Editing Sea level (section)

== Aviation ==
{{Main|Altitude in aviation}}

Pilots can estimate height above sea level with an [[altimeter]] set to a defined [[barometric pressure]]. Generally, the pressure used to set the altimeter is the barometric pressure that would exist at MSL in the region being flown over. This pressure is referred to as either [[QNH]] or "altimeter" and is transmitted to the pilot by radio from [[air traffic control]] (ATC) or an [[automatic terminal information service]] (ATIS). Since the terrain elevation is also referenced to MSL, the pilot can estimate height above ground by subtracting the terrain altitude from the altimeter reading. [[Aviation chart]]s are divided into boxes and the maximum terrain altitude from MSL in each box is clearly indicated. Once above the transition altitude, the altimeter is set to the [[international standard atmosphere]] (ISA) pressure at MSL which is 1013.25 hPa or 29.92 inHg.<ref>US [[Federal Aviation Administration]], Code of Federal Regulations [http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFar.nsf/3276afbe72d00920852566c700670189/da37f1d83828491d852566cf00615210!OpenDocument Sec. 91.121] {{Webarchive|url=https://web.archive.org/web/20090426130539/http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgFar.nsf/3276afbe72d00920852566c700670189/da37f1d83828491d852566cf00615210!OpenDocument |date=26 April 2009 }}</ref>