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Jim wrote

> 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.

If I understand, we are discussing the table (15 in my copy) that determines
distance from an object by using the vertical angle between its top and the
horizon.  In this case the base is below the horizon (or, I can see the mast
top but not the hull of a sailboat).

First, the sextant reading is adjusted for index error and dip, but nothing
else. Imagine the dip correction is what may yield negative values for the
angle.

As a non math-major I'll do my best to represent the formula from Bowditch
in ASCII:

D = (square root of (tan angle/0.0002419)squared)) + H-h/0.7349 - tan
angle/0.0002419

The constants 0.0002419 and 0.7349 account for standard terrestrial
refraction.

H is height of object above sea level in feet
h is height of observer's eye above sea level in feet

If I read the explanation correctly, the distance (D) is from the observer
to the object, not distance from object to observer's horizon.  (Perhaps the
dip correction helps out here as well?)

At any rate, will let those with appropriate skill sets chew on the
formula).

Hope this helps.

Bill

   

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