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Is there a table or formula to calculate the "reduction factor" for different heights above sea level. I am interested because when I am in Germany I take sights between 450 and 800 mtr. (1475-2620 feet) above sealevel. Or gives the local air pressure on that height already a sufficient correction?

Local air pressure is called "station pressure", at least in the US. https://www.weather.gov/bou/pressure_definitions

If your refraction formula has an input for pressure, use station pressure. However, it may be difficult to obtain. I don't know about Germany, but in the US the "barometric pressure" distributed to the public is actually altimeter setting. Even in Denver (1660 meters or 5400 feet above sea level), the "barometric pressure" you get from TV news is near the nominal sea level pressure.

As I write this, altimeter setting at Denver airport is 30.35 inches Hg. From that, and altitude of the airport, it's possible to get station pressure from this calculator:
  https://www.weather.gov/epz/wxcalc_stationpressure

At 1660 m and 30.35 inches Hg, station pressure is 840.9 mb or 24.83 inches Hg, which is 0.818 of the altimeter setting. In the formula (there's a link on calculator web page) you can see that station pressure is simply altimeter setting, multiplied by a value which depends only on the height of the station. In other words, station pressure at Denver is always 0.818 of altimeter setting.

The normal range of station pressure is small. Temperature variation has much more effect on refraction. For example, if temperature increases from 15 C to 20 C, that's a 1.7% increase in absolute temperature. A 1.7% pressure decrease has the same effect on refraction. But that's 17 mb or 0.5 inch Hg, which is rare.

-- 
Paul Hirose
sofajpl.com