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    Re: dip, dip short, distance off with buildings, etc.
    From: Frank Reed CT
    Date: 2006 Jan 13, 01:41 EST

    Michael B, you wrote:
    "My understanding is  that terrestrial surveyors  of old ( and RN surveyors of
    land  features)got their local horizontal datum from a highly portable
    reference  source, i.e. a leather bag of mercury. Exactly as the technique
    for finding  the position of the back garden using a paint roller tray full
    of oil. In  that type of situation, what's dip and near horizon refraction
    got to do with  anything...why not do as the oldies did and avoid the
    If  you're on land, sure, you can use an artificial horizon to get the local
    horizontal datum, just as you say. But that's only part of the issue. First,
    that clearly doesn't do you any good if you're on a boat. But second, even
    after  you've established the horizontal, all observations of distant objects
    are  significantly affected by terrestrial refraction, and it is a variable
    quantity.  So even if you have some alternate means of getting the horizontal, you
    still  have to deal with the issues of lapse rates and temperature
    All of these things --dip, dip short, range of visibility,  distance by
    measuring heights of objects beyond the horizon, etc.-- are  tabulated in the
    navigation manuals, like Bowditch. And navigators have used  these tables with
    little reason to doubt them in most cases. Not that they  haven't been warned. In
    fact, there is almost always some advice in the  navigation manuals that the
    navigator should watch out for variable refraction.  Unfortunately, this advice
    has been qualitative and sometimes contradictory.  Although "traditional
    navigation" has little remaining practical importance, we  do at least now have
    the luxury of going back and cleaning up some of the  material that has been
    done a bit sloppily in the past.
    42.0N  87.7W, or 41.4N 72.1W.

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