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Water: Data

NHDPlus Smoothed Layers

Source: http://www.horizon-systems.com/nhdplus/CatchmentSP.php Exit EPA Disclaimer
NHDPlus catchments form the basis of the NHDPlus data model.  Based on USGS 1:100,000 quad maps, NHDPlus catchments are highly complex polygons posing a challenge for analysis or display (see NHDPlus documentation Exit EPA Disclaimer).  Smoothed NHDPlus catchments and NHDPlus regions are a generalized version of the original NHDPlus catchment data, with roughly 85% fewer vertices.  These layers are useful for display purposes or for determining the NHDPlus region of a feature.  Note that the generalization process may result in inaccurate analysis results along the smoothed feature borders.  Users should weigh the performance gain verses accuracy loss when utilizing smoothed NHDPlus layers.

smoothed catchments example

Layers currently stored in the WATERS instance:

  • CATCHMENT_SM metadata details image link
    Generalized NHDPlus catchments.
  • NHDPLUS_REGION_SM metadata details image link
    Aggregation of generalized NHDPlus catchments into composite NHDPlus regions.

Storage Details for NHDPlus Smoothed Layers

  • Metadata for all smoothed layers is available via the GeoData Gateway Exit EPA Disclaimer by clicking on the metadata buttons above.
  • Smoothed layers are stored in the same coordinate space (NAD83) as RAD and NHD layers.
  • Smoothed layers are registered to ArcSDE allowing use by any ESRI client.

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