Space-Time Analytics of Tracks for the Understanding of Patterns of Life - Space-Time Integration in Geography and GIScience - page 372

￿ Moves : locations on a track where a mobile object stays shorter than the specified

threshold duration. Locations where an object moves have instantaneous velocity,
even though there may not be data available for velocity calculation. A track is
composed of temporally ordered observed locations at which the mobile object
of interest either stops or moves. In addition to the threshold duration, spatial
and temporal granularities can influence the determination of stops and moves
locations as well as locations along respective tracks.

Minimal spatial and temporal granularities are subject to observations and can
influence decisions on points of stops and moves. At a fine space-time granule, for
example sub-meter and second, detailed motion can be recognized, such as taking
a step forward and the point will be determined as a move. However, if the space-
time granule is at 10 m and 5 min, a step forward is likely to be within the granule
and therefore will be considered as a stop. The final decision of stops and moves is
subject to the defined space-time threshold, say 20 m in an hour, which will consider
a collection of points within 20 m in an hour as stops, but for a point which next
point is beyond 20 m in an hour will be consider a move point. Determination of a
proper space-time granule depends on the instrument taking observations (such as
GPS or twitter) as well as the application of interest.
There are two kinds of structures for the elements of paths, tracks, trajectories,
stops, and moves: time-oriented or space-oriented structures. For time-oriented
structures time is considered the first order principle of organization that connects
points of stops and moves over space to form different kinds of movement lines. At a
basic level there are locations of stops and moves that are recorded as observations.
Estimated pathways between temporally consecutive locations of stops and moves
are trajectories, while straight line connections between these locations form tracks.
Connections between these locations based on the actual route travelled results in a
path for the journey.
Alternatively, space is considered the first order principle of organization for
space-oriented structures. Space connects points of stops and moves over time to
form visit histories at locations. Some locations may be visited on a regular basis,
and others may be visited only occasionally. Frequency and timing of visits may
vary from time to time. Passing-by locations involves only points of moves, other
locations may have only stop points or mixed stops and moves. Moves by a location
may be at different speed or in different direction. Stops at a location may be long
or short. Passing by a location that has in the past been a traditionally stopping may
signal a change in spatial behavior (such as no longer eating at a given fast food
restaurant) or pattern of life (such as no longer needing to use day care services for
children). Changes in how an individual moves at locations can also be revealing,
such that speeding out of a work parking lot may be indicative of urgency.
Chronological time (a.k.a. calendar time) can serve the temporal structure
necessary for a time-oriented organization. Geographic space is two-dimensional
and is commonly referenced by x and y coordinates. Regular tessellation of space
is commonly used as the framework for discrete global grids, such as the Military
Grid Reference System, regular hierarchical triangulation (Dutton 2000 ), hexagonal