The incorporation of time into GIS has introduced
challenges in all realms of GIS research, from the development
of appropriate data structures and algorithms to analysis and
querying methodology and display. For users of any software,
however, the user interface is key for the software's productive
use. It is through the interface that (potentially complex) requests
are expressed and results of those requests are presented. The
current paper describes the GUI design for a Temporal GIS. The
specific example described here has been implemented as the user
interface of TEMPEST, a prototypical Temporal GIS. Emphasis in
the discussion is on the unique issues that arise in representing
time at the user-interface level, and effective strategies that
can be developed .
Data associated with space-time processes have long
been difficult to visualize and analyze, at least in a GIS context,
without time-based data models and interfaces. This necessary
emphasis on process -- analysis of data according to both spatial
and temporal components -- has created an imperative in the realm
of GIS research for efficient and user-friendly software that
deals with questions of time as well as it does with questions
of space. A prototype graphical user interface (GUI) has been
developed which facilitates analysis of events, patterns, and
processes within geo-referenced data. The representation of time
in a GIS user interface presents intriguing and unique challenges
to the designer, and it is these challenges and their proposed
solutions that will be the primary focus of this paper. The analogous
representations of locations and attributes are by no means meant
to be diminished in importance by this focus; in fact, representation
of these other dimensions is an ongoing and parallel research
The GUI described herein was designed within the
conceptual framework of the Triad model, first suggested by Peuquet
(1994), in which data are stored and presented based on three
"dimensions": where (location-based), what (object-based),
and when (time-based). The interface is intended to be the front
end of the TEMPEST GIS prototype, which, as its name suggests,
is being developed specifically for the representation and analysis
of spatiotemporal data (Peuquet and Qian, 1996). TEMPEST evaluates
queries according to what has been called the "dimensional
dominance" of the query (Langran, 1993). The "dominant"
dimension of a multidimensional query is that which is most constrained
by the query; for example, the query "what features have
ever existed at this location?" is most constrained by the
specification of location, its dominant dimension.
The design of the interface is driven by this typology
of historical or process-driven queries; with each type of question
comes different ideas for the integration of the vital temporal
component of the data represented and analyzed. The Triad model
provides an organizing structure for the development of individual
tools for the interface; the creation of various elements of the
GUI involved first distinguishing the operation as oriented toward
either spatial, temporal, or attribute information, then considering
how the operation might be related to the other dimensions.
Because the user of any information system interacts
with the system in every way through the user interface, its design
is of vital importance to the productive use of the system. In
essence, as Frank (1993) and Gould (1993) suggest that, to the
user, "the user interface is the system." Howard and
MacEachren (1996), in their typology of design for interfaces
to geo-referenced data, emphasize that a successful GIS "facilitates
creative thinking by allowing a user to change displays quickly
and in predictable ways," (p. 62) with a user-friendly, efficient,
and useful interface being the key to this success.
Following Gould's (1993) lead, the TEMPEST interface
design is focused first on "well-reasoned conceptual design."
Since issues of temporal query and display have not been fully
addressed in a GIS context, the proposition of a universal interface
for GIS (Raper and Rhind, 1990) requires conceptual extensions
for spatiotemporal data. These extensions are based on the types
of queries which might be made of spatiotemporal data, some of
which are outlined by Peuquet (1994). Three classes of temporal
queries are suggested: the change in object or feature (e.g. "where
was this object two years ago?"), changes in the spatial
distribution of an object or set of objects (e.g. "did any
land use changes occur in this drainage basin over the last 15
years?"), and temporal relationships between multiple geographic
phenomena (e.g. which areas experienced a landslide within one
week of a major storm?"). Effort was made in the TEMPEST
interface to recall existing design elements in familiar GIS GUIs,
drawing parallels between spatial and temporal constructs (Langran,
1992; Parkes and Thrift, 1990). For example, the concept of "size"
in a spatial sense is outlined by units of length, whereas in
a temporal sense it is outlined in units of duration. The familiar
click-drag operation of a mouse to outline a spatial length (say
a region on a displayed map) has been translated in the TEMPEST
GUI to outline a duration on a graphical timeline.
The general look of the interface is a reflection
of TEMPEST's reliance on the Triad model. The screen is divided
into three overlappable windows, one for each "vertex"
of the triad. Figure 1 shows a typical interface of TEMPEST in
action: (a) a "location" window displaying a spatial
representation (typically a map), (b) an "object" window
which lists not only spatial objects like lakes, thunderstorm
cells, and power lines, but also temporal objects like events
and conditions, and (c) a "time" window giving graphical
information about the temporal component of the data. The most
innovative graphical devices, by necessity, are found in the time
window. To facilitate understanding of the temporal analysis
capabilities of TEMPEST, the tools developed for time-based queries
are purposely analogous to those built for the more familiar location-
or attribute-based queries. Such features as a magnifying tool
for multi-(temporal-)scale queries and an area box tool for temporal
searches are borrowed and adapted from traditional GIS tools and
operators. The integrative approach of the TEMPEST data model
requires that the time-based queries be interdependent with the
location- and attribute-based queries, a need recognized by Langran
(1993) and Worboys (1994). A typology of these interrelationships
was presented in Peuquet and Qian (1996). The location window,
thus, is an interactive and dynamic spatial representation with
varying detail for varying scale, flexible symbolization schemes,
and a dynamic legend. The object window displays a dynamic hierarchical
"tree" of objects, customizable based upon such parameters
as location, duration, and size. All of the components of the
interface, like the components of the Triad model, are cooperative
A fundamental design consideration for any application,
GUI or otherwise is, of course, the intended audience : the user.
The TEMPEST GIS is designed for not only "public" applications
such as the presentataion and communication of multidimensional
information, but also "private" applications like the
cognitive, abstract formulation of hypotheses through interactive
and individual exploration and visualization (MacEachren, 1994).
Its ability to facilitate exploration of spatial and temporal
data and the formation of hypotheses during and after that exploration
is a priority in the interface design (Monmonier, 1990; MacDougall,
1992). Thus, prime consideration was given to the flexibility
and customization of the system through the interface such that
such exploratory (especially temporal) queries might be straightforward.
To this end, a conceptual distinction has been made
between linear and cyclical views of time. The linear view, more
familiar to humans in our everyday experience, is that time marches
inexorably forward and that changes occur steadily (like aging)
or suddenly (like an earthquake) , as observed. The cyclical
view, on the other hand, relates directly to conceptually perceived
or anticipated temporal patterns (like diurnal wildlife activity).
This distinction is manifested in the GUI in the temporal querying
tool, a graphical representation of time which can alternate between
a "time line" and a "time wheel" at the user's
request. The line representation (Fig. 2a) is effective when
querying observed raw data, with no presumption or imposition
of temporal rhythms or patterns. For example, a query about the
gradual change of an undisturbed meadow to old-growth forest
land over many decades would likely be linear and continuous (e.g.,
"from 1950 to 1990, what land cover changes did this acre
undergo?"), and best represented graphically by a time line.
The wheel representation (Fig. 2b), on the other hand, is useful
when querying spatiotemporal data which may have a known or anticipated
cyclical nature. The user may specify the period of the cycle
represented and choose to query only those dates which correspond
to a specified duration within the cycle. For example, suppose
a researcher is interested in the variation of rainfall each monsoon
season over several years. She would customize her query time
wheel to a yearly period and then select those months (or weeks
or days) of interest to limit her investigation. This flexibility
of investigative methods encourages creative exploration of the
databases for the formulation of new hypotheses as well as the
proof of existing ones.
The temporal querying tool described above is designed
separately from the display date tool, though they are both created
to be graphical representations of the temporal variable. The
display date is defined to be the real-world time that is represented
in the display. The display date is shown in the GUI both graphically
and textually in the display date tool (Fig. 3), an interactive
scrollable time line. A separate timeline is generated for each
spatiotemporal data layer (attribute). These multiple timelines
can be manipulated by the user in several ways: (1) They can be
controlled independently of one another, allowing a user to inspect
the spatial patterns and processes of one attribute while holding
the other(s) still in time. This might be of use to a researcher
examining the effects over time of a single event, like the diffusion
of certain cancers through space after a nuclear power plant accident.
(2) The timelines can be "bound" to one another, such
that the displayed layers move simultaneously, to inspect for
spatial correlation. For example, a meteorologist might wish
to bind a cloud-top brightness data layer to one of ultraviolet
radiation at the surface in order to examine how these phenomena
relate to one another. (3) A user can bind and "offset"
timelines such that the data layers move simultaneously, but with
a specified time lag. A researcher hypothesizing that landslides
occur approximately four days after a specific region experiences
a major precipitation event would find this feature of the interface
Among other tools specifically designed for the analysis
of temporal data is a time series graph (Fig. 4) showing changes
in an attribute over time at a specified location or region.
This is a natural extension of capabilities in other systems which
shows the change in an attribute over a certain spatial distance
at a specified time. The graph, not unlike the display date tool,
is scrollable and capable of displaying multiple time series simultaneously.
Here, too, the concept of "zooming" with a magnifying
tool has been borrowed from standard spatially-based GIS interfaces
for multiple temporal-scale inspections of the time series. Time
series analysis functions such as Fourier transformations and
filtering may be added to future versions of the interface.
The development of the interface for TEMPEST has
served to confirm that "the representation of phenomena in
time as well as space is significantly more complex and more difficult
than their representation in space alone" (Peuquet, 1994,
p.442), but that potentially powerful methods for this representaion
can be developed. Our goal was the incorporation and extension
of existing analogous query and display interface designs to a
GIS interface created to examine and analyze temporal as well
as spatial data. Priority was also given to the construction
of tools which encourage creative and exploratory thinking by
users. Our goal in this research was not only the practical implementation
of this interface as the front-end of the TEMPEST Temporal GIS
but also the suggestion of general strategies for the representation
of time in GIS displays. The specific interface described here
was created using a Sun UNIX workstation and the object-oriented
interface developers' language Tcl/Tk. Development of this interface
is ongoing and will continue to improve with creative input from
not only other designers but also, and especially, users of the
authors wish to thank Liujian Qian, Elizabeth Wentz, and Alan
MacEachren for their assistance and advice in the development
and design of the prototype interface. This research is sponsored
by the National Science Foundation under Grant FAW NSF 90-27,
and by the Environmental Protection Agency, Grant R825195-01-0.
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