# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

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Re: S-tables: where to have a look at them?
From: Robert VanderPol II
Date: 2017 Mar 20, 05:51 -0700

Long ago I was taught, and still believe, that DR and EP are related but different, analogous to the difference between t & LHA.

To quote from "Piloting and Dead Reckoning, 3rd ed." by Schfeldt et al:

"DR position ia a location defined by plotting a vector or series of consecutive vectors using the true course and distance, determined by speed thru the water, usually without consideration of disturbing factors like current.

Estimated Position (EP) is the most probable position of the a ship determined from incomplete or questionable data.  In practice it is often the DR position modified by some other data less complete  than a fix, such as a single LOP or sounding."

To quote from "Duttons, 12th Ed." by Dunlop et al,  last sentence of section #702:  "The effects of current are not considered in determining a DR position."

Section #714: "Estimated Position (EP) is the most probably position of a vessel determined from all available data, when a fix or running fix are unobtainable, and includes the effect of the estimate current."

I assume that for sailboats leeway would be treated in the same way as current.

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.

Stan
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