GPS Data

To collect GPS data automatically with a form submission, see https://dimagi.atlassian.net/wiki/x/ZynKfw. For troubleshooting, see GPS Limitations.

Deciphering the Data

GPS data in CommCare is captured in decimal degrees. It has 4 components, separated by a space character:

1. Latitude: in decimal degrees

2. Longitude: in decimal degrees

3. Elevation: in metres, and

4. Precision / Accuracy: in metres

For example, a data point collected in CommCare and viewed through form data may look something like 25.615311244889146 85.08323017699811 28.44 20.0, where:

1. 25.615311244889146 specifies the latitude in decimal degrees

2. 85.08323017699811 specifies the longitude in decimal degrees

3. 28.44 specifies the elevation in metres, and

4. 20.0 specifies the precision in metres.

Mapping the Data

GPS data in CommCare is captured in decimal degrees. It is easy to make a map using Google Earth.

Google Earth requires a kml file. You can generate a kml file using an Excel file and a free converter like this one.

1. Export data from CommCareHQ into Excel

2. If all of the coordinates are in one cell, select the GPS coordinates field and use the Excel "text to columns" feature to separate the GPS field into its 4 component fields (latitude, longitude, elevation, precision). You can delete the elevation and precision columns for a basic map.

1. Copy the latitude and longitude fields into a clean Excel file. The first number is latitude. Label the two columns in the top rows at latitude and longitude respectively.

2. Create additional columns with other information you want in the map (i.e. name, ID, etc.)

3. Save the Excel file as a xls (Excel 97-2003 Workbook) NOT xlsx

4. Upload the file into the converter

5. Open the resulting kml file in Google Earth

Extracting GPS Components

The location returned by a GPS question is a string composed of the latitude, longitude, altitude and accuracy, each separated by a space. In your export configuration, if you choose "Expand Checkbox Questions" then the GPS data will automatically be split into four columns.

If all of the coordinates are in one cell, the following expression will extract the latitude from the location: if(/data/location = '', '', selected-at(/data/location, 0))

Similarly, for longitude: if(/data/location = '', '', selected-at(/data/location, 1))

When using the Auto-Location capture feature, that GPS location is available through /data/meta/location and can be accessed by enabling "Show Advanced Questions" in a form export.

Calculating Distances Between GPS Coordinates

CommCare now supports distance calculation via the distance XPath function on both J2ME and Android phones. For example, the following expression returns the distance between two locations in meters:

distance(/data/location1, /data/location2)

Since the distance function returns -1 if either argument is an empty string, a more robust way to display the distance is shown below:

if(/data/location1 = '', '', if(/data/location2 = '', '', distance(/data/location1, /data/location2)))

Limitations to GPS

GPS positions provided via android devices are generated using multiple different methods, resulting in highly variable performance. Performance depends on the device, the cell network, availability of GPS satellites and line of sight to these satellites (http://en.wikipedia.org/wiki/GPS_signals ).

GPS devices need 3 pieces of data in order to find the current coordinates:

• Almanac data: very coarse satellite position information, good for 4 months and taking 12.5 minutes to download from a satellite; updated every 6 days

• Ephemeris data: fine-grained satellite position information, good for 4 hours and taking 18-36 seconds to download from a satellite; updated every 2 hours

• A lock on 3+ satellites 

A cold start happens when the device was off for more than 4 hours and new ephemeris data needs to be downloaded. A warm start happens when the ephemeris data is still valid and the device simply needs to get a lock on satellites. A device is "hot" if it has that lock. To accelerate the "time to first fix" (time from turning on the device to getting coordinates), a variety of techniques get used: (http://en.wikipedia.org/wiki/Assisted_GPS )

• A-GPS or assisted GPS 

  • The phone downloads ephemeris data from the cell tower (MSB)

  •  The phone sends its captured GPS data to the cell tower, letting the tower perform the calculations (MSA)

• The phone uses cell tower triangulation, totally ignoring the phone's GPS functionality - accuracy is usually reported as being more than 1000m. 

Some phones have incomplete GPS hardware, requiring a cell network to function - they depend on MSA as described above. The quality of the GPS antenna will affect how quickly a device will get a lock; so will trees, tall buildings or being inside concrete structures. Stand-alone devices usually have better GPS hardware and antennas.

Finally, new devices often support GLONASS in addition to GPS. GLONASS is a Russian network very similar to GPS. When combined, devices that support GLONASS and GPS will have very rapid satellite locks and high accuracy. GLONASS particularly excels in high latitudes (very south or very north of the equator).

Speed and accuracy of getting a GPS lock is highly variable. For example, the S10 has relatively good GPS hardware, including GLONASS and A-GPS support. However, it will still have issues when surrounded by concrete and getting a satellite lock will take longer when outside of cell network data coverage. It's not possible to "boost" the signal. However, you can ensure that your GPS unit is at the very least always "warm" and thereby reduce the acquisition time. For example, running something like Google's "MyTracks" app set to record the GPS location every 15 or 30 minutes should minimally impact the battery life but keep GPS should remain warm (Simply running MyTracks in the background is enough, it is not necessary to have it open all the time).