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geometric flow network that is traceable in ESRI's network solvers. This design is consistent with ESRI's <br />Arc Hydro approach to the flow network. <br />The other representation (described as the logical representation) is non-spatial, with a flow relationship <br />between reaches that is held in a separate table called NHDFIow, which is not described above or in the <br />NHD Hydrography object model description. This table has FromComID and ToComID columns that <br />define feature connectivity and flow direction. To support divergences and convergences, the NHDFIow <br />table supports many to many relationships. While the flow table information is redundant with the <br />geometric network that is included with the geodatabase (in the F1owDir value), this table is provided as <br />an alternative method of flow navigation that is independent of the GIS implementation. For example, <br />this table could be used by non-ESRI software to determine the order of stream reaches. <br />2.6 NHD Tools <br />To assist in the use of the NHD, the USGS has developed a toolkit as a set of extensions to ArcView 3.x. <br />This toolkit is available as a free download from http://nhd.usgs.~ov/tools.html. Since ArcView 3.x <br />cannot access a geodatabase, the toolkit can be used only with NHDinARC. The ArcView 3 Toolkit <br />includes the following extensions: <br />NHD Load/Unload Workspace, which loads, symbolizes and displays the themes and tables in <br />one or more NHD workspaces. A workspace is an ESRI term for the container of data such as <br />ArcInfo coverage or shapefile. <br />NHD Navigate provides tools for traversing the NHD drainage network through the use of the <br />NHD reach flow relationships. Using Navigate, the user may select a starting point for the <br />navigation, which is either a reach, or a point along a reach. At the user's option, the navigation <br />proceeds either upstream along the mainstem, upstream along all flow paths, downstream along <br />the mainstem, or downstream including all divergent flow paths. Other navigation options <br />include the selection of a maximum distance to travel or the selection of a particular stopping <br />point along the network. NHD Navigate displays the navigation results as a new theme, which is <br />added to the active view. <br />NHD Reach Indexing Tool (RIT) provides an interactive environment for creating point, line and <br />area events linked to NHD reaches. An event is information about a geographic feature such as <br />an observation (e.g., flow rate of a stream) occurring in a specified location and (possibly) time. <br />Because events reference location relative to features, they can be edited and maintained <br />separately and independently of the features themselves. The NHD RIT is a tool for associating <br />surface water entities, such as trout waters or use-impaired waters, with the NHD by creating <br />reach addresses (i.e., reach id and location along the reach in terms of relative distance from the <br />downstream end) for these entities. The NHD RIT uses ESRI's dynamic segmentation model to <br />build event tables, which store information about the NHD reaches that define the location of <br />each entity. This approach allows the user to link and map surface water attributes without <br />editing the underlying NHD coverage. <br />NHD Migrate is designed to provide NHD users with an easy to use tool for migrating tables of <br />point, linear, and areal events to NHD data that have been updated. NHD Migrate is optimized <br />for moving events between NHD of the same version (e.g. from medium resolution NHD to <br />medium resolution); however, it can be used to move from medium resolution NHD to high <br />resolution NHD. In particular, when moving from medium to high resolution, there are some <br />cases where the events will not move to the desired location on the reach, however the NHD <br />Migrate QA/QC features will assist the user in locating events that may need adjustment. <br />NHD Arc2Shape, which converts NHDinARC workspaces (coverage format) into NHDinSHP <br />workspaces (shapefile format). <br />Page 6 of 17 ~IZlverslde Technnlo~yv Inc. <br />4s':rkv Resuurce.s Ergmee: mg end ~C9 nsuNrng <br />