Altimeter using pressure sensor and WGS84 altitude correction

[jay3]

• use sensor pressure if available (Galaxy S3, Galaxy S4, Galaxy Note, Galaxy Note 2, Galaxy Nexus S, Nexus 4, Nexus 5) to compute a more precise altitude (and altitude variations)
• use Nmea to get a WGS84 geoid height and correct altitude Standard GPS altitude is the altitude above the WGS84 reference ellipsoid and not above sea level.