Android - ทำไมค่า getAltitude เป็นค่า 0 ครับ พยายามเก็บค่า location.getAltitude()

public class MainActivity extends Activity implements LocationListener {
	LocationManager locationManager;
	LocationListener locationListener;
	Context context;
	TextView t;
	Location location;
	String provider;
	Criteria criteria;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		t = (TextView) findViewById(R.id.txt);

		locationManager = (LocationManager)this.getSystemService(Context.LOCATION_SERVICE);
		 
        // Creating an empty criteria object
        criteria = new Criteria();
 
        // Getting the name of the provider that meets the criteria
        provider = locationManager.getBestProvider(criteria, false);
        
        if(provider!=null && !provider.equals("")){
        Location location = locationManager.getLastKnownLocation(provider);
        
        locationManager.requestLocationUpdates(provider, 20000, 1, this);
        
        onLocationChanged(location);
        }
	}

	@Override
	public boolean onCreateOptionsMenu(Menu menu) {
		// Inflate the menu; this adds items to the action bar if it is present.
		getMenuInflater().inflate(R.menu.main, menu);
		return true;
	}

	@Override
	public void onLocationChanged(Location location) {
		
		t.setText(Double.toString(location.getAltitude())+"/"+Double.toString(location.getLongitude()));
		Toast.makeText(getApplicationContext(),
				Double.toString(location.getAltitude())+"/"+Double.toString(location.getLongitude()), Toast.LENGTH_LONG)
				.show();

	}