An updated MODIS global urban extent product (MGUP) from 2001 to 2018 based on an automated mapping approach

Timely and accurate global urban maps are fundamental in monitoring urbanization process and understanding environmental degradation. Therefore, this paper proposed a locally adaptive and fully automated global mapping method and produced an updated 250 m MODIS global urban area product (MGUP) from 2001 to 2018. The proposed approach mainly consists of 1) automated samples extraction from existing global products, 2) locally adaptive samples selection and trained classification in each 5 degrees x 5 degrees grid, and 3) post-processing in terms of the spatio-temporal context. To validate the product, 9 groups of samples for every two years from 2001 to 2018, amounting to over 150,000 sample points, were collected manually from Landsat imagery as global validation dataset. Accuracy assessment indicates that MGUP has a F-score of 0.88, achieving better results than the contemporary global products, i.e., MCD12Q1.v5 (0.82), MCD12Q1.v6 (0.86), and CCI-LC (0.86). Analysis of urban expansion based on MGUP shows that the world's urban area increased to 802233 km(2) and accounted for 0.54% of the Earth's land surface in 2018. The total global urban area expanded by 1.68 times from 2001 to 2018. At continent level, urban density varies considerably, and the highest and lowest one is in Europe (1.78%) and Oceania (0.15%), respectively. At national level, large increment of urban area mainly occurs in North America, Asia, and South America; and countries having high growth rates are mainly developing countries in Africa and Asia. MGUP can be downloaded at https://www.researchgate.net/publication/339873537_MGUP_annual_global_2001_2018.

Automated summary

This paper proposes a locally adaptive and fully automated global mapping method, resulting in an updated global urban area product (MGUP) from 2001 to 2018. The MGUP achieved better accuracy than contemporary global products, and the urban area expanded by 1.68 times from 2001 to 2018.