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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 26, 16:34 -0400

```I searched through the rest of Bowditch 2002 for more references to the
Table 15 procedure, and found this odd paraphraph in Chapter 22, Article
2202:

"Distance by vertical angle between the waterline and the top of an object
is computed by solving the right triangle formed between the observer, the
top of the object, and the waterline of the object by simple trigonometry.
This assumes that the observer is at sea level, the Earth is flat between
observer and object, there is no refraction, and the object and its
waterline form a right angle. For most cases of practical significance,
these assumptions produce no large errors.
D = sqrt[(tan^2a/.0002419^2) + ((H-h)/0.7349)] - (tan a/.002419)
where D is the distance in nautical miles, a is the corrected vertical
angle, H is the height of the top of the object above sea level, and h is
the observer's height of eye in feet. The constants (0.0002419 and 0.7349)
account for refraction.".

I don't see why the Table 15 equation is given in this paragraph.  Looks to
me like a chunk of text is missing between the end of "...produce no large
errors" and the equation, and that in fact the second half, which contains
the equation, belongs to a missing title that should describe "Distance by
Vertical Angle Measured Between Sea Horizon and Top of Object Beyond Sea
Horizon".  Can someone check an older Bowditch?

Jim Thompson
jim2@jimthompson.net
www.jimthompson.net
--------------------
Outgoing email scanned by Norton Antivirus

```
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