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Author
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Topic: Sources of Error, poor results
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Jim
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posted January 06, 2020 03:44 PM
I take my sites on a lake with a Davis 25. Results vary. I understand the plastic sextant can be part of this but I think it's my skill level / technique is the culprit. I've looked at the different sources of possible error and I think Index Error adjustment and Index Correction Calculation are the biggest offenders if not done correctly. I do my best to eliminate index error by siting distant objects several miles away instead of looking at the sun. From what I can 'see' the index error looks removed. Is this OK or do I need to use the sun? Additionally, after adjustment I stop there without doing the Solar Index Correction calculation because I don't want to look directly at the sun even with shades.
From: USA
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David Burch
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posted January 06, 2020 04:07 PM
Jim, we can help with this, but need more data to proceed.
First I assume that the errors you are talking about are indeed large, meaning 10' or more, when you compare the plotted LOPs with your GPS position
Please tell us how large these errors are you are concerned about and how you determined them.
How are you doing the sight reduction?
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If you would like this help, then please do another sight session and record this information.
1) your GPS position as accurate as your phone will tell you
2) the time and date you start the sight session
3) the air temp and pressure
4) your estimate of your height of eye above the lake level (we can check that)
5) your estimate of the distance to the shoreline on the other side below the sun at the time of the sights
5a) your estimate of the distance to the objects you are using for index error check. Plus a description of them. If part of the lake shoreline, what is the true bearing to the objects?
6) take 3 or 4 measurements of the index correction both in the Toward and Away methods describe in our textbooks. send all 8 measurements. plus start and stop time of the index measuring session.
7) take about 5 sights of the sun at least 1 minute a part. be sure to turn the dial well off of the aligned position before starting a new one.
Send us the time and Hs of each of these
Then post all of this info here and we will have a look to see what we can learn.
From: Starpath, Seattle, WA
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Jim
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posted January 09, 2020 03:00 PM
I will get the information you have described soon, but for now it's been nothing but cloudy days. However, I would like to confirm the sextant adjustment process as this is something that seems to vary depending on the source. American Merchant Seaman's Manual goes in this order of adjustment - 1) Index Mirror perpendicular to frame. 2) Horizon Mirror perpendicular to frame. 3) Horizon Mirror parallel to Index mirror. 4) Then if there is still Index Error, use micrometer to bring images together and note the minutes. Another source I read has steps 2 and 3 reversed. Which is correct or does it matter the order? Thank you
From: USA
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