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STS493 Sofie d.B. et al.
increased data quality and hybrid forms of survey and sensor data. An example
is a travel survey in which respondents’ locations are stored whenever a device
is in motion. These data can be collected passively. The data may also be
shown to respondents for quality checks and for enrichment of sensor data
with stop motives and other context information. Active data collection is
much more demanding as it requires real-time data handling and a careful
design of a user interface.
Summarizing, there are various criteria from the person/company, sensor
and surveyor points of view that need to be confronted with costs and logistics
of sensor data collection and processing.

3. Mobile device sensors
We discuss the two forms of sensor data: self-initiated sensor data and
existing sensor data.

3.1 Mobile device sensors and wearable sensors
The following elements and sensors are supported by many contemporary
smartphones and tablets (see Mussmann and Schouten (2018) for details
about the sensors): 3D touch, motion sensors (accelerometer and gyroscope),
ambient light, Bluetooth, camera, camera flash, cellular (or GSM), fingerprint,
GPS, heart rate, humidity, magnetic field, microphone, NFC (near field
communication), pressure, proximity, screen, speaker, thermometer, vibration,
Wi-Fi and wireless charging.
Wearable devices are mobile devices that one can wear or attach to
clothes. Their functionality is more specific and targeted than smartphones
and tablets in order to reduce size and weight. The motivation to wear a device
is often health-related, but wearables do support other functions. Most
common wearables are activity trackers, fitness bands and smart watches,
which are intended for continuous use. Less common wearables are shoe clips,
smart glasses, clothes with sensors and jewellery with sensors, which are
intended for temporary use. Due to their more specific functionality, wearables
often require other mobile devices to communicate with users. A dedicated
app needs to be installed on a smartphone or tablet that provides a user
interface to set or alter wearable settings and to read summary data and
statistics. Wearable sensor data can be sent to the user directly or indirectly
through the producer of the wearable. In the latter case, often only edited and
aggregated data are available and the raw sensor data remain at the side of
the producer. Consequently, wearable sensor data lie somewhere in between
primary and secondary data collection. They are mostly self-initiated but often
are maintained and owned by another party. Due to the closer proximity to a
respondent’s body and the fact that they can be worn 24 hours per day and 7
days per week, wearables can measure data that smartphones and tablets

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