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Author
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Topic: Timing: AP position vs time of sights?
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bruce
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posted February 24, 2020 09:51 AM
What's the best practice for timing in calculating the navigational triangle using AP (assumed position)?
For example, I could plot my AP based on my DR position before I start my sight session. But then depending on how long my sight-session takes, my AP (and resulting navigational triangle) could be 1/2-hour or more out-of-date by the time I've recorded my sights.
Or should I plot my AP using my DR at the time of my sight(s), to ensure that my navigational triangle is as close to reality as possible? E.g., if my sight-session runs from 1745 to 1815, should I plot my AP based on my DR position at 1800 or something?
My instinct is that it should be the second approach, but I'm relearning this, so want to embed best-practices in my foundation.
Bruce
From: Everett, WA
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David Burch
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posted February 24, 2020 09:56 AM
If you do a series of sights between say 1745 and 1815, it does not matter which time you choose to do the sight reduction. Typically it might be the end of the season at 1815, but it could be any time. The key is that all sights must be advanced or retarded to the time you choose, and then your fix time is that selected time.
From: Starpath, Seattle, WA
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David Burch
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posted February 24, 2020 10:00 AM
A follow up. When you say calculate the nav triangle it seems to imply you are doing sight reduction by calculator or computer. In that case, each sight is computed at the exact time of the sight, then before the LOP is plotted it must be advanced or retarded to your chosen sight time.
Alternatively, doing this by hand with a calculator, you can choose one single sight time to use, then before doing each of the computations, one for each sight, then advance or retard the DR position to the sight time you selected.
From: Starpath, Seattle, WA
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bruce
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posted February 24, 2020 10:27 AM
Thanks (and, apologies in advance for what I'm quite sure are rookie questions as I dust off braincells I haven't used for 30 years)
Here's what I'm wondering: my navigational triangle for a sight (theory, not dependent on how I reduce the sight) is composed of the elevated pole, my AP, and the GP of the body in question. That navigational triangle is solved to yield an azimuth and distance to the GP, and using Ho I can plot an LOP on the azimuth, either "toward" or "away" based on my altitude intercept.
Is there a point at which my DR is "too stale" to be viable as an AP? My thought is that, as time goes by, the GP moves. So logically (?) it seems that if some significant amount of time goes by, the navigational triangle (Pn/AP/GP) I solved for the azimuth will no longer be adequately reflective of the actual position of GP at the time of the sight.
Or am I overthinking it?
I've read your blog post ("Does accurate DR matter?"), and I get it, but wondering how to integrate it into my thinking.
Bruce
From: Everett, WA
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David Burch
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posted February 24, 2020 11:17 AM
As i understand, we are looking at one sight only and that will be at some time... namely the time we did the sight. We also have a log book of DR positions, the latest of which is some time before this sight.
The standard procedure would be to do DR up to the time of the sight and then use that DR position for the sight reduction.
Note that when doing sight reduction by "computing the nav triangle" it means you would use the actual DR position. When doing sight reduction by books, we must use an assumed position AP.
From: Starpath, Seattle, WA
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