How does getAltitude() of Android GPS Location Works

Question

HI

I tried to implement a simple GPS tracker. Therefore is used

lm = (LocationManager) getSystemService(Context.LOCATION_SERVICE);
lm.requestLocationUpdates(LocationManager.GPS_PROVIDER, 1000, 0, this);  

Then i used the

public void onLocationChanged(Location location) {

method to read the altitude of my current location.

But i dont really know what Location.getAltitude() returns. The document says it returns the altitude. But is this in meters? or feets? if i put the phone on the desk next to me, this value changes between 500 and -500??

How does this really work???

Answer 1

As far as I'm aware its meters above sea level, however I don't think its very accurate.

Answer 2

I assume that your desk is located inside a building. If you get a GPS fix in a building you shouldn't expect the accuracy to be very high. Have to tried getting the accuracy of the fix by calling getAccuracy()?

Answer 3

The altitude value you get is in meters from the gps (WGS84) reference ellipsoid and not from the geoid.

From my own experience the GPS are really bad at altitude values.

I read this on the GPS Status FAQ:

GPS does not report the height above the mean sea level, rather the GPS system compares the height to the WGS84 reference ellipsoid which may be above or below the actual sea level. In different parts of the earth it can be off by more than 200 meters (depending on the mass distribution of Earth). For example the geoid's surface around Florida is above the mean sea level by a good 30-40 meters, which means that standing on the shore would show you -30m as altitude. This is normal, and not an error, and caused by the fact that the altitude is relative to an artificial reference surface and not to the sea level. If you are interested in this topic, I recommend to read Mean Sea Level, GPS, and the Geoid.