[GRASSLIST:5047] Re: 100KM raduis viewshed

[Dan Jacobson]

We have considered the earth's curvature, but um, "The Effect Of
Atmospheric Refraction On The Observed Elevation Angles Of Peaks"
http://tchester.org/sgm/analysis/peaks/refraction.html indeed affected
my results.  Without considering it some peaks visible in binoculars
would sink behind other peaks in d.3d's drawing.  So now I'm OK with

$ xc=235480 yc=2675355
$ r.mapcalc "nantou.bend=round(nantou-((x()-$xc)^2+(y()-$yc)^2)*.0000000675)"
which I found in a local textbook.
I suppose I won't fret about height and temperature, as they seem to
cancel out on the above webpage, though I could easily utilize FROM
and TO heights if there was a simple formula available.
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