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## 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 25, 08:45 -0700

```Of course, Gary. And thanks.

I put it that way to emphasize a point by making what I felt was an analogy -
i.e., EP has to move same direction and distance as DR. EP Advanced for
Course and Distance Run Between Sights would more like it.

Hewitt

On Mar 25, 2013, at 8:26 AM, "Gary LaPook"  wrote:

> But you don't start a new DR plot from an EP, an ESTIMATED position, you
only restart a DR track from a fix because of the uncertainty in an EP.
>
> gl
>
> --- On Mon, 3/25/13, Hewitt Schlereth  wrote:
>
> From: Hewitt Schlereth
> Subject: [NavList] Re: Fix Maximum Probability Positions
> To: garylapook---net
> Date: Monday, March 25, 2013, 8:14 AM
>
> John -
>
> It seems to me that on the one hand you abandon the traditional DR (for the
EP), then later on use the DR (to advance the LOP). The result is you haven't
>
> I can accept the EP as a likely position. But then, if that's where your
boat is, isn't it going to sail the same compass course as the course between
the two DRs? In other words, the EP and DR tracks will be parallel.
>
> Likewise, if the boat is at the EP at the time of the first sight, it is
going to travel the same distance to the time of the second sight as if it
were traveling from first DR to second DR. So would a boat located at any
other point on the LOP established by the first sight.
>
> But you only moved the first sight's LOP forward TO the second DR. That's
not far enough. The distance traveled from first to second sights must be the
same, whether the boat is at the EP or at the DR plotted for the time of the
first sight.
>
> At that time, your EP and every point on the LOP were ahead of the first DR,
so that's how the advanced LOP and every point on it (including the EP)
should lie relative to the DR at the time of the second sight. Otherwise, you
have not moved the EP the distance a boat located at it would have traveled
between the first and second sights.
>
> By moving the first LOP to the second DR, you advanced the EP the direction
it went all right, but you did not advance the EP the distance it went.
>
> It's okay to pick an EP, but you have to start a new DR from it.
>
> Hewitt
>
>
>
> On Mar 24, 2013, at 8:24 PM, "John Karl"  wrote:
>
>> Hewitt,
>> Yes, yours is a common definition of DR position, using only course and
speed.  I tried to make it clear in all my discussions, that the DR position
I was using was the best estimate the navigator had, or chose to have.
>>
>> Then the recent LOP, having much greater accuracy than the dead reckoning
"DR" point, is to be given greater weight is fixing the upgraded position.
With no other info, the MPP would be on that latest LOP at the point nearest
to the "EP."  The older LOP is not used at all in this plot; it played its
role in forming the origin of the current DR run.
>>
>> Your experience on runs of a few hours was excellent, giving very little
disagreement between your "DR" and your new LOP.  Over longer runs (days?)
and highly variable conditions this might not be the case.  The whole point
of the EPRF is that it gives the best position under all conditions,
including all angles between successive LOP pairs.  Plus it gives a realistic
quality check on the dead reckoning.  Since it's even simpler to plot, I
don't see any reason to use the traditional RF.
>>
>> Fair winds,
>> John
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>
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>
```
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