In this section, we look again at three-dimensional GIS—but the third dimension is time rather than a spatial dimension.
Time is usually considered an enemy of GIS. Time, among other things, ensures that a GIS is always out of date in some respect or other. The world around us changes constantly—because of both natural effects and human action. The bits in a computer’s memory persist in their states unless changed. Time marches on, gradually making our data obsolete or forcing us to update continually. Such updates are time-consuming and expensive. Imagine, for example, that a major feature of your database consisted of ortho photos, obtained from aircraft. How often do you re-fly the area so the photos reflect new development or changes to the environment? And at what cost?
Another issue with time and GIS is that very little attempt is usually made to preserve states of the database with an eye to analyzing changes later. If a piece of property is subdivided, or a road built, the affected databases may well be updated, but the time at which the update happens is usually lost, as far as any easy access by those who might want to compare “what is” with “what was.”
Again we face the issue of continuous versus discrete. The movement of time is a continuous phenomenon. Incremental changes happen within the unfolding of time. Are the changes large enough to warrant modification of the database? Consider a house in an historic district. The paint ...