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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?
>
>Dan
>
>Daniel K. Allen
>mailto:danallen46---bi.com
>http://home.attbi.com/~danallen46/

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
latitudes.
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
elevation.

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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