# NavList:

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

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Re: Fix Maximum Probability Positions
From: Hewitt Schlereth
Date: 2013 Mar 24, 17:58 -0700

John, what I mean by a DR is the boat's position based on the course steered
and distance run since the last fix. Just those two things.

My experience over the years was that over a period of four or five hours
courses were steered pretty accurately, especially on racing boats, where the
helm changed every half hour. One person steered a little high; the next a
bit low, etc., and resulted in the course aimed for.

The biggest variable I found was distance run. But it didn't take too long to
figure out the speedo or log was reading some percent high or low, once I had
several two-body fixes.

I stuck with what my friend called a "pure" DR because it was based on
measurements: 1) by the speedo or log for distance;  2) by my observing the
telltale compass as the helm went from hand to hand. Because this way of
keeping a DR rests on observable data, I always felt it reduced the DR
uncertainties as far as they could be. As time passed and fixes accumulated,
it became increasingly clear how closely the distances run and courses
steered approached the truth. In other words, this method required the least
amount of estimation on my part.

Hewitt

On Mar 24, 2013, at 3:42 PM, "John Karl"  wrote:

> Hewitt,
>
> Sounds like a semantics question here.  By DR position I mean the
navigator's best estimate of position when the second LOP was required.  In
all my figures the DR means the ship's current position from knowledge of
factors affecting the ships track and distance made good from the ship's last
>
> Maybe we are using different definitions of DR position.  There are many
ways of plotting running fixes, but I tried to use the simplest for
discussion.
>
> Does this make sense??
> John K
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