Mapping the Fourth Dimension

Our 3-dimensional world is the usual subject of 2-dimensional maps; their purpose is to accurately depict geographic extent on a readable/publishable surface, with or without thematic content.  The purpose of adding the 4th dimension of time is somewhat intuitive- maps of the same area through time can give a viewer an understanding of change in some feature.  The problem in depicting the 4th, temporal, dimension on a 2-dimensional surface is remedied by the use of various spatiotemporal visualization techniques, the most effective of which, it could be argued, is the map animation.  The simplest form of this variety of cartographic visualization is the display of two or more maps, adjacent to one another, showing the same geographic area at different times.  The change in the area depicted is evident in the difference(s) between the various maps.  While this method is effective, actual animation on a map is an even more salient method of displaying areal change through time.  

       
The above graphics represent two years- 1790 and 2000- and contain proportional symbols indicating the most populous cities in the U.S. at those times.  The two maps are actually the beginning and ending frames of an animation, which shows the same map for each 10 year period between 1790 and 2000, and thus gives the viewer an idea of where the major centers of population in the U.S. were for each decade.  The ease of doing so with a simple animation, cycling through each decade, is fairly large in comparison to creating and displaying something singular including separate maps, one for each decade.