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It is this distinction -- atmospheric vs barometric -- about pressure
that caused me to ask the related question about refraction corrections
at altitudes other than sea level.

-----Original Message-----
From: Navigation Mailing List
[mailto:NAVIGATION-L@LISTSERV.WEBKAHUNA.COM]On Behalf Of Dr. Geoffrey
Kolbe
Sent: Friday, March 01, 2002 12:27 AM
To: NAVIGATION-L@LISTSERV.WEBKAHUNA.COM
Subject: Re: Q: how to calculate refraction at higher altitudes on land?


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.

   

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