Urban-rural continuum

The <b>urban–rural continuum</b> classifies the global population, allocating rural populations around differently-sized cities. The classification is based on four dimensions: population distribution, population density, urban center location, and travel time to urban centers, all of which can be mapped globally and consistently and then aggregated as administrative unit statistics.<br>Using spatial data, we matched all rural locations to their urban center of reference based on the time needed to reach these urban centers. A hierarchy of urban centers by population size (largest to smallest) is used to determine which center is the point of “reference” for a given rural location: proximity to a larger center “dominates” over a smaller one in the same travel time category. This was done for 7 urban categories and then aggregated, for presentation purposes, into “large cities” (over 1 million people), “intermediate cities” (250,000 –1 million), and “small cities and towns” (20,000–250,000).<br>Finally, to reflect the diversity of population density across the urban–rural continuum, we distinguished between high-density rural areas with over 1,500 inhabitants per km2 and lower density areas. Unlike traditional functional area approaches, our approach does not define urban catchment areas by using thresholds, such as proportion of people commuting; instead, these emerge endogenously from our urban hierarchy and by calculating the shortest travel time.<br><b>Urban-Rural Catchment Areas (URCA).tif </b>is a raster dataset of the 30 urban–rural continuum categories for the urban–rural continuum showing the catchment areas around cities and towns of different sizes. Each rural pixel is assigned to one defined travel time category: less than one hour, one to two hours, and two to three hours travel time to one of seven urban agglomeration sizes. The agglomerations range from large cities with i) populations greater than 5 million and ii) between 1 to 5 million; intermediate cities with iii) 500,000 to 1 million and iv) 250,000 to 500,000 inhabitants; small cities with populations v) between 100,000 and 250,000 and vi) between 50,000 and 100,000; and vii) towns of between 20,000 and 50,000 people. The remaining pixels that are more than 3 hours away from any urban agglomeration of at least 20,000 people are considered as either hinterland or dispersed towns being that they are not gravitating around any urban agglomeration. The raster also allows for visualizing a simplified continuum created by grouping the seven urban agglomerations into 4 categories.<br><b>Urban-Rural Catchment Areas (URCA).tif </b>is in GeoTIFF format, band interleaved with LZW compression, suitable for use in Geographic Information Systems and statistical packages. The data type is byte, with pixel values ranging from 1 to 30. The no data value is 128. It has a spatial resolution of 30 arc seconds, which is approximately 1km at the equator. The spatial reference system (projection) is EPSG:4326 - WGS84 - Geographic Coordinate System (lat/long). The geographic extent is 83.6N - 60S / 180E - 180W. <b><br></b><b>Population_UR-continuum.xlsx [Excel file hosted on journal website] </b>- provides country-level populations for urban centers of different sizes and their catchment areas. Populations are summed using the Global Human Settlement (GHS) Population Grid for 2015 at a resolution of 1 km and the Global Administrative Unit Layers. The region and country-income grouping for each country is included.<br>Further details are in the <b>ReadMe_data_description.docx</b><br>

**城乡连续谱（urban–rural continuum）** 旨在对全球人口进行分类，将农村人口分配至不同规模的城市周边区域。该分类体系依托四大维度构建：人口分布、人口密度、城市中心区位，以及至城市中心的出行耗时。所有维度均可实现全球一致性制图，并可聚合为行政单元级统计数据。
研究团队借助空间数据，依据抵达各候选城市中心的出行耗时，将所有农村点位匹配至对应的参考城市中心。研究采用按人口规模从大到小排序的城市中心层级体系，以确定某一农村点位的“参考”城市中心：在同一出行耗时类别中，邻近更大规模城市中心的点位优先级高于邻近较小城市中心的点位。该分析最初覆盖7类城市规模，为便于展示，进一步将其归为三大类：**大城市（人口超100万）**、**中等城市（25万~100万人口）**以及**小城市与城镇（2万~25万人口）**。
最后，为完整反映城乡连续谱内的人口密度多样性，研究团队将农村区划分为两类：每平方公里人口超1500人的高密度农村区域，以及人口密度更低的区域。与传统功能区划分方法不同，本研究未采用通勤人口占比等阈值指标来定义城市集水区，而是通过城市层级体系与最短出行耗时计算，内生得出城市集水区的范围。
**城乡集水区（Urban-Rural Catchment Areas, URCA）.tif** 是包含30类城乡连续谱类别的栅格数据集，用于可视化展示不同规模城镇周边的集水区范围。每个农村像素点均被分配至某一出行耗时类别：距离7类城市集聚区之一的出行耗时小于1小时、1~2小时，以及2~3小时。这7类城市集聚区具体包括：① 人口超500万的特大城市；② 人口100万~500万的大城市；③ 人口50万~100万的中等城市；④ 人口25万~50万的中等城市；⑤ 人口10万~25万的小城市；⑥ 人口5万~10万的小城市；以及⑦ 人口2万~5万的城镇。剩余所有距离至少2万人口的城市集聚区超过3小时行程的像素点，则被归为腹地或分散型城镇，即未依附任何城市集聚区的区域。该栅格数据还支持将7类城市集聚区归为4大类后，生成简化的城乡连续谱用于可视化展示。
本数据集采用GeoTIFF格式，波段交织存储并采用LZW压缩，适配地理信息系统（Geographic Information Systems）与各类统计分析软件使用。数据类型为字节型，像素值取值范围为1~30，无数据值为128。空间分辨率为30角秒，在赤道处约合1公里。空间参考系统（投影）为EPSG:4326——WGS84地理坐标系（经纬度）。地理覆盖范围为北纬83.6°至南纬60°，东经180°至西经180°。
**Population_UR-continuum.xlsx [期刊官网托管的Excel文件]** —— 提供了不同规模城市中心及其集水区的国家级人口统计数据。该数据以2015年分辨率为1公里的**全球人类居民点（Global Human Settlement, GHS）**人口栅格与全球行政单元图层为基础，汇总得到各城市中心的人口规模。文件中同时包含各国的区域分组与国家收入层级分类信息。
更多详细信息请参见**ReadMe_data_description.docx**