Study on Spatial Thermal Environment in Lanzhou City Based on Remote Sensing and GIS

Abstract

The urban thermal environment is a thermal phenomenon of whole spatial environment of a city. The research of urban spatial thermal environment will find the changes of urban spatial structure and urban scale, and favor sustainable urban development and improve the quality of human habitation environment. Moreover, the spatial pattern of urban thermal environment is not only the dominant content in assessing city ecological environment, but also an important base for city planning and environment monitoring. Lanzhou is a typical valley-city, whose main part has been formed and developed in the valley. The yellow river crosses it from northwest to southeast. Its transmeridional length is about 35 kilometers, and the distance from its south to north is between 2 kilometers to 10 kilometers. Its spatial limit cause undesirable urban thermal environment with the urbanization process. Thermal remote sensing imagery has been widely used to reveal urban thermal phenomenon since the last two decades. To study the urban thermal environment changes, it is usually expected to use the thermal data of different years with the same date. However, it is generally difficult to find the available data, which will make the comparison of such thermal image data more difficult due to seasonal difference. In order to solve the problem, this paper introduced a method which can reduce the seasonal difference if the images acquired on different date. Based on the Lanzhou TM infrared data of 6/20, 1986 and 8/27, 2002, we first processed the thermal infrared bands of different time respectively through several image enhancement technologies. To examine the changes of the urban thermal environment between different times, the two thermal images were normalized to reduce seasonal difference. Based on the instantaneous radiant temperature of the land surfaces that had been processed, all the patches were divided into 5 levels, they were: low radiation temperature, lower radiation temperature, middling radiation temperature, higher radiation temperature and high radiation temperature. Then we analyzed the changes of the area and the spatial distribution of different radiation temperature levels. The result of research indicated that the middling radiation temperature was dominant in Lanzhou in 1986, however it was replaced by higher radiation temperature in 2002. Moreover, the proportion of high radiation temperature had increased remarkably.