HUC8 - CONUS Shapefile

This resource contains a shapefile of HUC-8 (eight digit Hydrologic Unit Codes) for the Continental United States (CONUS).

The Watershed Boundary Dataset (WBD) is a comprehensive aggregated collection of hydrologic unit data consistent with the national criteria for delineation and resolution. It defines the areal extent of surface water drainage to a point except in coastal or lake front areas where there could be multiple outlets as stated by the \"Federal Standards and Procedures for the National Watershed Boundary Dataset (WBD)\" “Standard” (http://pubs.usgs.gov/tm/11/a3/). Watershed boundaries are determined solely upon science-based hydrologic principles, not favoring any administrative boundaries or special projects, nor particular program or agency. This dataset represents the hydrologic unit boundaries to the 12-digit (6th level) for the entire United States. Some areas may also include additional subdivisions representing the 14- and 16-digit hydrologic unit (HU). At a minimum, the HUs are delineated at 1:24,000-scale in the conterminous United States, 1:25,000-scale in Hawaii, Pacific basin and the Caribbean, and 1:63,360-scale in Alaska, meeting the National Map Accuracy Standards (NMAS). Higher resolution boundaries are being developed where partners and data exist and will be incorporated back into the WBD. WBD data are delivered as a dataset of polygons and corresponding lines that define the boundary of the polygon. WBD polygon attributes include hydrologic unit codes (HUC), size (in the form of acres and square kilometers), name, downstream hydrologic unit code, type of watershed, non-contributing areas, and flow modifications. The HUC describes where the unit is in the country and the level of the unit. WBD line attributes contain the highest level of hydrologic unit for each boundary, line source information and flow modifications.

本资源包含适用于美国本土（Continental United States, 简称CONUS）的8位水文单元代码（Hydrologic Unit Codes, 简称HUC）的形状文件（shapefile），即HUC-8数据集。
流域边界数据集（Watershed Boundary Dataset, 简称WBD）是一套符合国家划定标准与精度规范的综合性聚合水文单元数据集。其定义了地表水汇至单一汇点的区域范围，但沿海或湖滨区域除外——此类区域存在多个出水口，符合《国家流域边界数据集（WBD）联邦标准与规程》（"标准"）（http://pubs.usgs.gov/tm/11/a3/）中的相关规定。流域边界仅基于科学的水文原理划定，不偏袒任何行政边界、专项工程、特定项目或机构。本数据集覆盖全美范围的12位（第6级）水文单元边界，部分区域还包含代表14位与16位水文单元（Hydrologic Unit, 简称HU）的额外细分单元。
在精度要求上，美国本土的水文单元最低按1:24000比例尺划定，夏威夷、太平洋流域及加勒比海地区按1:25000比例尺划定，阿拉斯加按1:63360比例尺划定，均符合《国家地图精度标准》（National Map Accuracy Standards, 简称NMAS）。目前，在有合作方及对应数据的区域，正开发更高分辨率的边界数据，并将其整合回WBD数据集。
WBD数据以多边形数据集及定义多边形边界的对应线要素数据集形式交付。WBD多边形要素的属性字段包括：水文单元代码（HUC）、面积（以英亩与平方千米为单位）、名称、下游水文单元代码、流域类型、非汇水区以及径流改造情况。HUC用于标识该单元在全国的位置及所属层级。WBD线要素属性则包含各边界对应的最高级别水文单元、线要素来源信息以及径流改造情况。