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features represent bodies of water (e.g., lakes, reservoirs, ponds), paths through which water flows (e.g.,
<br />streams, rivers), and other hydrographic entities (e.g., coastlines, wells, springs, dams, bridges, pipes).
<br />There are three basic types of features: points, stored in the `HydroPoint' feature class; lines, stored in the
<br />`HydroLine' feature class; and areas, stored in the `HydroArea' feature class. `HydroPoint' contains
<br />gaging stations, wells, etc. `HydroLine' contains features that comprise the surface water drainage
<br />network and coastline-stream/river, canal/ditch, artificial path, connector, and pipeline. Artificial path is
<br />aline that represents the transport of water through a water body such as a lake. The lines representing
<br />network features, through which water flows in a known direction, are oriented in the direction of flow,
<br />where the vertices along the line are ordered from the upstream end of the line to downstream end. The
<br />lines representing coastlines are usually oriented so that the sea or ocean is to the right. Areas represent
<br />areal (or polygonal) features that contain water, or through which water flows, such as wide rivers.
<br />Basic to the understanding of the data is the concept of a reach, composed of one or more features with
<br />similar hydrologic characteristics. Reaches provide the framework for linking water-related data to the
<br />NHD surface water drainage network. For streams, a reach is typically comprised of one or more line
<br />segments (digitized, connected points), where the end-points of the reach correspond to physical features
<br />including confluences, bridge crossings, or other distinguishable breaks in the line. Each group of line
<br />segments is assigned the same reach code. Reaches appear to be used only for stream features, although
<br />NHD documentation describes reach code for point features (discussions with USGS personnel indicate
<br />that the documentation is in error).
<br />The Feature class is a base class that includes the Shape attribute for all derived (more specific) classes.
<br />The Feature class stores the geometry (i.e., location information), of a feature. This is a standard ESRI
<br />class that is not specific to the NHD. Details of Feature classes are provided in a series of tables in
<br />Appendix A. The features are as follows:
<br />• Hydrography Class is the base class for all NHS features, allowing a unique identifier for each
<br />feature to be assigned.
<br />• HydroPoint Class is used to store point features (e.g., wells, headgates).
<br />• HydroLine Class is the base class for line features, which represent flow (e.g., canal, stream
<br />segments).
<br />• NHDLine Class is used to store line features that do not represent flow (e.g., coastline).
<br />• NHDFlowline Class is used to store line features that represent flow (e.g., canal, stream). Note
<br />that flow lines (stream reaches) are often maintained through water bodies to allow the network
<br />tools to function -when drawing streams, this can result in lines being drawn through water
<br />bodies if linework is shown on top of the water bodies.
<br />• NHDWaterbody Class is used to store water bodies (e.g., lakes), using the average capacity
<br />conditions to define the extent of the feature.
<br />2.5 Network Representation
<br />The NHDinGEO includes two flow representations. One representation is spatial (or geometric), with
<br />features connected at nodes and the coordinates ordered from upstream to downstream. If features are
<br />topologically connected and have a F1owDir value of "With Digitized", the features will be part of a
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