NetMap and the NHDplus have compatible goals and each provides value added facilities to the other. However, the NHD/NHDPlus and NetMap are not equivalent systems.
The NHD/NHDPlus functions as the national spatial library of streams and rivers of the United States and consequently must maintain a strict data structure that includes a rigid delimited set of river flow lines or hydrography. In contrast, NetMap's flow lines (or hydrography), including the Virtual Watersheds they are embedded within, are designed to be customizable and flexible, both in spatial extent and data structure, to provide the greatest adaptability and utility in scientific applications.
NHDplus provides a common national hydrography platform that is used for archiving channel-associated data (in certain agencies), for sharing analysis tools that work with the NHD/NHDplus data structure, and for distributing data and modeling results. NetMap was originally created to provide analysis capabilities that the NHD/NHDplus lacked for specific resource management, risk mitigation and conservation applications in particular geographic locations. These included customizable synthetic river networks - virtual watersheds (derived directly from DEMs of any resolution, or combination of resolutions) designed to:
- maintain digital information at the finest spatial scales,
- provide flexibility in river network generation and river-terrestrial discretization,
- support routing across networks and surfaces such as road systems, debris-flow corridors, flood plains, and riparian zones, and
- disseminate specialized tools for predicting fish habitats, floodplains, in-stream wood recruitment, thermal loading, road impacts, soil erosion, and landslide and debris flow hazard assessments, and in particular to evaluate connections among them.
NetMap and NHDplus data structures are different and each has different strengths. The NHDplus utilizes event tables that allow data to be registered precisely along NHD reaches, allowing users to geo-register disparate data to channel networks. With NHDplus, spatially distributed information about the land areas draining to a channel network, such as mean slope, or proportion of area in forest cover, can be summarized and associated to locations on the NHD. The NHDplus also contains a set of climate and hydrology variables. Multi-kilometer NHD reaches are linked to the terrestrial environment via flow directions, but at relatively large spatial scales.
NetMap creates a surface-channel network via a set of linked nodes at the finest spatial grain available (typically the grid spacing of the source DEM) but that can extend into the ephemeral and zero-order portions of a channel network, to include, for example, debris-flow corridors used for hazard assessments. Each node has an associated database of user-requested attributes (e.g., survey data, channel geometry, channel gradient, discharge statistics, and hillslope erosion indices etc.). Continuity of the network is established through upstream-downstream connections between nodes. Each node, or larger reach segments (commonly between 100 and 200 m), support fine scale discretization and analysis terrestrial environments. All riverine and terrestrial locations are geo-referenced to one another allowing for comprehensive analyses of connectivity among landforms, processes and land uses.
In Alaska where high resolution IfSAR DEMs are becoming available to create improved maps of surface topography, NetMap’s development of more accurate hydrography (such as in the Matanuska-Susitna watershed) can be coupled to, and enhance, development of the NHDplus. During development of NetMap’s node based hydrography coupled to the DEM required for NetMap tools, an NHDplus hydrography can also be created with an event table data structure at the longer channel length scales typical of NHDplus. Common NHDplus attributes, including of climate and hydrology, are placed in event tables. DEM rasters are included and associated with NHDplus hydrography, via flow directions. New WDB boundaries are created at the scale of NHDplus segment lengths that are either consistent with the synthetic hydrography or existing WBD boundaries can be enforced onto the DEM via “burning-in” and “walls” (as is done with existing NHDplus datasets in the lower 48).
Where synthetic hydrography already exists, NetMap will mirror existing stream lines and utilize existing flow direction/accumulation and can reproduce a NHDplus data structure consisting of event tables, NHD attributes and associated raster files. Although the NHDplus as constructed can stand alone (similar to NHD/NHDplus anywhere), users of a derived NHDplus also have the option of utilizing NetMap’s tools to conduct various types of analyses associated with routing (downstream, upstream, downslope, upslope), floodplain mapping, habitat modeling, erosion assessments and hazard analyses. NetMap is being designed to work with NHD/NHDPlus tools.
NetMap’s value added facilities include the derivation of detailed hydrography extending to the ephemeral and unchanneled portions of landscapes, connectivity and routing analyses, and advanced tools for resource management, risk mitigation and conservation, with transfer of that information to the NHDplus. NHDplus’s value added facility is the communication of NetMap’s attributes and analysis results to the common national hydrography platform used for distributing data and modeling results across large and diverse stakeholder groups.
NetMap compatibility with the NHD/NHDplus includes:
Where NHD exists: there at least two options for translation between NetMap's attributes and analysis results to the NHD. Either a cross talk tool is used to transfer data across the two stream layers or a NHDplus compliant raster dataset is built, including with "burning-in" and "walls" to couple hydrography to the DEM.
Where NHDplus exists: Existing NHDplus datasets can be translated to a NetMap channel node (data structure) and a D-8 flow path network. Thus, the NHD flow path network and flow directions are used directly with NetMap.
Where NHDplus is being derived directly from DEMs: Where NetMap is creating the original synthetic hydrography (such as in the Matanuska-Susitna watershed, Alaska), one of the derived products is a stand-alone NHDplus hydrography and associated raster files. Users of the NHDplus have the option of using NetMap tools.