# NavList:

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

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Re: Raw data for bubble
From: Peter Fogg
Date: 2007 Mar 10, 14:48 +1100

```Responding to my query:
> > Do you mean that you use Excel to draw a graph?

Bill wrote:
> Yes. I will enter my observation altitudes (generally Ho as opposed to Hs or
> H) in a spread sheet column and times of observations a second column. Then
> I will enter calculated altitude (Hc) at 4 minute intervals in a third
> column, along with time of Hc calculation in a forth column.
>
> Using chart wizard I'll plot Ho vs. column-two time, then Hc vs. column-four
> time on the same chart/graph.  You can graphically connect the Hc dots and
> move the slope line up or down to observation data to spot outliers.
>
> I also like to have Excel add a linear regression slope to the observations
> and compare that to the actual slope (Hc vs. time).  Often quite different.

Thanks Bill. I intend to have a go at this.

>  > ...An example of how this can be useful is
> > when you are trying to make an observation at prime vertical - unlike
> > a meridian passage the body just zooms past this due east or west
> > point of azimuth. It is the only way I know of being sure to be able
> > to adopt that precise moment.

Bill inquired:
> Would you please go into more detail on the above paragraph?

If you can manage to make an observation with an azimuth of precisely
090d or 270d then you can establish longitude with that one sight,
just as latitude can be established with a meridian sight. But how to
ensure that the sight is recorded at the right moment?

The moment of prime verticality can be calculated for the body
(usually the sun) but its a bit tricky to make that the moment when
the altitude is recorded. By making a series of sights over a few
minutes before and after this moment then plotting the slope the
appropriate moment can be adopted for sight reduction. As a bonus all
the other advantages of this slope analysis of the individual sights

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