Altitude Readings

All GPS receivers get signals from satellites and from them they calculate their position on earth, this position is in 3 dimentions North / South, East / West and Altitude (hight above sea level), GPS receivers usually output NMEA sentences which are lines of readable text like this :-

$GPGGA,170834.00,4124.8963,N,08151.6838,W,1,05,1.5,280.2,M,-34.0,M,x.x,xxxx*75

Each NMEA sentence carries different data that can be used by the user, the GGA sentence includes altitude data :-