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    Re: Table A4 + elevation?
    From: George Huxtable
    Date: 2003 May 3, 00:43 +0100

    Doug, I don't think you are getting full use out of table A4, though it
    won't make a big difference to the result.
    That table wasn't intended to cope with corrections at an altitude; just
    with the sort of pressure variations that weather causes. It could cope
    with navigation on the Great Lakes, however.
    Your 2100 ft altitude takes it well out of its intended range. 28 inches
    isn;t a very good approximation for pressure at 2100 ft; I make it more
    like 27.5. The inches of Mercury scale, on the right, goes down no lower
    than 28.6 inches. Quite a difference. All is not lost, however.
    Take table A4, or better, a photocopy of it. Look at the horizontal line
    across the diagram, just below the number 970 at the left. Draw in another
    horizontal line, 1.25 inches below it (using a ruler, not the "pressure in
    inches" scale on the right!). It will run right across the numbers in the
    table, but don't worry about that. Mark that line with the number "935" at
    the left. That new line corresponds to the expected pressure at your height
    of 2100 ft. under normal weather conditions.
    Now look at the sloping lines in the diagram. Extend them, down and to the
    left, until they meet the new horizontal line. Now put a dot on that
    horizontal line at a position corresponding to your air temperature. That
    will tell you which correction zone, between the sloping lines, you ought
    to be in: maybe H or perhaps J. Choose the appropriate correction table.
    As I said, it won't make a lot of difference. I've only spelled it out
    because you asked.
    Yours, George.
    contact George Huxtable by email at george@huxtable.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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