Database Reference
In-Depth Information
8.2.1 Visualizing Trajectories
Themost common types of display for the visualization of movements of discrete
entities are static and animated maps and interactive space-time cubes (STC).
STC is a unified representation of space and time as a 3D cube in which two
dimensions represent space and one dimension represents time. Spatio-temporal
positions can be represented as points in an STC and trajectories as three-
dimensional lines. When multiple trajectories are shown, the displays may suffer
from visual clutter and occlusions. The drawback of STC, besides occlusion,
is distortion of both space and time due to projection. It is also quite limited
with respect to the length of the time interval that can be effectively explored. To
compensate for these limitations, map and STC displays are often complemented
with other types of graphs and diagrams.
Common interaction techniques facilitating visual exploration of trajectories
and related data include manipulations of the view (zooming, shifting, rota-
tion, changing the visibility and rendering order of different information layers,
changing opacity levels, etc.), manipulations of the data representation (selec-
tion of attributes to represent and visual encoding of their values, for example,
by coloring or line thickness), manipulations of the content (selection or filter-
ing of the objects that will be shown), and interactions with display elements
(e.g., access to detailed information by mouse pointing, highlighting, selec-
tion of objects to explore in other views, etc.). Multiple coexisting displays
are visually linked by using consistent visual encodings (e.g., same colors) and
exhibit coordinated behaviors by simultaneous consistent reaction to various user
interactions.
Figure
8.1
gives examples of map and STC displays and demonstrates some
basic interaction techniques. The map in Figure
8.1
a shows a subset of the Milan
data set consisting of 8,206 trajectories that began on Wednesday, April 4, 2007.
To make the map legible, the trajectory lines are drawn with only 5% opacity. A
temporal filter, as in Figure
8.1
c, can be used to limit the map view to showing
only the positions and movements within a selected time interval. Thus, the
display state in Figure
8.1
b corresponds to the 30-minute time interval from
06:30 A.M. till 07:00 A.M. The time filter can also be used for map animation:
the limiting time interval is moved (automatically or interactively) forward or
backward in time, making the map and other displays dynamically update their
content according to the current start and end of the interval.
Figure
8.1
b also demonstrates the access to various attributes associated
with a trajectory, such as start and end time, number of positions, length, and
duration. When the mouse cursor points on a trajectory line, the attributes of
this trajectory are shown in a pop-up window as well as the time when the car
was in the position at the cursor.
