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Re: S-tables: where to have a look at them?
From: Stan K
Date: 2017 Mar 20, 20:07 -0400
From: Stan K
Date: 2017 Mar 20, 20:07 -0400
Bob,
I agree completely. I almost said "For the case of sight reduction, you reduce a single sight...", but then I figured that would be taken for granted. (I guess I was wrong.) Yes, there are many ways to improve your DR position to an EP besides celestial.
I never intended to imply that you were confusing DR and EP. I just mistakenly thought that we were using different terms for the same thing. However, the way you do it and the way I was taught are a bit different, but make no practical difference. Whether you reduce/plot from the DR position, the EP, or any other position relatively close to these, we should all end up with the same LOP (assuming the same sight reduction method, one that does not use an AP).
Stan
-----Original Message-----
From: Robert VanderPol II <NoReply_RobertVanderPolII@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Mon, Mar 20, 2017 7:01 pm
Subject: [NavList] Re: S-tables: where to have a look at them?
From: Robert VanderPol II <NoReply_RobertVanderPolII@fer3.com>
To: slk1000 <slk1000@aol.com>
Sent: Mon, Mar 20, 2017 7:01 pm
Subject: [NavList] Re: S-tables: where to have a look at them?
Stan:
We agree on the definition of DR.
I think your difinition of EP is a bit more limited than the one I use or that I can find authoritative definitions for. I do not feel that a line of postion is the only way to improve a DR to an EP. Historical knowledge of your own boat's leeway in differing conditions and points of sail/motoring allow you to adjust for leeway. Current predictions are available for many areas without reference to measurement in the here and now. Correlation of soundings with charted depths can also improve on your DR.
Knowlege of your knotmeter's slippage in varying conditions could also contribute to jmproving the DR or EP. I could see arguements that this is part of DR, compensating for mismeasurement by your instruments.
Regardless, your post yesterday implied that I was confusing the DR & EP (or I inferred that you did not know the difffence between the 2, whatever). I did not confuse the 2, I chose EP because that is what I normally would use to start the reduction. If I was not making any estimates of current and/or leeway and had no sounding data then I would use DR. Being prone to sloth it's very likely I would occasionally use the DR or rarely even just an out and out guess on occasion.
Peace,
Bob II
Re: S-tables: where to have a look at them?
From: Stan K
Date: 2017 Mar 20, 14:55 -0400Bob,The way I was taught, your DR position is determined strictly by planned course, speed, and time elapsed, to determine estimate distance traveled and position. Current/leeway is not considered in determining a DR position, so if you do get a good fix by some method, you can then compare your DR position to your fix and determine the effects of current/leeway. You reduce a single sight from your DR position, draw the LOP on your DR plot, then draw a line from your DR position perpendicular to the LOP. The point where the line intersects the LOP is your EP, a position more probable than your DR position, but not as good as a fix. However, once the current/leeway is known (or at least estimated), the EP can be adjusted accordingly.StanRe: S-tables: where to have a look at them?
From: Stan K
Date: 2017 Mar 19, 21:07 -0400Bob's response pretty much covers it.The only thing I might add is about other tabular methods that allow plotting from a DR position (what Bob calls an EP) rather than an AP. The only mainstream one (besides the variants of H.O. 211) that is included in Celestial Tools is H.O. 214, but only in the delta d, delta t, delta L mode.
Stan
