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Author
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Topic: LOP and Fixes
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Miguel
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posted April 20, 2007 01:59 PM
Hello,
I have solved problems A and B on page 4.8. In A I found 35*34'N 144*30'W and in B 34*17'N 146*14'W
The respectives answers are 35*35'N 144*32'W (A) and 34*16.8'N 146*16'W (B). Is a difference of about 1.5' (average difference) acceptable? This correspond to 1 to 2 Miles.
Thank you for your help
Miguel
From: Rochester,MN
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David Burch
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posted April 21, 2007 03:59 PM
The instructions to this quiz ask you—or at least imply—that you should strive for agreement to within 1 mile, which is 1' on the lat and a little more on the longitude (about 1.2' at lat 35).
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if you are restricted to the normal sight reduction procedures, then you are left to plot from the AP, and thus precision in plotting is crucial, which is why we have this very valuable exercise. You don't really have many options. You can blow up the scale, but the a-values may then be off your page when you do that.
When you have other options and can actually do the sight reduction (and subsequent plotting) from your DR position, or from a first iteration fix position, then you can do much better.
One way to really home in on the plotting, is to plot the fix with standard system and scales and get a lat and lon.
Then do the sight reduction again, using this fix position as the DR or AP for the sights. This means you are doing it with a calculator or with tables that have special procedures that allow for reduction from a DR position.
Once you have the a-values and Zns from this new position, set up a custom plotting sheet with the 3 inches now equal to 6' instead of 60' with the center of the page equal your fix. this means the lines will be at odd spacings, ie center lat could be 34°26' and the next line would be 6' up at 34°32' and so on. Then plot again and you will get a much better insight into your accuracy. Now 1 mile will be about half an inch, not just a couple mm.
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In the next issue of the Navigator's Newsletter ( http://www.navigationfoundation.org ) there will be an article on how to use the NAO sight reduction tables to do a sight reduction from your DR position. This will allow for higher precision work using tables alone.
From: Starpath, Seattle, WA
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