Mobility Features

Author: Thomas Nilsson (tnni@dtu.dk)

Setup

Add the package to your pubspec.yaml file and import the package

No permissions are required to use the package, however, a location plugin should be used to stream data.

We recommend our own plugin https://pub.dev/packages/carp_background_location which works on both Android and iOS as of August 2020.

import 'package:mobility_features/mobility_features.dart';

Step 1: Choose parameters

MobilityFactory mobilityFactory = MobilityFactory.instance;
mobilityFactory.stopDuration = Duration(seconds: 20);
mobilityFactory.placeRadius = 50.0;
mobilityFactory.stopRadius = 5.0;

Optionally, the following configurations can be made, which will influence the algorithms for producing features:

  • The stop radius should be kept low (5-20 meters)
  • The place radius somewhat higher (25-50 meters).
  • The stop duration can also be set to any desired duration, for most cases it should be kept lower than 3 minutes.

Features computation is triggered when the user moves around and change their geo-position by a certain distance (stop distance). If the stop was long enough (stop duration) the stop will be saved. Places are computed by grouping stops based on distance between them (place radius)

Common for these parameters is that their value depend on what you are trying to capture: Low parameter values will make the features more fine-grained but will trigger computation more often and will likely also lead to noisy features.

For example, given a low stop duration, stopping for a red light in traffic will count as a stop. Such granularity will be irrelevant for many use cases, but may be useful if questions such as 'do they take the same route to work every day?' are to be answered.


StreamSubscription<MobilityContext> mobilitySubscription;
MobilityFactory mobilityFactory = MobilityFactory.instance;
MobilityContext _mobilityContext;

void initState() {
    ...
    mobilityFactory.stopDuration = Duration(seconds: 30);
    mobilityFactory.placeRadius = 20;
    mobilityFactory.stopRadius = 5;
}

Step 2: Set up streaming

Location data collection is not directly supported by this package, for this you have to use a location plugin such as https://pub.dev/packages/mubs_background_location.

From here, you can to convert from whichever Data Transfer Object is used by the location plugin to a LocationSample.

Next, you need to subscribe to the MobilityFactory instance's contextStream to be be notified each time a new set of features has been computed.

Below is shown an example using the mubs_background_location plugin, where a LocationDto stream is converted into a LocationSample stream by using a map-function.

/// Start the streaming of location data and mobility features
void streamInit() async {
    /// Get the location data stream (specific to mubs_background_location)
    Stream<LocationDto> dtoStream = locationManager.dtoStream;
    
    /// Start the location service (specific to mubs_background_location)
    await locationManager.start();
    
    /// Convert from [LocationDto] to [LocationSample]
    Stream<LocationSample> locationSampleStream = dtoStream.map((e) =>
        LocationSample(GeoLocation(e.latitude, e.longitude), DateTime.now()));

    /// Provide the MobilityFactory instance with the LocationSample stream
    mobilityFactory.startListening(locationSampleStream);
    
    /// Start listening to incoming MobilityContext objects
    mobilityFactory.contextStream.listen(onMobilityContext);
}

Step 3: Handle features

A call-back method is used to handle incoming MobilityContext objects:

/// Handle incoming contexts
void onMobilityContext(MobilityContext context) {
  /// Do something with the context
  print('Context received: ${context.toJson()}');
}

All features are implemented as getters for a MobilityContext object.

/// Location features
context.places;
context.stops;
context.moves;

/// Derived features
context.numberOfPlaces;
context.homeStay;
context.entropy;
context.normalizedEntropy;
context.distanceTravelled;

Example

The example application included in the package shows the feature values, including separate pages for stops, moves and places.

Feature errors

When a feature cannot be evaluated, it will result in a value of -1.0.

Examples:

  • The Home Stay feature requires at least some data to be collected between 00:00 and 06:00, otherwise the feature cannot be evaluated.

  • The Entropy and Normalized Entropy features require at least 2 places to be evaluated. If only a single place was found, this will result in an Entropy of 0.

Theorical Background

For mental health research, location data, together with a time component, both collected from the user’s smartphone, can be reduced to certain behavioral features pertaining to the user’s mobility. These features can be used to diagnose patients suffering from mental disorders such as depression.

Location Features

The mobility features which will be used are derived from GPS location data are:

Stop A collection of GPS points which together represent a visit at a known Place (see below) for an extended period of time. A Stop is defined by a location that represents the centroid of a collection of data points, from which a is created. In addition a Stop also has an arrival and a departure time-stamp, representing when the user arrived at the place and when the user left the place. From the arrival- and departure timestamps of the Stop the duration can be computed.

Place A group of stops that were clustered by the DBSCAN algorithm. From the cluster of stops, the centroid of the stops can be found, i.e. the center location. In addition, it can be computed how long a user has visited a given place by summing over the duration of all the stops at that place.

Move The travel between two Stops, which the user will pass though a path of GPS points. The distance of a Move can be computed as the sum of using the haversine distance of this path. Given the distance travelled as well as departure and arrival timestamp from the Stops, the average speed at which the user traveled can be derived.

Derived Features

Home Stay The portion (percentage) of the total time elapsed since midnight which was spent at home. Elapsed time is calculated from the departure time of the last known stop.

Location Variance The statistical variance in the latitude- and longitudinal coordinates.

Number of Places The number of places visited today.

Entropy The entropy with respect to time spent at places.

Normalized Entropy The normalized entropy with respect to time spent at places.

Distance Travelled The total distance travelled today (in meters), i.e. not limited to walking or running.

Libraries

mobility_features