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On Sat, 4 Nov 2006 13:14:10 +1100, Peter Fogg wrote:

> The experiment you are working you way towards carrying out
> involves observations made over a distance parallel to the
> ground, close to the ground. Refraction will depend on your
> atmospheric conditions there, and could be different to the
> refractive effect from sighting from eye height skywards.
>
True, over longer distances or special conditions, and that
explains a slight disadvantage in the US custom of using a
vertical surveying rod for stadia derived distances.  The method
of using a horizontal two meter subtense bar, common in Europe,
has more potential accuracy ... according to the texts.

So:  Place the sextant horizontally on a small but sturdy
platform on a good surveying tripod.  Then both laser beams will
be in the same layers of air.  Do not have either beam run close
to a building or big tree, as there is such a thing as
horizontal deflection also, and there are warnings against that
practice in geodetic surveying.  Depending on the weather, you
may find the beams quiver a bit due to local mirage.  Night
observations are the best, especially if the ground and air
temperatures are close to being equal, and the beam will be more
steady.

--
Richard, 2006-Nov-03 9:28:15 PM GMT - 6 hours for summer

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