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In NavList 3849 Gary LaPook wrote:

> Back in March there was a discussion of various formulas to use to
> predict refraction. I have found some other formulas that may be more
> precise. The U.S. Naval Observatory used to publish its "Almanac For
> computers" but stopped it after 1991. That publication has several
> formulas for refraction:
>
> (1)     R' = cot(a + (7.31/(a + 4.4)))
>
> (2)    R = R' - 0.06sin(14.7R' + 13)
>
>
> R is in minutes and a is the observed altitude. "At sea level, with
> air temperature of 10�C and pressure of 1010 mb, R' of eq. (1) is
> accurate to 0.07' and R of eq. (2) is accurate to 0.015' for an
> altitude range of 0� - 90�."
>
>
> "For nonstandard conditions the following formula is accurate to about
> 0.2' for temperatures in the range of -20� to + 40� C and pressures of
> 970 - 1050 mb.
>
> (3)   R = R'((P - 80)/930)/(1 + 8x10 ^ -5(R' + 39) T - 10)))
>
>
>   
I found another formula for temperature and pressure which I used for 
correcting the observations with the demi-cross and compared that to (2):

(4)      R = (0.267 x P / T)/(TAN(Altitude + 
((4.848x10^-2)/(TAN(Altitude)+0.028))))

In our case (alt = 4m, P = 1018mbar, T = 12�) there was only 0.05 arc 
minutes difference with (2)



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