This photograph illustrates the development of cities by expansion of residential areas around the fringes of the existing urban area. As we progress through the sequence, try to imagine the changes in the spectral characteristics that would result.

The first stage is the invasion of agricultural land - most commonly pastures - by housing. The landcover of a sparsely wooded grassland is transformed initially into bare soil as the buildings and infrastructure are mechanically imposed. The spectral signature of this area has changed from green vegetation (pink through to crimson in FCC image) to bare soil, which is cream to white in FCC image. As the new housing is occupied, the signature is changed by the presence of asphalt roadways and newly growing gardens, lawns and nature strips. The house roofs and the roadways still dominate each pixel.

The last stage is reached when the new suburb is perhaps 10 years old or more. Now the green vegetation signature of established and expanding trees and gardens dominates the pixel, and it begins to resemble that of the agricultural landcover it has irreversibly replaced. Except that as we shall see in this Chapter, the surrounding agricultural landcover is usually rain fed and it waxes and wanes with the seasons. In this arid country, we like to control the condition of our surrounds, and so we maintain the greenness of our vegetation with artificial rain or watering.