# NavList:

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

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Re: Poor Line of Position Computer/popularity of slide rule sight reduction
From: Gary LaPook
Date: 2010 Jan 15, 11:26 -0800
Which you can compare to the Bygrave method which uses the same manipulation of the device repeated three times to derive first an intermediate value, second the azimuth angle and then third, the computed altitude. In addition to this you do two math steps, first subtracting your assumed latitude (you can use your DR if you choose) from ninety degrees to form co-latitude and then you add (or subtract) the intermediate value to the co-latitude and use the result on the slide rule in calculating azimuth angle and altitude. Using the flat version, the thrice repeated manipulation consists of placing the red cosine scale, printed on the clear plastic sheet, over the cotan scale and lining up a value on the red cosine scale with a value on the black cotan scale and then looking at another value on the cosine scale and reading out a value from the adjacent cotan scale.

You can find more detail at my new website:

gl

Hein Bodahl wrote:
```On 15.01.2010 14:54, Brad Morris wrote:

```
```How does the Poor Line of Position computer compare in
accuracy to a Bygrave or to the more accurate tabular methods?
```
```
>From the book:
http://ia351430.us.archive.org/3/items/simplifiednavig00poorgoog/simplifiednavig00poorgoog.pdf

"With this instrument the calculations
are actually performed with a few direct and
simple movements, and with an accuracy equiva-
lent to that obtained with four place logarithmic
tables."

```
```What about the speed of solution?
```
```
"A comparison of these three methods shows
the following number of book openings and aux-
iliary figures necessary in each to obtain the re-
quired altitude and azimuth:

Logarithmic 8 openings and 45 figures.
Aquino 2 openings and 64 figures.
Mechanical 1 opening and 4 figures."

and

"With this instrument the calculation of the al-
titude, as required by the Saint Hilaire method,
is reduced to four (4) simple and direct move-
ments, while the azimuth may be found by two
(2) movements. The entire calculation is made
on the instrument, the work is straight-forward
and requires the writing down of one figure only."

Could maybe give an idea.

Just ordered the book on amazon.
Hope it has the foldout of the computer.

Hein

```

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