Space-Time Analytics of Tracks for the Understanding of Patterns of Life - Space-Time Integration in Geography and GIScience - page 380

Fig. 20.10 Resampling tracks to match locations and time intervals to facilitate calculation of
space-time deviance between two tracks
Determination of space-time differences between any two tracks is not trivial.
GPS observations were likely taken at different time intervals and at different times.
Even if a person leaves home at the same time and takes the same route for a 2 day
period, GPS points of stops and moves along the track for the first day will not
match exactly the track for the second day. Resampling is one technique that can
be used to adjust the shifts in location and time of GPS observations (Fig. 20.10 ).
Resampling along tracks allows all tracks in consideration to have the same number
of GPS locations during a period of interest for comparison. The resampled time
interval should be determined by spatial and temporal variability in the application
domain of interest. For daily patterns, identification of hourly changes in space and
time may be sufficient. For seasonal patterns, daily or weekly changes may be more
meaningful than hourly variations.
Assuming that the time resampling interval is T (say an hour) in Fig. 20.10 ,the
mean center of all GPS points taken T ˙ T/2 (e.g. 30 min before and after an
hour) will be assigned to the location to time T i (e.g. on the hour). When there
are no GPS points taken within T ˙ T/2 (e.g. 30 min before and after the hour),
the mean center calculated for T i 1 (e.g. the previous hour) is assumed to be the
representative location at T i (e.g. on the hour). In addition, when there is no GPS
location in the initial hours of the day, the first known GPS location is assumed to be
the locations for these initial hours. An example would be if an individual left home
at 9 am, comparably later than 7 am on the previous day. The approach assumes that
the individual was at home at 7 a.m. to 9 a.m. The resampling procedure results in
the same number of locations and the same number of time intervals on all tracks in
consideration for comparative analysis of space-time difference.