National Hydrography Dataset (NHD)

The National Hydrography Dataset (NHD) is a comprehensive set of digital spatial data that contains information about surface water features such as lakes, ponds, streams, rivers, springs and wells. Within the NHD, surface water features are combined to form reaches, which provide the framework for linking water-related data to the NHD surface waterdrainage network. These linkages enable the analysis and display of these water-related data in upstream and downstream order.

The NHD is based upon the content of USGS Digital Line Graph (DLG) hydrography data integrated with reach-related information from the EPA Reach File Version 3 (RF3). The NHD supersedes DLG and RF3 by incorporating them, not by replacing them. Users of DLG or RF3 will find the National Hydrography Dataset both familiar and greatly expanded and refined.

While initially based on 1:100,000-scale data, the NHD is designed to incorporate and encourage the development of higher resolution data required by many users.

The NHD data are distributed as tarred and compressed ARC/INFO workspaces. Each workspace contains the data for a single hydrologic cataloging unit. Cataloging units are drainage basins averaging 700 square miles (1,813 square kilometers) in area. Within a workspace, there are three ARC/INFO coverages plus several related INFO tables. There is also a folder containing the metadata text files.

The NHD data support many applications, such as: making maps; geocoding observations (i.e., the means to link data to water features); modeling the flow of water along the Nation's waterways (e.g., information about the direction of flow, when combined with other data, can help users model the transport of materials in hydrographic networks, and other applications); and cooperative data maintenance.

国家水文数据集（National Hydrography Dataset，NHD）是一套综合性数字空间数据集，涵盖湖泊、池塘、溪流、河流、泉眼与水井等地表水体特征相关信息。在该数据集中，地表水体特征会被整合为河段，以此为框架将涉水相关数据关联至国家水文数据集的地表排水网络。此类关联可实现涉水数据按上下游顺序的分析与可视化展示。
国家水文数据集基于美国地质调查局（United States Geological Survey，USGS）数字线划图（Digital Line Graph，DLG）水文数据，并整合了美国环境保护署（Environmental Protection Agency，EPA）第三版河段文件（Reach File Version 3，RF3）中的河段相关信息。该数据集通过整合而非替代的方式，取代了数字线划图（DLG）与第三版河段文件（RF3）。数字线划图（DLG）与第三版河段文件（RF3）的使用者会发现，国家水文数据集既保留了原有使用习惯，又实现了大幅扩展与优化。
尽管国家水文数据集最初以1:10万比例尺数据为基础构建，但其设计目标是兼容并推动多类用户所需的高分辨率水文数据的开发。
国家水文数据集以打包压缩后的ARC/INFO工作区格式进行分发。每个工作区承载单个水文编目单元的相关数据，而水文编目单元指平均面积约700平方英里（合1813平方千米）的流域。每个工作区内包含三个ARC/INFO覆盖图层以及若干关联的INFO数据表，此外还设有存储元数据（metadata）文本文件的文件夹。
国家水文数据集可支撑多类应用场景，例如：地图制图、观测数据地理编码（即实现数据与水体特征关联的技术手段）、沿国家水系的水流模拟（例如，结合其他数据的水流方向信息，可辅助用户开展水文网络内的物质输运建模等相关应用）以及协同数据维护。