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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

    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---.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/

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