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At 16:42 02/07/2012, you wrote:
>Geoffrey:
>
>I have only the vaguest knowledge of land surveying techniques, but
>I am forced to ask two questions:
>
>1.  Do land surveyors do astronomical observations to establish
>position any more?

I could respond by asking if professional sailors do astronomical
observations to establish position any more? ;-)

>  Every surveying crew I observe these days seems to have one of
> these high-precision GPS devices.   In fact, did they ever do it
> except, for example, to establish the position of some island in
> the Pacific?    Every illustration of surveying techniques I've
> seen seem to be based on distances and angles from known starting
> points (called, I believe, traverses).  The Great Survey of India,
> a 60-year effort to map the boundaries of the subcontinent, was
> strictly trignometric; if celestial observations were part of the
> surveyor's repitorie, would they not have been used here?

Ah yes, but you need to know the azimuth angle of those traverse
lines. And using a theodolite to do a timed observation of the sun to
obtain a reference azimuth is a very quick and accurate way of doing
it - even in these days of highly accurate GPS receivers. The NA is a
very available and very suitable ephemeris for doing such work.

Geoffrey

   

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