Geography Reference
In-Depth Information
time-stamped spatial coordinates. Where an individual frequently visits and how
the person usually travels to these places are fundamental space-time elements to
understanding the person's pattern of life. Therefore, the first step of space-time
analytics is to distinguish stops and moves on tracks that the person took.
A convenient way to differentiate which GPS points correspond to stops from
those GPS points that were taken when one was in motion is based on how long
one stayed at these locations, which is reflected in differences of time-stamps or by
spatial clusters of GPS points (Palma et al. 2008 ). For tracks taken by individuals,
clusters may not be apparent due to the number of sample points taken in periods of
short duration. Therefore, we propose two scale parameters, a duration threshold (£)
and a distance threshold (•) to determine stops on a track. Duration thresholds, e.g.
greater than 5 min, may be used to include short duration stops such as the use of
drive-through windows at business establishments. Longer duration thresholds may
reflect locations for major activities, such as £ D 30 min or longer for shopping or
dining. A distance threshold is used to determine a geographical size of stop events.
A smaller distance threshold may filter GPS sampling noise and determine stop
events consisting of small locational variations, e.g. waiting at a bus stop in urban
space, whereas a larger distance threshold can determine stay events that potentially
include movement activities such as playing at a park.
Figure 20.6 illustrates the algorithm for differentiating stops and moves from
collected GPS points based on two threshold values. A GPS track TR is composed
of a sequence of 3- or 4-dimensional GPS points, Tr Df p 1 , p 2 , p 3 , :::, p k g where
k denotes the number of points in the track, p k Df x, y, (z), t g where (x, y) are
longitude and latitude, z is altitude, and t is timestamp. Based on the distance
threshold, the algorithm finds anchor points ( pa ), which differentiate between stops
and moves, and each stop or move is composed of a set of GPS points between two
consecutive anchor points. To find anchor points, we begin with the first and the last
points of a track as anchor points ( pa 1 , pa n ). Then, we calculate a segment distance
between two GPS points starting from the first point and check if it exceeds the
distance threshold. In Fig. 20.6 , the distance between p 1 and p 2 exceeds the distance
Fig. 20.6
A stop detection algorithm based on duration and distance thresholds
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