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    Re: S-tables: where to have a look at them?
    From: Robert VanderPol II
    Date: 2017 Mar 20, 15:38 -0700


    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.


    Bob II

    Re: S-tables: where to have a look at them?
    From: Stan K
    Date: 2017 Mar 20, 14:55 -0400

    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.

    Re: S-tables: where to have a look at them?
    From: Stan K
    Date: 2017 Mar 19, 21:07 -0400

    Bob'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. 

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