30 sight. These are not corrected - I used my 0844 DR position for the calculation. My DR position is within a mile of what is plotted on page 76.I'm not sure how to the corrections for the Hc calculations. I can correct the a-values using the techniques for the Automatic Advancement of LOPs but that gives you a correction to the a-value for plotting, as near as I can tell. How does that apply to the Hc correction? Do you add or subtract it from the Hc to get the corrected Hc?I'm sorry but this whole area has me confused. As I said, I can use the raw data, plot the DR plots accurately, and use the sights selected in the book to get fixes that agree with the book fixes to within 1 to 5 miles, but I do not grasp this Analysis section at all.Confused in Florida From: apalachicola