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To link HydroBase and NHD for structure point features (primarily key diversions, as scoped for this <br />task), the following procedure can be followed: <br />Prepare the NHD data for the South Platte Basin, as described in Section 3.1, Merging NHD <br />Data Files. <br />Use linear referencing to associate each key diversion structure in HydroBase with the NHD <br />reach, using the proximity of the structure from the shapefile (which was generated from <br />HydroBase with ArcCDSS). This identifies the reach with which a structure is associated and the <br />location along the reach (measure) to where the structure is snapped. This will result in an event <br />theme containing diversion structure attributes that allow a link to HydroBase (e.g., WD ID). <br />Note that if the HydroBase stream data (water sources in the HydroBase stream and wd water <br />tables) are first associated with NHD (see section 3.2, above), these data could be used to make a <br />better association, where proximity may not be the best estimate. <br />Create a map and manually check key structures. <br />To allow a link to NHD from HydroBase, add to the HydroBase structure table an <br />NHD_ReachCode (String) field, as a foreign key to NHDF1owline.ReachCode. Add a measure to <br />the HydroBase structure table. A measure is the location along the reach in percentage of the <br />length of the reach from the downstream end. <br />By following the above procedure and adding data to HydroBase, GIS software can use NHD and the <br />event theme to determine diversion structures above, below, or between points on the NHD stream <br />network. <br />4. Comments and Concerns <br />The USGS has invested substantial resources in developing the NHD. However, as discussed in Section 2 <br />above, using NHD for more than simple maps requires an understanding of the NHD data model, <br />including its features and limitations. Additionally, the data model itself continues to evolve, and <br />consequently, the tools that use the data continue to evolve. Real-world applications that use NHD must <br />therefore have some flexibility to allow "refresh" with new NHD and software. Personnel involved with <br />these applications should have a sound understanding of NHD. <br />The background provided in this memorandum can be used to educate CDSS team members. The <br />recommended procedures have been researched and tested to some degree. However, the data quality <br />resulting from the recommended procedure will vary depending on the level of verification. For example, <br />linking key diversion structures to NHD will allow using the NHD network features to determine <br />diversions upstream or downstream of a point. However, if the referencing of the diversion structures to <br />the NHD linework causes errors, the corresponding upstream/downstream analysis also will contain <br />errors. Quality assurance of the spatial data ultimately will yield better results. <br />The following issues have been compiled as a part of this NHD evaluation, but none has been resolved <br />despite repeated conversations with USGS personnel: <br />No comprehensive document or data dictionary is currently available for NHDinGEO. <br />NHD databases do not use version numbers or release notes. The refresh procedure is currently <br />being defined by the USGS. <br />The reach concept does not apply to points. <br />Manual corrections must be sent to USGS. Only the USGS corrects errors in NHD, not the user. <br />The same reach has a different ComID and the same ReachCode across resolutions. When <br />combining different resolutions they are maintained as separate networks in separate feature <br />Page 12 of 17 ~fifversFde TeChnnlG~yv #nc. <br />4s':rkv Resuurce.s Ergmee: mg ane ~C.u ^sc?lrng <br />