The way we represent relevant components of the real world in our models determines the kinds of questions we can or cannot answer. Besides representing an object or field in 2D or 3D space, the temporal dimension is of a continuous nature. Therefore, in order to represent it in a GIS we have to discretize the time dimension. Spatio-temporal data models are ways of organizing representations of space and time in a GIS. Several representation techniques have been proposed in the literature. Perhaps the most common of these is the “snapshot state”, which represents a single moment in time of an ongoing natural or man-made process. We may store a series of these “snapshot states” to represent “change”, but we must be aware that this is by no means a comprehensive representation of that process. In spatio-temporal analysis we consider changes of spatial and thematic attributes over time. We can keep the spatial domain fixed and look only at the attribute changes over time for a given location in space. We might be interested how land cover has changed for a given location or how land use has changed for a given land parcel over time, provided its boundary has not changed. On the other hand, we can keep the attribute domain fixed and consider the spatial changes over time for a given thematic attribute. In this case, we might want to identify locations that were covered by forest over a given period of time. Finally, we can assume both the spatial and attribute domains are variable and consider how fields or objects have changed over time. This may lead to notions of object motion - a subject receiving increasing attention in the literature. Applications of moving object research include traffic control, mobile telephony, wildlife tracking, vector-borne disease control and weather forecasting. In these types of applications, the problem of object identity becomes apparent. When does a change or movement cause an object to disappear and become something new? With wildlife this is quite obvious; with weather systems less so. But this should no longer be a surprise: we have already seen that some geographic phenomena can be nicely described as objects, while others are better represented as fields. Mapping time means mapping change. This may be change in a feature’s geometry, in its attributes, or both. Examples of changing geometry are the evolving coastline of the Netherlands, the location of Europe’s national boundaries, or the position of weather fronts. Changes in the ownership of a land parcel, in land use or in road traffic intensity are other examples of changing attributes. Urban growth is a combination of both: urban boundaries expand with growth and simultaneously land use shifts from rural to urban. If maps are to represent events like these, they should be suggestive of such change. Three temporal cartographic techniques can be distinguished: Single Static Map Specific graphic variables and symbols are used to indicate change or represent an event. We can apply the visual variable “value” to represent for example the age of built-up areas. Series of Static Maps A single map in the series represents a “snapshot” in time. Together, the maps depict a process of change. Change is perceived by the succession of individual maps depicting the situation in successive snapshots. It could be said that the temporal sequence is represented by a spatial sequence that the user has to follow to perceive the temporal variation. The number of images should be limited since it is difficult for the human eye to follow long series of maps. Animated Maps Change is perceived to evolve in a single image by displaying several snapshots one after the other, just like a video clip of successive frames. The difference from the series of maps is that the variation can be deduced from real “change” seen taking place in the image itself, not from a spatial sequence. For the user of a cartographic animation, it is important to have tools available that allow for interaction while viewing the animation. Seeing an animation play will often leave users with many questions about what they have seen. And just replaying the animation is not sufficient to answer questions like “What was the position of the northern coastline during the 15th century?” Most of the general software packages for viewing animations already offer facilities such as “pause” (to look at a particular frame) and ‘(fast-)forward’ and ‘(fast-)backward’, or step-by-step display. More options have to be added, such as the possibility to go directly to a certain frame based on a task command like: “Go to 1850”.
Completed (GI-N2K)