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TOR VERGATA UNIVERSITY UNSUPERVISED CLASSIFICATION ...

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Introduction 5<br />

Since the late ‗70s, with the first Landsat missions, satellite remote sensing is<br />

increasingly being used as a timely and cost-effective source of information in a<br />

urban planning [3], [4].<br />

During the last two decades, significant progress has been made in developing and<br />

launching satellites with instruments, in both optical/infrared and microwave<br />

regions of spectra, well suited for Earth Observation with an increasingly finer<br />

spatial, spectral and temporal resolution [5]. With the recent availability of<br />

commercial Very High Resolution (VHR) remote sensing multispectral imagery<br />

from sensors such as IKONOS and QuickBird, it is possible to identify small-scale<br />

features such as individual roads and buildings in the urban environment (roessner<br />

et. al, 2001) and have a specific potential for more detailed and accurate mapping<br />

of urban areas [6], [7]. Imagery from these sensors is an important source of timely<br />

data, which can be used for the creation and/or updating of GIS vector layer [8].<br />

However, VHR images analysis is one of the most challenging areas for the remote<br />

sensing community due to its high spatial and spectral diversity. From the physical<br />

composition point of view, several different materials can be used for the same<br />

built-up element (for example, building roofs can be made of clay tiles, metal,<br />

concrete, glass, plastic). On the other hand, the same material can be used for<br />

different built-up elements (for example, concrete can be found in paved roads or<br />

building roofs [9]. Fig. 2 shows how the same surface (two types of roads) can be<br />

made from different material and two different surfaces (a road and a building) can<br />

be made with the same material (concrete). Misclassification occur between<br />

buildings and streets, which are caused by spectral similarities between materials<br />

covering these surfaces and the influence of shadow.<br />

A different spectral behavior could be explained also from the effect of the<br />

atmosphere, which operates at different wavelengths.

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