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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.
>
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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/
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