Some Concepts That Underpin GIS - Introducing Geographic Information Systems with ArcGIS - page 20

(3.05,7.62) (11.58,7.62)

(0,5.49)

(11.58,5.49)

(10.06,3.05)

(0,0) (10.06,0)

FIGURE 1-9 Locations of garden stakes

The stakes are in the same place that they were before. Their positions are simply referred to by a different set of
numbers . Geographic coordinate or datum transformation is nothing more or less than this, except that
the mathematical operations applied to each number are usually more involved.

Physical Dimensionality
All matter exists in four dimensions—roughly characterized as left-right, toward-away, up-down, and
time. The first three are conceptually clumped together and called “spatial dimensions.” Time is treated
as a dimension here because we want to talk about “position” in space and time. 18

Any physical object occupies space and persists in time. This is true of the largest object in the universe
and the smallest atom. However, when the measure of one or more dimensions of an object is small, or
insignificant, with respect to the measure of others or is tiny with respect to its environment, it is useful
to describe and represent objects by pretending that they occupy fewer than three spatial dimensions.
For example, a sheet of paper can be considered a pseudo-two-dimensional object; it has thickness
(up-down), but that dimension is miniscule compared with left-right and toward-away. A parking
meter in a city could be considered a pseudo zero-dimensional object, because the measures of all its
dimensions are insignificant with respect to its environment. (The parking meter would certainly not be
considered a spatially zero-dimensional object by its designer, manufacturer, or the driver who uses it.
Therefore, the pseudo-dimensionality of an object depends on its context.)

18 In physics, time is treated as the fourth dimension because it is inexorably bound up with the other three according
to Einstein's theories (special in 1905 and general in 1916) of relativity. This connection need not concern us except
for some esoteric technical matters related to time on global positioning system satellites (for one, time on a satellite
runs faster because it is farther from Earth, where there is less gravity).