Database Reference
In-Depth Information
Position-related dynamic attributes can also be visualized in separate temporal
displays such as a time graph or a time bars display. An example of a time
bars display is given in Figure
8.3
a. The horizontal axis represents time. Each
trajectory is represented by a horizontal bar such that its horizontal position
and length correspond to the start time and duration of the trajectory. Note that
temporal zooming has been applied: a selected interval from 06:30 A.M. till
12:00 P.M. is stretched to the full available width. The vertical dimension is
used to arrange the bars, which can be sorted based on one or more attributes of
the trajectories (start time in our example). Coloring of bar segments encodes
values of some user-selected dynamic attribute associated with the positions
in the trajectories. This may be an existing (measured) attribute or an attribute
derived from the position records, that is, coordinates and times. Examples of
such derivable attributes are speed, acceleration, and direction. To represent
attribute values by colors, the value range is divided into intervals and each
interval is assigned a distinct color or shade. In Figure
8.3
a, shades of red and
green represent speed values; red is used for low speeds and green for high. The
legend on the left explains the color coding. Interactive linking between displays
allows the user to relate attribute values to the spatial context: when the mouse
cursor points on some element within the time bars display, the corresponding
spatial position is marked in the map by crossing horizontal and vertical lines
and the trajectory containing it is highlighted (Figure
8.3
b). In this example
we see that the car whose trajectory is highlighted moved at 06:54 A.M. to the
northeast with a speed of 1.2 km/h.
The use of this kind of dynamic link is limited to exploration of one or a
few particular trajectories. To investigate position-related dynamic attributes in a
large number of trajectories, the analyst can apply filtering of trajectory segments
according to attribute values. Figure
8.3
c-d illustrate how such filtering can be
done in a highly interactive way. The color legend on the left of the time bars
display is simultaneously a filtering device: the user can switch off and on the
visibility of any value interval by clicking on the corresponding colored rectangle
in the legend. In Figure
8.3
c, the user has switched off all intervals except for
that with speeds from 0 to 5 km/h. As a result, the trajectory segments with the
speed values higher than 5 km/h have been hidden. The filter affects not only
the time bars display but also the map (Figure
8.3
d). It is possible to combine
several segment filters based on values of different attributes.
The points satisfying filter conditions can be extracted from the trajecto-
ries into a separate data set (information layer) consisting of spatial events,
that is, objects located in space and time. This data set can be visualized and
analyzed independently from the original trajectories or in combination with
them. In Figure
8.3
e, the yellow circles represent 19,339 spatial events con-
structed from the points of the car trajectories where the speeds did not exceed
5 km/h. The filtering of the trajectory segments has been canceled so that
