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Top1. Introduction
Urbanization and industrialization are the key factors for social and economical development and represent a specific response to economic, demographic and environmental conditions. During the last few decades, in many European regions, abandonment of agricultural land has induced a high concentration of people in densely populated urban areas. This phenomenon has been observed throughout the world. In 1950, only 30% of the world population lived in urban areas. By 2000 that proportion rose up to 47%, and by 2030 the estimated number will be around 60% (United Nations Population Division, 2001).
Such a rapid industrialization and expansion of urban areas has caused strong and sharp land cover changes and significant landscape transformations, which significantly impact local and regional environmental conditions. Nowadays, the increase of people concentration in densely populated urban areas is considered as a pressing issue in developing countries. For example, following land reform initiated in 1987, vast areas of China have been involved in a rapid urban expansion with new urban settlements (Cheng & Masser 2002), so that in a few years, several cities rapidly have become big centres or regional nodes.
The analysis of city size distribution deals with different disciplines such as planning, geography, economy, demography, ecology, physics, statistics, etc., because the evolution of a city is a dynamic process involving a number of different factors. An issue of great importance in modelling urban growth includes spatial and temporal dynamics, scale dynamics, man-induced land use changes. Although urban growth is perceived as necessary for a sustainable economy, uncontrolled or sprawling urban growth can cause various problems, such as loss of open space, landscape alteration, environmental pollution, traffic congestion, infrastructure pressure, and other social and economical issues. To face such drawbacks, a continuous monitoring of urban growth evolution in terms of type and extent of changes over time is essential to support planners and decision makers in future urban planning. In this discipline there is a partially wrong common opinion that soil consumption can be reduced adopting constraints. Constraints cannot be the only way to preserve the environment. In a lot of cases, after several years, it is allowed to partially remove them changing completely the original spatial planning objectives.
Soil is a limited resource which allows biological productivity and exchange of material and energy; often there is confusion between soil and territory. Soil is a sort of thin interface within which great part of the processes that govern life occur. Territory is a wider concept strongly related to government aspects, while soil is mainly associated with safeguard policy.
Since 1998 a program has been developed in Germany, for the reduction of soil consumption. Current Chancellor, Angela Merkel, in that period Minister for the Environment in the government led by Helmut Kohl, approved an ambitious program with the goal of reducing soil consumption from 130 to 30 hectares per day within 2020.
Since 2004 in the United Kingdom, new urbanizations have to be located in brown-field sites and the Density Direction established a mandatory minimum density of 30 dwellings per hectare in new areas of expansion..
In Italy, where eight square meters of soil are consumed every second, several laws have been proposed to the parliament in order to reduce the phenomenon.
All programmes and laws need quantifiable indicators and detailed methods, techniques and procedures for their calculation.
The aim of this research is to test methods and techniques to monitor urban expansion; the main purpose is not to obtain high-resolution images, but quantifying the phenomenon in order to perform a valid monitoring.
A critical point to understand and monitor urban expansion processes is the availability of both (i) time-series data set and (ii) updated information relating to current urban spatial structure to define and to locate evolution trends. In such a context, an effective contribution can be offered by satellite remote sensing technologies, which are able to provide both an historical data archive and up-to-date imagery. Satellite technologies represent a cost-effective mean to obtain useful data, which can be easily and systematically updated worldwide. Nowadays, medium resolution satellite images, such as Landsat, can be downloaded free of charge from NASA web site.