Geography Reference
In-Depth Information
The Time
Dimension
OVERVIEW
3-D: 2-D (Spatial) Plus 1-D (Temporal)
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 Time, among other things, ensures that a GIS is always out
of date in some respect or other.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 on the house
deteriorates slowly. When is the record of the appearance of the house changed from excellent to good to
fair to poor? So, Question 1 is how and how often do you update your database. Question 2 is what does
it take to trigger a change. Suppose the house is repainted a different color. Suppose that an extension is
added. Suppose the house is demolished.
These are merely the complications time poses for our two-dimensional GIS. Looking to the future,
can GIS be harnessed to allow analysis of changes to the environment over time? Certainly, old maps can
be scanned in and interpreted. Such data can be compared with more recently installed GIS data.
Those interested in using GIS for history should read Anne Kelly Knowles' book
Past Time, Past Place
(Esri Press, 2002).
But consider: Old maps have dates on them. Suppose that you are interested in making comparisons
between conditions now and those 10 years ago in a venue that has been using, and continually
updating, a GIS database. How would you know what changed when?
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