SSN analyses can be based on any digital hydrography network but these require topological reconditioning with the STARS ArcGIS toolkit to meet the compatibility requirements before conducting a spatial analysis. The reconditioning process may be time consuming and requires considerable GIS skills, so the NSI network was developed to expedite SSN analysis by reconditioning the NHD-Plus Version 2 medium resolution (1:100,000-scale) hydrography layer for all streams in the contiguous United States using the STARS toolkit. Use of NHD-Plus to create the NSI network also enables users to tap into large databases of reach descriptors developed for NHD-Plus that serve as useful predictor variables in spatial analyses. The NSI network is fully back-compatible with the original NHD-Plus layer after completion of a SSN analysis.
Shapefiles of the NSI network are available here for download by NHD production units; as are shapefiles of prediction points that can be used to make predictions at unsampled locations and map the results of SSN analyses. Each shapefile has specific metadata, but for general metadata see the General Flowline Metadata document or the Prediction Points Metadata document. A comprehensive description of the technical procedures applied to the NHD-Plus hydrography layer, as well as additional steps that are required to prepare datasets for SSN analysis are described in the NSI User Guide.
Citation to use: Nagel, D., S. Wollrab, S. Parkes-Payne, E. Peterson, D. Isaak, and J. Ver Hoef. 2017. National Stream Internet hydrography datasets for spatial-stream-network (SSN) analysis. Rocky Mountain Research Station, U.S. Forest Service Data Archive, Fort Collins, CO. DOI: pending.