Hydrologic Unit Code (HUC) 12

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.流域边界数据集（Watershed Boundary Dataset, WBD）是一套符合国家划定标准与分辨率规范的综合性聚合水文单元数据集。根据《国家流域边界数据集（Watershed Boundary Dataset, WBD）联邦标准与规程》中的相关标准条款（http://pubs.usgs.gov/tm/11/a3/），该数据集明确了地表水汇流至指定点的区域范围，沿海或湖滨区域除外——此类区域可能存在多个出水口。流域边界仅基于科学水文原则划定，不偏向任何行政边界、专项工程、特定项目或机构。 本数据集覆盖全美全域，提供12位编码（第6级）的水文单元边界。部分区域还包含代表14位、16位编码水文单元（Hydrologic Unit, HU）的额外细分边界。最低精度要求下，美国本土48州采用1:24000比例尺划定水文单元，夏威夷、太平洋盆地及加勒比地区采用1:25000比例尺，阿拉斯加采用1:63360比例尺，所有精度均符合《国家地图精度标准》（National Map Accuracy Standards, NMAS）。现有合作方与数据源支持的区域正在开发更高分辨率的边界数据，并将其回纳入WBD数据集。 WBD数据集以多边形及定义多边形边界的对应线要素形式提供。WBD多边形属性包含水文单元编码（Hydrologic Unit Code, HUC）、面积（以英亩和平方千米为单位）、名称、下游水文单元编码、流域类型、非汇水区及径流改造情况。水文单元编码用于标识该单元在全国的位置及其层级。WBD线要素属性包含每条边界对应的最高层级水文单元、线数据来源信息及径流改造情况。