Hydrography depicts surface water, including rivers and streams, lakes and ponds, canals, coastlines, and many other water features. The Hydrography layer combines two data sets: the 2014 National Map Hydrographic Geodatabase (1:1,000,000 scale) for small to medium scales, and the 2016 National Hydrography Datataset (NHD) (1:24,000 scale) for large scale mapping. Hydrography is depicted in two layers. Rivers & Streams depicts linear hydrography, including rivers, streams, canals, pipes, and artificial paths (line networks through larger water bodies). Waterbodies dipicts area hydrography, including larger rivers, lakes, ponds, seas, bays, estuaries, etc.
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The National Hydrography Dataset (NHD) is used to portray surface water. The NHD represents the drainage network with features such as rivers, streams, canals, lakes, ponds, coastline, etc. The principal components of the NHD are:
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- NHDFlowline: This is the fundamental flow network consisting predominantly of stream/river and artificial path vector features. It represents the spatial geometry, carries the attributes, models the water flow, and contains linear referencing measures for locating events on the network. Additional NHDFlowline features are canal/ditch, pipeline, connector, underground conduit, and coastline.
- NHDArea: This feature class contains many additional features of water polygons. One of the more important is the stream/river feature. It represents the aerial extent of the water in a wide stream/river with a basic set of attributes. They typically contain NHDFlowline artificial paths that are used to model the stream/river. Artificial path carries the critical attributes of the stream/river, whereas NHDArea represents the geometric extent.
- NHDWaterbody: Basic waterbodies such as lake/pond features are represented here. They portray the spatial geometry and the attributes of the feature. These water polygons may contain NHDFlowline artificial paths to allow the representation of water flow. Other NHDWaterbody features are swamp/marsh, reservoir, playa, estuary, and ice mass.