a matter of class!
Objects of similar natures have similar spectral properties. That
means that the electromagnetic radiation reflected by objects of
the same nature is similar overall and these objects will thus have
similar spectral signatures. Since the spectral signatures of the
objects observed by satellites are converted into different colours
in digital images, objects of the same kind will appear in closely
This property has been used for years to interpret aerial photographs
and the images supplied by Earth-observing satellites. The interpreter
places in the same category all the objects in an image that seem
to have the same or closely related colour.
Since the colours in a digital image are merely a conventional
transposition of numerical values, it is also possible to exploit
the computers computational power to classify the pixels by
their numerical values, which is to say, in the final analysis,
by the corresponding objects spectral properties. This is
the basic principle of image classification.
From their orbits satellites see only the upper part
of an objects outer envelope. For example, satellites see
only the roof of a house. They will thus be able at best to classify
buildings by their roofing materials, whereas the interpreter is
probably more interested in the buildings functions (residential,
commercial, industrial, etc.). This limitation is especially bothersome
when it comes to identifying manmade elements of the landscape.
Spectral signature analyses prove much more relevant for analysing
land, crops, natural vegetation, etc. It is very important to take
this basic principle into account in setting the categories of a
classification scheme. These categories should always be based on
the types of land cover rather than land use. For example, a car
park and a buildings tarred roof correspond to very different
types of land use, but their spectral signatures are so close that
they will most likely be classified in the same land-cover category.