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    Re: sextant index error measurement
    From: Bill B
    Date: 2006 Nov 04, 01:03 -0500

    Richard wrote:
    > 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.
    I had lunch with an old college buddy a while back that was US Navy and now
    owns a surveying company.  On topic he did mention, if I recall correctly,
    that in some parts of the USA  traditional methods (optical?) had to take a
    break during the hottest part of the day due to refraction.  I failed to
    mention that as I trust it will affect both beams almost uniformly, and it
    is better to be thought an idiot than to open one's mouth and prove it. Of
    course that has not stopped me in the past ;-)
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