Research On Global Urban Extraction Method Based On MODIS Satellite Data

In the 21st century,the urbanization process has further intensified.The largepopulation and human activities in cities put a heavy burden on the environment.Timely and accurate extraction of the urban area is not only essential for monitoring the process of urbanization,but also provides the necessary basic data for environmental research.Having the advantage of rich band information,high time resolution,wide coverage,long-term earth observation and free global data acquisition,MODIS is selected as an important data source for global urban extraction.However,the current MODIS urban products need to be further improved.This study proposes a fully automatic urban extraction method based on local adaptive strategy,which aims to solve the problem on the generation of training samples,which is a challenge in global urban extraction.Another ambition of this research is to improve the mapping effect of MODIS urban products.The main steps of the proposed method include:1)generation of training sample pool,2)training sample extraction based on K-L divergence,3)construction of local classifier,and 4)post-processing of classification based on spatio-temporal information.With the proposed method,an annual urban product MGUP(MODIS Global Urban Extent Product)is produced,which maps the global urban area between 2001 and 2020.For validation,more than 150,000 samples are collected in 9 years around the world.The precision of MGUP products is evaluated by year,city level and continent with F-score as the comprehensive index.In general,the F-score of MGUP is 0.88,which is better than MCD12Q1.v5(0.82),MCD12Q1.v6(0.86)and CCI(0.86).Among various evaluation methods,the comprehensive mapping effect of MGUP products is the best.According to MGUP,a dynamic analysis and evaluation of global urban expansion is conducted from 2001 to 2020.Statistics show that the global urban area reaches809,135 km~2 in 2020,accounting for 0.54%of the global land surface.The urban area in2020 increases to 1.70 times of the global urban area in 2001.In the analysis of urban expansion for different continents,it is found that the urban area of all continents(except Antarctica)continues to grow.The urban expansion of Africa,Asia,Europe and South America generally presents a trend of rapid urban expansion,while that of North America and Oceania tends to be stable.The areas that occur large urban expansion are mainly concentrated in North America,Asia,parts of South America and western Europe.The distribution of global urban area growth rate is quite different from that of urban expansion area.From 2001 to 2020,the countries with higher urban area growth rate are mainly concentrated in developing countries of Africa and Asia.