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Re: Q: how to calculate refraction at higher altitudes on land?
From: Richard B. Langley
Date: 2002 Mar 1, 14:14 -0400
From: Richard B. Langley
Date: 2002 Mar 1, 14:14 -0400
The American Meteorological Society defines "barometric pressure" as identical to "atmospheric pressure." The pressure at mean sea level obtained by "reducing" the observed station pressure is called the sea-level pressure. Perhaps there's different usage of the terms in Britain. -- Richard Langley Professor of Geodesy and Precision Navigation On Fri, 1 Mar 2002, Dr. Geoffrey Kolbe wrote: >Hello Dan. > >I think your "high quality Chelsea barometer" is going to be your best bet >in finding your altitude, here is why. A lot of people think that >barometric pressure is the same thing as atmospheric pressure. It is not. >Atmospheric pressure is the actual air pressure at your location. >Barometric pressure is the atmospheric pressure that would be indicated if >your location was at sea level. Barometers are (or should be) corrected for >altitude so as to give sea level air pressures at their location. Mariners >tend to use the terms interchangeably, which is understandable as at sea >level they are interchangeable. > >On a day where there is a wide spread high pressure system over your >location, (no wind, blue sky) ring up a local weather centre and ask them >for the barometric pressure over your area. Synoptic charts and weather >maps which show isobars actually show barometric pressure isobars, not >atmospheric pressure isobars. Now, assuming your "high quality Chelsea >barometer" has NOT been corrected for altitude, the difference between your >barometer and the weather centre will give you your altitude - after a >little calculation. Going through this exercise at a sea level location >first will enable you to check your barometer out for calibration. > >Geoffrey Kolbe. > >At 10:03 28/02/02 -0800, you wrote: >> >>I am specifically trying to determine the elevation of my house. >> >>The topographic maps of my area are decades old. They do not show >>our streets or houses, and in fact the area has been graded to some >>extent as well, so even finding my exact location via GPS and then >>looking on the map only gives a ballpark figure. >> >>Since SA has been turned off, the GPS gives a range of elevations >>from 580 feet to 650 feet. The Garmin GPS 3 tends to wander over >>this range. I am trying to get a better value. >> >>I have a high quality Chelsea barometer, a Garmin GPS, and many >>sextants. I have high mountains behind my house, lots of clouds, >>and the only kind of horizon I'll ever see is an artificial one. >>In the winter, the sun comes up behind the mountain at about 11AM >>and goes down about 12:30PM! >> >>Given these constraits and tools, can I get a better estimate of >>my elevation? >> > >Border Barrels Ltd., Newcastleton, Roxburghshire, TD9 0SN Scotland. >Tel. +44 (0)13873 76253 Fax. +44 (0)13873 76214. > =============================================================================== Richard B. Langley E-mail: lang@unb.ca Geodetic Research Laboratory Web: http://www.unb.ca/GGE/ Dept. of Geodesy and Geomatics Engineering Phone: +1 506 453-5142 University of New Brunswick Fax: +1 506 453-4943 Fredericton, N.B., Canada E3B 5A3 Fredericton? Where's that? See: http://www.city.fredericton.nb.ca/ ===============================================================================