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Subject: Re: Table A4 + elevation?
From: George Huxtable (george@XXX.XXX)
Date: Fri May 02 2003 - 19:43:49 EDT
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.
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contact George Huxtable by email at george@XXX.XXX 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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