Android: How to get accurate altitude?

Question

I need to get an accurate measurement of altitude using GPS only.

I tried Location.getAltitude(), but that is terribly inaccurate. Any advice?

Answer 1

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.

Answer 2

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.

Answer 3

There are other ways to get the altitude than by GPS. You can use the barometer but as there isn't that many devices with a barometric sensors yet (only the new ones). I will recommend to use a web service to acquire the desired data.

Here is a question which should help you through: Get altitude by longitude and latitude in Android

Answer 4

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:

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)

Answer 5

For newcomers I made a library that wrap LocationManager into rxjava observables and add some observable helpers to get sea level altitutde from Nmea/GPGGA mre info here

Answer 6

There are libraries, such as the open-source CS-Map which provide an API that do these lookups in large tables. You specify the coordinates, and it will tell you the height offset that needs to be applied to the ellipsoidal height at that location to get the "real-world" orthometric height.

Note: Having used CS-Map before, it isn't exactly straight-forward 5-minute job to plug it in. Warning you in advance that it is more complicated than calling a single API with a set of lat-long coordinates and getting back a height. I no longer work at the company where we were doing this kind of work, so unfortunately cannot look up the code to say exactly which API to call.

Doing a google search for it right now (2019), it seems CS-Map has been merged into MetaCRS in OSGeo, the "Open Source Geospatial Foundation" project. That same search led me to this old CS-Map wiki as well as the PROJ GitHub page, where PROJ seems to be similar project to CS-Map.

Answer 7

I would recommend using NmeaListener, as sladstaetter suggested in his answer. However, according to NMEA documentation, "$GPGGA" is not the only sentence you can get. You should look for any "GGA" sentence ($--GGA).

Regular expressions are great for that, e.g.:

@Override
public void onNmeaReceived(final long timestamp, final String nmea) {
    final Pattern pattern = Pattern.compile("\\$..GGA,[^,]*,[^,]*,[^,]*,[^,]*,[^,]*,[^,]*,[^,]*,[^,]*,([+-]?\\d+(.\\d+)?),[^,]*,[^,]*,[^,]*,[^,]*,[^,]*$");
    final Matcher matcher = pattern.matcher(nmea);
    if (matcher.find()) {
        final float altitude = Float.parseFloat(matcher.group(1));
        // ...enjoy the altitude!
    }
}