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The USGS ArcView Toolkit has additional options such as navigating from and to reach midpoints, <br />rather than only from junction to junction. The USGS continues to support the ArcView Toolkit for use <br />with NHDinARC. For NHDinGEO the USGS relies on ESRI to provide similar tools. ArcMap includes <br />the Utility Network Analyst toolbar that can be used to: <br />Display arrows to show the flow direction. <br />Perform trace operations. Nine trace tasks are available: <br />a. Find all network features that lie upstream of a given point in the network. A user- <br />specified tolerance is used to snap the point to the network. <br />b. Find all network features that lie downstream of a given point. <br />c. Find the total cost of all network features that lie upstream of a given point. The cost is <br />an attribute value weighted by the length of the line or another attribute of the feature. <br />d. Find an upstream path from a point. <br />e. Find the common features that are upstream of a given set of points. <br />£ Find all of the features that are connected to a given point. <br />g. Find all of the features that are not connected to a given point. <br />h. Find loops that can result in multiple paths between points. For example, in a water <br />supply network, identify nodes that feed themselves. <br />i. Find a path between two points. <br />The ArcInfo level of ArcMap also includes the Route Events GeoProcessing Wizard (which is separate <br />from the Network Analyst) that can be used to associate user events (user-defined features) to the NHD <br />reaches. Other functionality that is provided in the USGS toolkit such as the data migration tool is not yet <br />available with NHDinGEO. The U.S. Forest Service is developing additional tools for ArcGIS, but it is <br />unclear if and when these tools will be released to the public. A known limitation in the current ArcGIS <br />Network Analyst tools is that only full reaches can be selected -anew object model is being <br />implemented in ArcGIS 9.x to address this and other issues. <br />3. Approach <br />The approach used to link HydroBase and NHD, in particular to identify key structures in the stream <br />network, is as follows: <br />Gain an understanding of NHD, in particular the unique identifiers that could be used to link to <br />HydroBase (see background above). <br />Download and evaluate data for Colorado to understand the limitations of the data in terms of <br />coverage, scale, and relative number of features in NHD compared to number of features in <br />HydroBase. <br />Document a recommended design and procedure to link HydroBase with NHD (provided in this <br />memorandum, below). <br />Provide recommendations to the State and incorporate feedback in the design and approach. <br />Implement the NHD and HydroBase linkage for the SPDSS key structures. <br />The remainder of this memorandum focuses on technical issues necessary to determine recommended <br />design and procedures, in order to accomplish the third item, above. <br />3.1 Merging NHD Data Files <br />Page 7 of 17 ~IZlverslde Technnlo~yv Inc. <br />4s':rkv Resuurce.s Ergmee: mg end ~C9 nsuNrng <br />