Flow direction from the Hydrologic Derivatives for Modeling and Analysis (HDMA) database -- Africa

This dataset contains the Flow Direction (FD) for Africa from the Hydrologic Derivatives for Modeling and Analysis (HDMA) database. The FD data were developed and distributed by processing units. There are 19 processing units for Africa. The distribution files have the number of the processing unit appended to the end of the zip file name (e.g. af_fd_3_2.zip contains the FD data for unit 3-2).The HDMA database provides comprehensive and consistent global coverage of raster and vector topographically derived layers, including raster layers of digital elevation model (DEM) data, flow direction, flow accumulation, slope, and compound topographic index (CTI); and vector layers of streams and catchment boundaries. The coverage of the data is global (-180º, 180º, -90º, 90º) with the underlying DEM being a hybrid of three datasets: HydroSHEDS (Hydrological data and maps based on SHuttle Elevation Derivatives at multiple Scales), Global Multi-resolution Terrain Elevation Data 2010 (GMTED2010) and the Shuttle Radar Topography Mission (SRTM). For most of the globe south of 60º North, the raster resolution of the data is 3-arc-seconds, corresponding to the resolution of the SRTM. For the areas North of 60º, the resolution is 7.5-arc-seconds (the smallest resolution of the GMTED2010 dataset) except for Greenland, where the resolution is 30-arc-seconds. The streams and catchments are attributed with Pfafstetter codes, based on a hierarchical numbering system, that carry important topological information.

本数据集包含来自水文建模与分析衍生数据集（Hydrologic Derivatives for Modeling and Analysis, HDMA）数据库的非洲区域流向（Flow Direction, FD）数据。流向数据按处理单元开发并分发，非洲区域共计19个处理单元。分发所用的压缩包文件名末尾会附加处理单元编号，例如af_fd_3_2.zip即包含3-2号处理单元的流向数据。HDMA数据库提供了覆盖全球且一致性优异的栅格与矢量地形衍生图层，其中栅格图层涵盖数字高程模型（Digital Elevation Model, DEM）、流向、汇流累积量、坡度以及复合地形指数（Compound Topographic Index, CTI），矢量图层则包含水系与汇水区边界。该数据集的空间覆盖范围为全球（-180°~180°，-90°~90°），其基础DEM数据由三套数据集融合而成：HydroSHEDS（多尺度航天飞机高程衍生水文数据与地图）、2010年全球多分辨率地形高程数据（Global Multi-resolution Terrain Elevation Data 2010, GMTED2010）以及航天飞机雷达地形测绘任务（Shuttle Radar Topography Mission, SRTM）。在北纬60°以南的全球绝大多数区域，数据栅格分辨率为3弧秒，与SRTM的分辨率保持一致；北纬60°以北的区域（格陵兰除外）分辨率为7.5弧秒（即GMTED2010数据集的最高分辨率），而格陵兰区域的分辨率则为30弧秒。水系与汇水区图层附带基于分层编号系统的普发斯特特编码（Pfafstetter codes），该编码承载了关键的拓扑信息。