Methodology

• Stage 1: Blind Pass Processing
  Using well-tested GIS techniques, the NED database is being processed into a Stage 1 EDNA product. The bulk of this work is being done by the National Weather Service's Severe Storms Laboratory (NSSL) in Norman, Oklahoma. Additional processing is currently being provided by the USGS WRD office in Sacramento, California.

  The NED is processed using blind-pass techniques to remove spurious depressions from the DEM. The resulting flow direction and flow accumulation data sets are also being developed, as well as an initial synthetic streamlines database. The Stage 1 reach catchments (drainage areas corresponding to each "link" in the synthetic streamline coverage) are also being derived. These catchments will be immediately useful in the NWS's AMBER implementation. This national catchment database is being attributed with catchment identification numbers according to Pfafstetter,which allows for efficient upstream aggregation and downstream tracing.

  This blind-pass development provides the Stage 1 drainage basin delineations and synthetic streamline coverages which will be transferred to stakeholders involved in the Stage 2 assessment. QA/QC to be done at this stage includes initial conflation of the synthetic streams with the NHD and comparison of derived drainage basin boundaries with existing digital basin boundary coverages.

• Stage 1B: Seamless Process
  

  The EDNA Stage1B process involves collecting the raw data from the EDNA cooperators, performing QA/QC checks on the raw data, and preparing the data for Stage II of the project. ArcInfo amls are executed to create the seamless database, and ArcView tools are utilized to determine seamless accuracy. The Stage 1B database development provides seamless drainage basin delineations and synthetic streamline coverages that are passed on to Stage II cooperators for quality checks and resource applications.

• Stage 2: Vector Editing
  The delineations produced in Stage 1 are passed on to appropriate cooperators, who will provide an intensive QA/QC. The derived watersheds will be overlain on 1:24,000 map sheets (as DRGs) and the watershed boundaries will be revised using standard vector editing techniques. These revised boundaries will provide the Stage 2 delineation. Watershed areas found to be in conflict with the DRGs will be flagged as problem areas. An additional QA/QC will be performed through comparison of the synthetic streamlines derived from EDNA with the 1:100,000 scale National Hydrography Dataset (NHD). Derived streamlines in conflict with the NHD will be flagged, as well.

  Arcview tools have been developed at EDC to facilitate the Stage 2 processing of the data. Discussion of the Stage 2 tools is available by following this link. Locations of current cooperators are displayed in this graphic

• Stage 3: Raster Editing
  The results from the Stage 2 delineation will be incorporated into the EDNA development process. Drainage basin areas and synthetic streamline locations found to be in error at Stage 2 will be reanalyzed and, if necessary, the DEM will be reprocessed to ensure that the newly derived streamlines and basin boundaries are consistent with those developed in Stage 2. Delineations derived in this fashion retain the desirable relationship between the DEM and the derivatives Following completion of the Stage 3 boundaries, the entire suite of the EDNA derivatives will be produced. These include the hydrologically correct DEM, flow directions, flow accumulations, slope, aspect and compound topographic index.