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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: Bowditch Table 15
From: Jim Thompson
Date: 2005 Jan 24, 12:29 -0400

```I look forward to the explanation too.  I have never been comfortable with
Table 15.  I just spent the morning working paper examples and searching the
Nav-L archives for prior discussions about Table 9, but it remains unclear.
My muddle is posted here, but I will fix it when I understand what is going
http://jimthompson.net/boating/CelestialNav/CelestNotes/Bowditch15.htm

Is the entering value (H-h) supposed to be the difference between the height
of the object above sea level (say a 100' lighthouse) and the height of eye?

When I do drawings, the vertical angle that an observer would shoot between
the top of an object over the horizon and the horizon has to increase for
decreasing height of eye as the observer climbs down a 100' mast, but the
table suggests that vertical angle increases (keep "Distance by Vertical
Angle" constant, and change [H-h] appropriately as you look at Table 15).

The solution ""Distance by Vertical Angle" cannot be the geographic range,
or distance between observer and object over the surface of the earth: I ran
some examples by adding distance to the horizon for an observer at various
heights of eye, to the distance from horizon to a 100 ft lighthouse.  But
"Distance by Vertical Angle" cannot be the distance from horizon to the 100
ft lighthouse either, according to my examples.

To further confuse matters, see the difference in definition for the
entering value height of object in the NGIA online calculator at
http://164.214.12.145/NAV_PUBS/APN/Tables/T-15.pdf.  I emailed NGIA to
obtain their underlying formula for that calculator.

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
--------------------
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> -----Original Message-----
> From: Trevor J. Kenchington
> ...I am having trouble with Table 15...

```
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