[OT] altimeter calibration?

[raveracer77 at yahoo.ca (Rave Racer 77)]

    I've never heard of altimeters being set by using radar to measure 
distance over ground but it might be possible.  The way light aircraft and 
older large aircraft do it is by measuring static air pressure and comparing 
that to a Mean Sea Level pressure setting that you can adjust on the 
altimeter.  29.92 inches of Mercury is standard atmospheric pressure. 
Inches of Mercury (in/HG) is how all of my altimeters were measured.  If 
you're on the ground at 1000ft above sea level and your local pressure is 
29.92 then you should read 1000ft on your altimeter.  If you know your 
altitude when you hop in the aircraft then you can find you current pressure 
by adjusting your altimeter to your local altitude and reading the pressure 
measurement.  There are local stations all around the country based at small 
airports, navigation aids and other beacons that transmit a recorded weather 
message that gets updated regularly.  This is a good place to get your 
pressure if you're in an unknown area in uncontrolled airspace.  When you 
get above a certain altitude, I think 10,000ft, you're supposed to set your 
altimeter to standard atmospheric pressure so that everyone is using the 
same basis for measurement.  This is more for commercial aviation to avoid 
large differences in pressure settings on long distance flights where two 
aircraft may think they have altitude separation but don't because of the 
different pressures at different points of departure.  Hope that helps.

      Rave Racer
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___________________________________________
----- Original Message ----- 
From: "Spewey" <spewey@xxxxxxxxxxx>
To: <scirocco-l@xxxxxxxxxxxx>
Sent: Saturday, September 15, 2007 12:25
Subject: [OT] altimeter calibration?


> Hey military/airplane people,
>
> I've got a question for you:  once when I was out sailing on a weekday
> on Lake Pepin, two Minnesota National Guard C-130s flew repeatedly up
> and down the lake at about 1000 ft.  Big birds low down wing to wing.
>
> The lake is about 23 miles long and 3 miles wide, actually a wide spot
> in the Mississippi River, probably the biggest open water for a hundred
> miles.  It was an awesome sight but I presume it was not done for
> jollies since they kept going back and forth.
>
> Someone said that maybe they were calibrating their altimeters by flying
> over a flat surface but I wondered how that would make sense if the
> river level changes.  Is that what they were doing?
>
>
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