Urban is a spatial regional system. Each component in the urban space forms a mutual connection and restriction of organisms. British scholar K.J.Button considered that city is a network system that located within a limited space where a variety of economic market, house, labor, land and transportation intertwined together. At the same time, the urban environment also changing in the urbanization process. With the development of satellite technology, the application of RS in monitoring the development of urbanization has become a new trend.
Urban ecological environment is a huge open system; ecological environment change is comprehensive reflection of many factors. Using remote sensing to study the effect of urban sprawl of urban ecological environment, general only extract small amount of ecological environmental indicators. At present, the urban ecological environment for remote sensing monitoring index mainly includes: vegetation index and vegetation coverage, primary productivity, land surface temperature and water channel density.
Vegetation is an important component part of urban ecosystem. Through selective absorption and reflection of solar radiant by vegetation to improve the factors of city natural environment by adjusting the latent heat and sensible heat exchange, refreshing the air in cities and reducing the pollution etc, which play an important role in air purification, ecological environment protection, disaster prevention, city appearance improvement and soil erosion prevention.
In recent years, as aggravating of global warming and the speeding up of urbanization, a large ratio of natural surface was replaced with the urban surface, which produce more significant thermal effect and the increasing urban heat, resulting in urban population increased morbidity and mortality. As a result, many researchers tend to treat urban heat island thermal environment as an index to evaluate urban environment. At present, the research of urban thermal environment mainly concentrated in the urban heat island effect and the change of urban thermal environment in the processing of urban expansion. Usually, researchers use land surface temperature measured by satellite remote sensing instead of air temperature to study the thermal environment of the city.
Change detection refers to the change area is extracted from the image algorithm, is a way of monitoring implementation. At present, the urban expansion monitoring using remote sensing images, the dynamical mechanism of the development of urban expansion model. The supervised classification, also called the training classification method, is the process that identifies the unknown pixels by the sample pixels which have been confirmed the class they belong.
Monitoring of Land Use Land Cover (LULC) is the most important and fundamental part in urban remote sensing, which is essential step of further study of the development of Urban area. At present, there are several significant study aspects of LULC: Land classification, land use dynamic change mechanism, driving forces of land use change, land use change and the environment changes, the relationship between the transformation situations of all kinds of land use.
Urbanization Remote Sensing analysis of City Suzhou, CHINA
Urban Ecological Environment
The normalized difference vegetation index (NDVI) as an important information, a state of description of vegetation is acknowledged as the large area scale characterize the important index of the ecological environment quality. Because the NDVI data has close relative with the precipitation, air temperature, leaf area index and the transpiration rate, urban heat island and many other factors, which leads to the uncertainty of NDVI. For this reason, the LST should also be investigated.
Applying the tool in the ENVI, we can get the NDVI of the study region. Usually, a concept FVC would be introduced to study changes in vegetation cover using images for different dates.
Pattern of urban thermal field and the strength of the heat island effect in significant level reflect the quality of the urban ecological environment, which can be used as an important information comprehensive evaluation and the construction of ecological city. Due to the most obvious influence of urban heat island effect on the surrounding environment is the change in temperature; the temperature change could reflect the status of the thermal field of a city.
There are several procedures required to produce luminance temperature maps: radiometric calibration of raw data, extract the study region, calculate the top-of-atmosphere (TOA) radiance (Lλ), after that produce the feature sensor temperature T. For the radiation of any feature smaller than the blackbody radiation rate, the feature sensor temperature already can reflect the distribution the urban thermal field.
Through the process we can get the LULC maps of study region, statistic the area of each category, we can analysis the change of land use of study region. Also, extract the built-up area to produce the urban expansion variation, combined with the statistic data of study region; we can generate the expansion intensity, rate and driving factor that prompt the urbanization process.
The NDVI and LST maps of Shanghai in 1995, 2005 and 2015
From the above NDVI and LST maps, we can see the high quality vegetation area decreased significantly. Through the statistics of each class area, the result shows the average vegetation coverage was declined.
The LST graphs give the urban thermal field distribution of study region. It is obviously the high temperature area increased heavily during the study period. Also, the high temperature area generally matched with the urban area which means the urban heat island phenomenon had show up at the beginning of the study period.
Combined with the NDVI maps, we can find the LST values are inversely proportional to the NDVI values, which means increased the vegetation coverage density is an effective solution for the urban heat island effect.
Many scholars have proved that urban sprawl is an important factor makes the change of the surrounding environment. Except the statistics analysis of NDVI and LST respectively, combine the output together could be used to reflect the ecological environment variation through the urbanization.
At present, multi-scale and multi-source remote sensing data provide adequate information data resource, with the increasingly mature of remote sensing software, processing method and model, the monitoring of large scale urban expansion and ecological environment variation has met the requirement of urban planning.