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each stream table record must correspond to one linearly-referenced stream in NHD. Ideally, HydroBase <br />streams could be digitized by selecting end-points on the NHD. Additional discussion with the State is <br />required to clarify their requirements. A similar approach has been discussed for the ISF DSS; however, <br />the ISF reach activity is possible without extensive effort because the ISF reaches are easier to use <br />because they are already digitized and can be linked to NHD. <br />A recommended non-automated procedure for linking HydroBase stream features to NHD using linear- <br />referencing is provided below. A similar approach could be taken for springs, aquifers, groundwater, <br />lakes, and reservoirs. <br />Prepare the NHD data for the area in question (see previous section). <br />Review HydroBase stream and wd water table contents in order to define unique physical <br />features, as used in HydroBase. For example, use the stream and wd water records to identify <br />unique streams that are allowed to cross water district boundaries. <br />Identify stream linework corresponding to stream and/or wd water records in HydroBase. For <br />example, NHD could be first used to identify the main stem stream reaches by specifying the end- <br />point at the headwater (selected from NHD) and the point on the state line, or beyond (if NHD are <br />available beyond the state line). Smaller streams then could identified by selecting end-points on <br />the NHD features. By moving upstream and focusing on one district at a time, it should be <br />possible to utilize NHD linework while matching HydroBase records. The result will be an event <br />theme that includes attributes to reference NHD reaches and attributes to link to HydroBase. <br />Run queries to generate additional attributes indicating the down-stream direction of streams, to <br />store in HydroBase. This information is stored with the NHD data tables and may not be <br />necessary in HydroBase. However, NHD upstream/downstream information is based on NHD <br />reaches, not HydroBase streams. Therefore, the derived data may be useful to facilitate <br />HydroBase queries. The attributes also could be added to the event theme attributes. <br />Create a table in HydroBase to store the attributes of the event theme created in Step 3. This table <br />could be a new table or possibly could be a modification of the existing stream table. A decision <br />should be made after discussions with the HydroBase DBA. Also it will be possible to distribute <br />the resulting stream layer as a geodatabase. <br />One issue with the above approach is that if new NHD data are released (or if 24K data are made <br />available where only 100K data were available previously), then it may be necessary to regenerate the <br />data records that link HydroBase and NHD. Although the reach codes should be consistent, the measures <br />along the reaches may vary in different data sets. <br />Digitizing stream features from NHD was not anticipated in this task, instead being envisioned for SPDSS <br />Task 109. However, the activities in Task 109 have been postponed or eliminated, pending an evaluation <br />of CDSS administration tools. Nevertheless, it is possible to utilize NHD's network features as required <br />for point data queries, as described in the following section. <br />3.3 Linking NHD with HydroBase Point Data <br />The above procedure describes linking HydroBase streams to NHD, which is needed to perform spatial <br />queries using HydroBase attributes (e.g., find diversions upstream of a point on the HydroBase "XYZ <br />stream"). However, it is possible to perform spatial queries using NHD without linking NHD and <br />HydroBase streams. The latter approach relies on the network data in NHD. In both cases, linear <br />referencing to the NHD is used. <br />Page 11 of 17 ~fifversFde TeChnnlG~yv #nc. <br />4s':rkv Resuurce.s Ergmee: mg ane ~C.u ^sc?lrng <br />