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Author
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Topic: DR plotting - tiny errata, and a quandary
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bruce
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posted March 02, 2020 08:52 AM
I'm working through the celnav book and went into chapter 4 (plotting and chart work) thinking it would be fun and easy. I still keep a running deck-log/DR-plot on my boat, even in familiar waters, just because I enjoy the discipline. My dock-mates find that amusing.... but I do. I'm also pretty comfortable with setting up and using a UPS.
I had no problems with Exercise 4.5 My LOPs came out just fine, with the resulting fixes easily within 0.5' of the answers in the back.
I had a minor nit with the exercise in 4.3 - in part two, two of the points (a and c) share the same longitude, and differ by 32' of latitude. "In theory", that means they are exactly 32nm apart (32' of latitude, with no easting or westing.) But the comment says the three points should form a triangle with each side 31' long. (I'm working through the Kindle version of the 2nd edition, if that makes a difference)
Then I got to the DR plotting exercise (4.6) I got three of the five "wrong", but in exactly the same fashion, and I can't figure out what I did.
#2 I got the DR position within a couple of tenths
#3 I got the DR position within a couple of tenths
#1, #4, #5, I got the latitude within a couple of tenths, but in all three cases I was significantly off on longitude (2.3', 1.3' and 1.9', respectively).
I've re-done them a couple of times, and get the same result each time. I've checked my UPS setup, I'm using the right scale for measuring longitude, I've checked my bearings and distances, and I can't figure it out.
The only "pattern" I see is that in all three cases, the end-point of the DR track is to the east of the starting point, and my coordinates in all three cases are due *east* of the coordinates in the answers.
Not looking for anyone to check my work, I'll set up a new UPS tonight and work through them again, just to see if I can identify what I'm doing wrong but, in the meantime, I'm wondering... is there a "typical" pattern of systemic error that people run into when running a plot like this, that I might be blindly allowing into my process?
Bruce
From: Everett, WA
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bruce
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posted March 03, 2020 02:44 PM
Update
I sharpened my pencil and re-worked these.
Literally, sharpened my pencil. I was using my normal everyday Pentel throwaway pencil, which produces (apparently) a fairly thick line. And (apparently) my imprecision with it led to some tolerance-stacking. When I reworked the problems with clean/sharp plot lines... they came out right. I guess that's a lesson.
The third one (#5) is still a quandary. I've now worked it three times and get exactly the same result each time: 23*38'N 44*21'W
The listed answer is 23*38N 44*22.9'W, so while my latitude is correct, I continue to be making 1.9 minutes of longitude error somewhere.
From: Everett, WA
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David Burch
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posted March 04, 2020 07:02 AM
If you are plotting on Universal Plotting sheets, one thing to check is that the sheet itself is printed properly. That is, compare 60' of lon read from the compass rose is the same as red from the nomogram.
From: Starpath, Seattle, WA
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bruce
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posted March 04, 2020 09:39 AM
That's something I didn't think of. thanks!
(I'm trying not to obsess about that one problem, but it bugs me to not know where my error is.)
From: Everett, WA
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