NavList:

Frank wrote

> Just to clarify, I do not recommend any particular constants  for Table 15.
> The calculation underlying Table 15 is HIGHLY susceptible to  variability in
> terrestrial refraction. I would recommend doing this calculation  with a range
> of constants to get some feeling for the possible  variation.

Frank

Sorry if you feel I misrepresented you, or that you recommend one "constant"
I presented for testing.  They represent my attempts at reducing some mean
values or "rules of thumb" you and others have posted into alternative mean
working values for T15 calculations.  It was not my intent to imply
endorsement, but rather to give credit to my sources.
===============================
From my notes:

To determine the affects of refraction instead of a vacuum, a constant can
be used
R/(1-C)
R = 3440.1

For example in Bowditch Table 15, first term:
tanA / 0.0002419 = tanA * 4133.939644
4133.939644 = R / (1-C)
C = 0.16783981

Lift due to terrestrial refraction

Frank's rule of thumb:  Lift in minutes = Dnm * 0.15
0.15 /60 = 0.0025
D  is distance in nautical miles
Lift in feet = 6076.1 * ( tan (.0025 D )) * D
==================================

Clearly, as recently as Alex's beach shots, we see that terrestrial
refraction  is anything but constant.  On the water we may not have the
luxury of a full input of up-to-the minute data; or the know how to
interpret if we did.  So we work with mean values.

It is my hypothesis that the mean values for terrestrial refraction used in
Bowditch T15 do not provide sufficient "lift" at 578+ feet above sea level,
and will not get better at sea level.  Therefore my request that readers
test the proposed "constants" under a variety of conditions.

Bill





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