PostGIS Raster : Extending PostgreSQL for The Support of ... - CoDE

Appendix F
Continuous Surfaces
One type of continuous surface, that is derived from those characteristics that define a surface, is
a surface where each point in the surface is its measured from a fixed reference point. These include
elevation (the fixed reference point is a sea level and the measure at each point is its height above the
sea level) and aspect (the fixed reference point is one of four directions: north, east, south and west
and the measure at each point is the direction of a normal at that point).
Figure F.1: Elevation represented in raster format [33].
Another type of continuous surface includes phenomena that progressively vary across a surface
from a source. Examples of such phenomena include fluid and air movement. These surfaces are
characterized by the way in which the phenomenon moves. The first type of movement is through
diffusion in which the phenomenon moves from areas with high concentration to those with less
concentration until the concentration level evens out. Surface characteristics of this movement type
include salt concentration moving through the ground or water, contamination moving away from a
hazardous spill or a nuclear reactor and heat from a forest fire. In this type of continuous surface,
there has to be a source, so-called source-concentration surface. The concentration is always greater
near the source and decreases following a function of distance and the intermediate substance.
In the source-concentration surface above, the concentration at any position is a function of the
capability of the phenomenon to move through the environment. Another type of concentration surface
is governed by the inherent characteristics of the moving phenomenon. For example, the movement of
the noise from a bomb blast is governed by the inherent characteristics of noise and the environment
that it moves through. Mode of movement can also limit and directly affect the surface concentration
of a feature, as is the case with seed dispersal from a plant. The means of movement, whether it be
bees, man, wind, or water, all affect the surface concentration of seed dispersal for the plant. Other
concentration surfaces include dispersal of animal populations, potential customers of a store (cars
being the means of locomotion and time being the limiting factor) and the spreading of a disease.
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