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    Re: Leap second today
    From: James N Wilson
    Date: 2012 Jul 2, 17:59 -0700
    Several decades ago I learned how to take Sun shots as part of a surveying course. We used a transit with an eyeshade, but I don't know where the astronomical data came from. It definitely wasn't the Nautical Almanac, which I learned about many years later.
    Jim Wilson
    On 2 Jul 2012 12:39:40 -0700 "G Becker" <george@gwbeckerpls.com> writes:


    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? 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?

    2. If land surveyors were to attempt to do high-precision location via celestial sights, would they use the NA for celestial data or a publication giving celestial data to even higher precision?

    We're talking about the Nautical Almanac, which is intended for celestial navigation of ships. I suspect an almanac with data that, with perfect observations, gives location to within 0.1 nautical mile is quite sufficient for the task.


    I just deleted a long diatribe on the state of Land Surveying today.. instead, I'll attempt to answer to your two questions. By the way, I'm a licensed land surveyor.

    1a. No. when they seldom did, this was the reference ephemeris ( http://www.rollanet.org/~eksi/Handbook.htm )2008 was the last year published.

    1b. Yes. Usually via a sun shot. The software in my data collector has a sun shot option built in. The problem with modern total stations is they suggest you not point it toward the sun due to the delicate optical electronics. (go figure, maybe a sales ploy)

    1c. I'm not familiar with the India Survey.

    2a. See 1a.

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