Space-Time Analytics of Tracks for the Understanding of Patterns of Life - Space-Time Integration in Geography and GIScience - page 383

be black throughout. Otherwise, the spatial difference from the track on Day i 1 at
Time j will be stored in the cell (Day i ,Time j ).
Figure 20.12 shows a space-time-track image based on an individual's GPS
tracks for 215 days. There are two key re-occurring features in the image. One
feature consists of repetitive sets of two bright lines starting around Day 35 to
Day 215. The bright lines correspond to spatial differences between Fridays and
Saturdays as well as Sundays and Mondays. The dark lines in-between reflect the
greater similarity between tracks recorded for Saturdays and Sundays. The second
re-occurring feature is the progression of the scattering bright spots developed over
these days in the late night hours. These late-night outings could be for social events
or a night job, which can be further explored by mapping stops and moves on these
days and times to determine places that the individual visited. The space-time-
track image also includes incongruent features. Examples are marked by circles
in Fig. 20.12 . These incongruent features could be indicative of new or occasional
activities.
Furthermore, the daily total of spatial difference (e.g. the row sum in the space-
time-track image) reflects the cumulative difference of a track compared to the track
taken on the previous day. If an individual more or less took the same route to work,
the person's daily track would be quite similar and therefore resulted in a small
cumulative difference along a track. On the other hand, if one was wandering around
town, the person's daily tracks could be very different and the cumulative difference
would be large. Figure 20.13 shows a time series of daily cumulative differences of
tracks based on the values in the space-time-track image in Fig. 20.12 . The time

Fig. 20.13 A time series of daily accumulative differences among tracks