Image Processing Reference
Fig. 6.1 The vegetation-impervious surface-soil (V-I-S) model of urban cover composition
(Ridd 1995 )
structures can be treated in various ways, for example grouped with impervious surface
(especially dark impervious) in areas of multiple stories as commercial and apartment
buildings, or with water (especially if water features are rare, etc.).
At this point it is important to emphasize the environmental/ecological basis of the
model. Consider the ways in which the three terrestrial cover types and water differ from
one another in response to energy and moisture in the ecosystem (see Table 6.1 ).
It is the variation in energy and moisture response, as they interact with the four
surface materials, or sub-categories of them, that gives the model substance as a
foundation for environmental or human studies of urban systems. Consider the differ-
ence between a central business district (CBD) and a residential area or a city park in
terms of temperature. Consider the differing moisture regimes, runoff rates, moisture
storage, and the effect of evapotranspiration on cooling. Consider the differences in
human habitability and diurnal population dynamics.
The model may be applied at various scales of observation: sub-pixel, pixel,
groups of similar pixels, or even the city/settlement as a whole. Depending on the
interplay between spatial resolution (pixel size) and spatial complexity of the urban
environment (object size), a given pixel may be “pure” (100%) V, I, or S, or, more
commonly, mixed - made up of some combination of the three types and/or water.
Pure pixels (other than water) occur at the vertices of the diagram. Mixed pixels lie
elsewhere in the diagram. A point in the center of the ternary diagram consists of
33% of each of the three cover types. Sampling sites for calibration may be a group