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After an earlier message from Guy-

| "Having just enjoyed a wonderful weekend at Lake Tahoe elevation of
| 6225 ft. I was thinking about what my height of eye would be if I
were
| taking sites at this elevation? Not that I would be lost on the Lake
it is
| 22 miles long by 12 miles wide."

Bill commented-

| It struck me in the shower this AM I may have missed one point that
had been
| discussed a year or two ago.  That being reduced refraction in the
thinner
| air.  I don't recall the consensus--if there was one.
|
| The broadcast barometric pressure will be adjusted to sea level.  At
6225
| feet multiply the broadcast BP by .823 for local BP.  The adjusted
BP will
| be off the almanac table, but gives you a starting point.

Bill's second-thoughts (and I wonder if the shower helps to produce
them) point to a better answer to Guy's question than his
first-thoughts did.

Refraction depends on air density, so for atmospheric refraction, Guy
could simply take his sea-level value for refraction (adjusted for
local temperature if he thinks fit) and then muliply it by 0.823.

But Guy's question referred to height of eye, so was he asking about
an appropriate dip correction, I wonder? If he gets a long enough
view, along the lake, to see a true horizon, that is.

Dip, mostly, just depends on the radius of the Earth and the height of
eye, so altitude above sea level doesn't really affect it. Except that
refraction, on the lower few feet of the atmosphere, between the level
of the observer's eye and water-level, normally reduces that geometric
dip by about 1 part in 12, (by 8%, say), so 92% remains, and this is
the value you will find in the dip tables. That refraction component
depends greatly on the local air-temperature gradient above the water
surface, and is very variable.

With the reduced local air-pressure at Lake Tahoe, that refraction
component would itself be reduced to 83% of its sea-level value, so
the correction to geometrical dip would now be 6.7% of the geometric
dip, so the net result at Tahoe would be 93.3% of it, compared with
the 92% that was listed in the tables. That is, the effictive dip for
a farticular height of eye, will increase, by a factor of 93.3 / 92,
or 1.4%, above the book-value. In almost all circumstances, that's
quite negligible.

George.

contact George Huxtable at george@huxtable.u-net.com
or at +44 1865 820222 (from UK, 01865 820222)
or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.


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