Overview of the geographical distribution of districts. (a) The geographical map of 174 divisions across 4 places (Changchun City, Dehui City, Yushu City, and Nong’an County). The number in each region with many divisions denote its administrative division codes in 201716. (b) The geographical position of divisions in the earth view, as the southeast part of Songliao Basin, Northeast China. The spatial map was created using OpenStreetMap online platform (http://www.openstreetmap.org/) (© OpenStreetMap contributors) under the license of CC BY-SA (http://www.openstreetmap.org/copyright). More details of the licence can be found in http://creativecommons.org/licenses/by-sa/2.0/. The earth view was created using Google Earth Pro platform with version 7.3 for non-commercial use (https://www.google.com/earth/). Line graphs were drawn using Tableau Software for Desktop version 9.2.15 (https://www.tableau.com/zh-cn/support/releases/9.2.15). The layouts were modified with Keynote version 6.6.2 (http://www.apple.com/keynote/).