Android: How to get accurate altitude?

There are two issues with using altitude of a smartphone / tablet GPS:

  1. The altitude is the altitude above the WGS84 reference ellipsoid. It is not the altitude above ground level or sea level. Here is more detail on that: http://www.gpspassion.com/forumsen/topic.asp?TOPIC_ID=10915. This error can be corrected; here is a description how to do that by hand: http://www.unavco.org/edu_outreach/tutorial/geoidcorr.html. The web article links to a calculator to get the Geoid height for correction; I do not know if there is also a web service available for this computation.
  2. The GPS altitude is terribly inaccurate for relatively cheap GPS receivers. Here is an article on that: http://gpsinformation.net/main/altitude.htm. One method to cope with this kind of inaccuracy is to filter the altitude data. I used a circular array data structure to remember the last few (I used 4) altitude readings and compute the average. This sufficed to get a relatively accurate reading of vertical speed for my application.

Another way would be parsing NMEA strings. The $GPGGA sentence already contains the corrected altitude data above sea level.

So, simply create a listener to NMEA-strings for your LocationManager and parse the messages:

private GpsStatus.NmeaListener mNmeaListener = new GpsStatus.NmeaListener() {
    @Override
    public void onNmeaReceived(long timestamp, String nmea) {
        parseNmeaString(nmea);
    }
};

public void registerLocationManager(Context context) {
        mLocationManager = (LocationManager) mContext.getSystemService(LOCATION_SERVICE);
        mLocationManager.addNmeaListener(mNmeaListener);
}

private void parseNmeaString(String line) {
        if (line.startsWith("$")) {
            String[] tokens = line.split(",");
            String type = tokens[0];

            // Parse altitude above sea level, Detailed description of NMEA string here http://aprs.gids.nl/nmea/#gga
            if (type.startsWith("$GPGGA")) {
                if (!tokens[9].isEmpty()) {
                    mLastMslAltitude = Double.parseDouble(tokens[9]);
                }
            }
        }
    }

You can either replace the altitude of the last Location object received through a location listener, or parse the whole new location through NMEA.

Another approach is to measure the altitude from the barometer.

By using the pressure you can calculate the user’s altitude. I’m uncertain of the precision level of this and whether it is more accurate than the other answer’s approach.

By using the hypsometric formula you can calculate the altitude:

hypsometric formula

Variable definition:

P0: Sea-level pressure 
P: Atmospheric pressure 
T: Temperature 

You can get the pressure in android from the environment sensors

SensorManager.getAltitude(SensorManager.PRESSURE_STANDARD_ATMOSPHERE,pressure)

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