Space-Time Analytics of Tracks for the Understanding of Patterns of Life - Space-Time Integration in Geography and GIScience - page 376

Fig. 20.7 GPS points and extracted stop-event distribution for a month (9/1/2009 to 9/30/2009)

threshold; therefore, the segment cannot formulate a stop. Moving to the next point,
the distance between p 2 and p 3 is less than the threshold value, and thus the segment
is a candidate for formulating a stop. When such a segment is identified, the first
point becomes an anchor point ( pa 2 ). Then we calculate the segment distance
between the anchor point and its following points until the distance exceeds the
threshold value. When the threshold is exceeded (a red dashed-line between p 2 and
p 9 ), the last point ( p 8 ) becomes an anchor point ( pa 3 ). Lastly, if the duration between
two anchor points ( pa 2 and pa 3 ) is larger than the duration threshold, GPS points
enclosed by those anchor points formulate a stop event; otherwise, anchor points
will be released.
In our case study for stop analysis, we used GPS points of one individual
collected over a 6-month period between 5/1/2009 and 10/31/2009. Figure 20.7
illustrates GPS points (left) and the stop-events distribution identified by the
duration-distance thresholds algorithm, where £ D 1sand• D 20 m. In this case, the
duration threshold is set to 1 s in order to extract all possible stops. The algorithm
extracted stops with short-duration near the street intersection suggesting stops due
to traffic lights. It also identified several long stops such as one in a residential area
possibly for one's dwelling place, one in a commercial district located at south-east
corner of an intersection (B), and one at a golf course nearby a lake (A).
After the stop locations are extracted from the GPS point data, the similarity
between 2 days of stop points can be calculated for temporal analysis. For example,
over the time range of a dataset, one day of stops can be compared to its previous
day of stops to explore if any weekly stop pattern emerges, or one day of stops can
be compared to the same day the week before to investigate if a pattern of stops is
congruent or incongruent for a specific day of week. To measure similarity, various