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    Re: Index corr., Octant as dipmeter
    From: George Huxtable
    Date: 2004 Nov 22, 08:52 +0000

    Jared Sherman wrote, about anomalous dip, first quoting from my earlier message
    ><on land has little to do with it.>>
    >Thank you, George. My understanding was that part of those "changes" was
    >also the fact that the land and water heat and cool very differently with
    >solar radiation, so that the air masses above them also heated and cooled
    >VERY differently, i.e. which is why hang gliders find thermals often based
    >what is under them, i.e. water, land, bright areas, dark ones.
    >If variations in the temperature of the air mass over the water are
    >important, how can it not matter that the air masses "here" and "there" are
    >going to be at very different temperatures, i.e. from one being over the
    >land and the other over the water, where air temperatures will be radically
    >To what altitude are these changes in air temperature problematic?
    Yes, of course, solar heating of the ground has an important effect, on
    land. It can give rise to mirages, or to what look like reflecting pools on
    a tarmac road surface. Sand, for example, can quickly get too hot to walk
    on, and the rising hot air can distort the path of light passing over it.
    Depending on how the ground varies in different directions, then the local
    dip will also vary, in different directions. That's on land, but from on
    land you are seldom observing altitudes above a sea horizon, except as a
    shore exercise, perhaps.
    If an altitude is being measured above the horizon from a coast, on land,
    then the observer needs to be near the water's edge; not back some way from
    the shoreline. Otherwise he will get some very strange instances of dip, as
    the Sun heats the ground under his light-path.
    But at sea, where a horizon is being observed from on board, it's only in
    odd circumstances that low land will intervene along the path, for the
    observer to look over when he sees the horizon. If the observer is THAT
    close to land then he has more troubles on his plate than celestial nav.
    will solve! And if land did intervene in that way, I would distrust any dip
    corrections; but you have to do your best with whatever information is
    In the Red Sea, I understand, navigation is bedevilled by the presence of
    many local low reefs, and where these dry out and are heated by the Sun,
    then that's likely to give strange values of dip for a body seen over that
    reef. And in those special circumstances the dipmeter will provide an
    imperfect correction, because, as Jared pointed out earlier, it's averaging
    dips in two opposite directions. So, as usual, the navigator has to
    interpret his information with some caution.
    Jared asked-
    "To what altitude are these changes in air temperature problematic?".
    That's an easy one to answer. From zero height to the observer's height of
    eye, whatever that may be. Changes above that height have no effect on the
    dip: none at all.
    contact George Huxtable by email at george---.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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