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» Online Classroom   » Celestial Navigation   » Public Discussion of Cel Nav   » running fix problem 6.6 -- interpolating Z

   
Author Topic: running fix problem 6.6 -- interpolating Z
Rob Pearce


 - posted February 01, 2005 02:23 PM      Profile for Rob Pearce           Edit/Delete Post 
I hope this doesn't get posted twice, but my internet went down when I did it the first time. On problem 6.6 running fix practice

I got
21 24 N
125 25 W

Answer in book is

21 28.1
125 26.3

is that close enough. I have been getting closer results on other problems, but I did 6.6 three times and ended up in thes same place each time. The text says the answers are a result of computer calculations not tables and therefore our answers might not agree. I just want to make sure the error I got is acceptable, before I continue.

David Burch


 - posted February 01, 2005 05:49 PM      Profile for David Burch           Edit/Delete Post 
This is 6.6 part 1, and no, this is not close enough. it is some 4 miles off which is too much. Please check the intermediate answers that are located on page A-9 and A-10 and read the notes carefully on each of those pages.

This should find your errors as the intermediate answers are there for every step of the solutions. Just start from the beginning on each of the lines and see where your values are different.

Please let us know what you find.

From: Starpath, Seattle, WA
Rob Pearce


 - posted February 02, 2005 02:15 PM      Profile for Rob Pearce           Edit/Delete Post 
Hi again:

I figured 4 miles is too much. I went through and checked the intermediate answers and still ended up in the same place 3 times.......grrrr. I did problems 2, 3, and 4 in the same section and I was with in 1 - 1. 5 minutes, or less. I will try again tonight on problem 4.

Rob Pearce


 - posted February 02, 2005 02:16 PM      Profile for Rob Pearce           Edit/Delete Post 
oops, I mean I will try again on problem 1
Rob Pearce


 - posted February 03, 2005 08:46 AM      Profile for Rob Pearce           Edit/Delete Post 
I redid 6.6 (1) -for the fourth time- and it plotted the correct position. I can't figure out what I did wrong the first 3 times!

I would like to clarify the explinations given in the notes for A-9 and A 10.

My understanding regarding selecting Z is that if the minutes in the Dec are over 30 then on uses the Z from the next higher dec degree. For example if the Dec is 14 54.4 S then I would choose Z from the 15 S column same LHA row and Hc and d would still come from the 14 S column. Is that correct? That is what I have done.

David Burch


 - posted February 04, 2005 08:30 AM      Profile for David Burch           Edit/Delete Post 
Yes, that is correct. You must always choose the Hc that corresponds to the exact degrees part of dec, because the correction to Hc is based on that value, but you are free to look at the dec minutes part and judge for yourself what is best to take for Z, either the Z for the exact dec deg or the Z for dec deg + 1.... the dec minutes must be above 30' for sure and generally this won't switch to a different Z value till you get to higher dec minutes, closer to 50+ or so....

However, if the sigght is a high one, ie Hc above 70° or so, then you will find that Z changes more rapidly with dec and this can be a more important adjustment to make. For high sights you may find the next Z is as much as 3° differnt and if your DR is not close to your AP (ie large a-values)then the final fix could be quite a bit off.

From: Starpath, Seattle, WA
Rob Pearce


 - posted February 04, 2005 01:57 PM      Profile for Rob Pearce           Edit/Delete Post 
As always, thanks for the help . I found 6.6 (3) somewhat amusing-all the lines were crunched close together on the universal plotting sheets.

As I work the problems I find my biggest errors are simple arithmetic ones, and sometimes I write down the wrong info from the table. As I back check things I find my boo boos. I am enjoying this course

David Burch


 - posted February 04, 2005 03:38 PM      Profile for David Burch           Edit/Delete Post 
Glad to hear you are enjoying the course. Thanks.

These running fix problems in 6.6 are in fact problems from the USCG cel nav exam, essentially unmodified. They are not particularly realistic, but they do offer some practice.

And yes, your observation is to the point. The main task in modern cel nav is learning to check and double check yourself. The beauty of the process is errors will indeed eventually show up and be obvious, but it might be 20 or 30 minutes of paper work down the line. Therefore we must go slow and check our work at each step, before moving on.

From: Starpath, Seattle, WA


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