NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Table A4 + elevation?
From: George Huxtable
Date: 2003 May 3, 00:43 +0100
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. ================================================================