When a plane reaches its designated airport, the pilot needs to contact the tower to get their DASI (Digital Altimeter Setting Indicator) number. This number is created by adding local barometric pressure with a pressure correction for the landing strip's station elevation. This is typically done using the National Advisory Committee for Avionics' report 1235, a chart detailing the relationship between pressure and altitude, to account for pressure variation for the station.
The model 370 is able to give the user that DASI number.
The standard for altimeter settings is 29.92" Hg.
- 29.92" Hg is the standard for sea level pressure
- 29.92" Hg is set into the altimeter by the pilot when flying above 18,000 feet in altitude
- When flying below 18,000 feet the local sea level pressure is to be used
Example for determining elevation:
Stow Airport in Stow, MA
Station elevation = 268 feet
Barometric pressure = 29.78" Hg (1009.4 mB)
From the NACA 1235 report, 268 feet above sea level is a 0.727" Hg decrease in atmospheric pressure
268 feet is 0.68 the distance from 200 feet to 300 feet, which results in the 0.727" Hg change
The DASI number sent to the pilot is 29.5256. This number is entered into the altimeter, and the altimeter will now display the plane's altitude relative to the airport. The pilot will not nose dive into the landing strip nor shut off the engines 50 feet above it.
References:
https://aviation.stackexchange.com/questions/2991/where-did-the-standard-pressure-29-92-come-from
http://naca.central.cranfield.ac.uk/reports/1955/naca-report-1235.pdf