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    Re: Determining Elevation with an Altimeter
    From: Brian Whatcott
    Date: 2002 Feb 28, 21:22 -0600

    At 11:18 PM 2/27/02, you wrote:
    >Does anyone have any clever ideas about determining one's elevation above
    >sea level using a sextant?  What if one knows one's
    >latitude and longitude exactly -- would that help?
    >Daniel K. Allen
    The most accurate (instant) altitude is likely to be found by using a
    pressure altimeter. This has a setting scale for equivalent sea level pressure,
    and this setting can be obtained from the nearest airfield with a tower.
    European altimeters are usually set with a millibar scale, and US ones are
    equiped with a scale graduated in inches of Mercury.
    The indication is resolved to about +/- 25 ft but does not represent an
    actual altitude above the sea level pressure as set, but rather an
    idealized altitude/pressure relation which holds reasonably well at mid
    This International Standard Atmosphere is modeled as a smoothly declining
    pressure and temperature with altitude.  The temperature lapse rate is
    given as -6.5degC per km to 11 km. This implies relatively dry air. Here
    are some values.
    Pressure in millibars    Temp in degC    Altitude (ft)
    1013.2 (=29.92inHg)      15                          0
      942.1                               11                   2000
      875.1                                7.1                 4000
      811.9                                3.1                 6000
    If you phone a local airfield tower or FAA station and get the equivalent
    sea level pressure setting or "QNH" setting for a local airport of known
    elevation, and if its temperature is similar to the value expected for its
    elevation, and when you use this setting, the altitude indicated on your
    altimeter is similar to the  value you would expect for the temperature at
    your position, then the altimeter is as close as it can get to reading true
    Repeating the process and averaging the values sharpens the precision.
    The altimeter can be checked by carrying it to an airport and with the
    present QNH set, verify that the airfield elevation indication agrees with
    the chart.
    I suppose that if you capture the data stream to a PC  from a GPS for
    elevation over a period and average that, it would provide useful accuracy too.
    Brian Whatcott
       Altus OK                      Eureka!

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