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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: Raw data for bubble
From: Peter Fogg
Date: 2007 Mar 9, 15:51 +1100

Bill wrote:

> The second paragraph is what I mean. Take the Hc (calculated elevation, not
> Ho--observed elevation) from your AP every 4 to 5 minutes and plot that
> slope. Fit that slope to the observations.  Then you are working against a
> known and can more easily detect outliers.

This is a new method for me. I calculate the 'Change of Altitude in 5
Minutes of Time' as a function of azimuth and latitude. The main thing
is to end up with the actual (apparent) rise or fall as a line, used
to compare the sights made against. They are plotted on graph paper,
altitude vertically, time horizontally. If the sights were perfect
their slope would be parallel to the calculated slope. The extent that
they do not fit that slope is an indication of error, but only of
random error.

> Exactly. I have used Excel to compare the the slopes of calculated altitude
> (Hc) versus  Ho (observed altitude).  When comparing the observations to the
> Hc slope, the stray sheep often stick out like a sore thumb.  They are much
> harder to identify compared to the linear regression slope for obvious
> reasons.  The two slopes can be significantly different.

Do you mean that you use Excel to draw a graph?

Another advantage of this slope is that once the pattern of sights has
been reconciled as best as possible with the actual slope, any point
along this line can be adopted as the altitude/time combination
adopted for sight reduction. 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.

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