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    Re: Corrections for use of artificial horizon at 1,000' MSL?
    From: Fred Hebard
    Date: 2012 Mar 13, 11:49 -0400

    Hi Alex, glad to see you back on board.
    
    With regard to a barometer, you have to make sure the barometer is not 
    adjusted to sea level.  The lapse rate for barometric pressure is not linear 
    with elevation, but an elevation of 2000 feet drops atmospheric pressure 
    about 2 inches of mercury.  Above about 2000 feet, most barometers do not 
    measure.  An old-fashioned barometric altimeter can give you a direct 
    reading.  They are not very accurate, but are more than accurate enough for 
    the elevation correction.
    
    Fred Hebard
    
    
    
    
    On Mar 13, 2012, at 11:27 AM, Alexandre E Eremenko wrote:
    
    > 
    > Dear Scott,
    > When observing with the  artificial horizon, you
    > 1. Correct for the index error.
    > 2. Divide the result by 2.
    > 3. Correct the result for refraction for the altitude
    > obtained on step 2. (Yellow page in the Almanac).
    > 4. Use table on p. A4 of the Almanac to correct for the
    > temperature and barometric pressure at your altitude.
    > For this you can use a barometer, or use the weather broadcast
    > from the nearby airport, taking the difference of your altitude
    > from the airport into account, if it is substantial.
    > 
    > You enter all tables with the altitude obtained on Step 2.
    > 
    > The result is your observed and corrected altitude.
    > 
    > Your altitude of 960 feet over the sea level probably does not matter much 
    for refraction, and most of the correction for it, will come from table
    > A4. Only in high mountains you will probably need an additional correction.
    > 
    > Myself, I use A4 correction only in winter, or when doing lunars,
    > using a home barometer, or my airport pressure.
    > 
    > 
    > Alex.
    > 
    > On Tue, 12 Mar 2012, Scott Statz wrote:
    > 
    >> 
    >> I live in Madison, WI, at about 960' mean sea level.
    > I am teaching myself with the help of CELESTIAL NAVIGATION BY H.O.249
    > by John E. Milligan and am still working my way through the book.
    > When using an artificial horizon, do I need to be concerned with
    > a special correction for refraction
    > (anything else at this altitude or in my circumstance)?
    > I understand its not necessary at sea level and there are concerns when taking a sight at low angles.
    >> 
    >> Thanks
    >> 
    >> Scott
    >> 
    >> 
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    > 
    > 
    
    
    
    
    

       
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