EDNA Stage 3 Processing Steps (Conflation)
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.
The Stage 3 flow chart illustrates the processing steps involved in creating a hydrologically correct DEM and conflating NHD attributes to the corrected synthetic streams. These steps are followed after mismatches have been identified (and flagged) between the NHD streams and the original NED synthetic streams. Topographic information, in the form of Digital Raster Graphics (DRG), are then used to verify the correct stream path.
Once the problems have been identified and verified, the next step involves the editing of the DEM. Edits are made to individual cells or groups of cells to produce subsequent flow characteristics (and ultimately stream lines) that more closely match those shown by the NHD. When all edits are completed, new flow direction, flow accumulation, and stream lines are generated and compared to the NHD prior to continuing to the conflation stage of processing. In order to keep a record of the edits made to the DEM, a difference grid (original, filled DEM minus the edited, filled DEM) is created. In addition to this grid, a vector coverage is created from this grid. Attributes are included in this coverage that describe the reason for making the DEM edit.
One the hydrologically correct DEM has been created, conflation of the NHD attributes (e.g. reach code, name, reach date, etc.) can take place.
EDNA Stage 3 Processing Steps (Conflation)
Conflation can be described as a process by which two separate coverages are processed so that they can share common attributes. Two different processing approaches have been investigated thus far within the EDNA project.
The first process is automated and carried out through the implementation of an AML (Arc-Macro Language) script. This AML uses arc midpoints and incremental nearness tolerances to transfer attributes from the NHD to the synthetic streams. The AML also identifies those arcs that meet certain criteria, which identifies them as candidates for possible errors. Error coverages are created within the AML, which helps the analyst locate and fix errors.
Within this process the synthetic stream lines are segmented, where appropriate, in order to best match the NHD stream arcs on a one-to-one basis. Figure 1 illustrates the results of this conflation process. Figure 2 shows an example of the error coverages generated by the conflation aml.
A second process for conflation has also been developed that does not segment the original stage 1 synthetic streams. This process uses the same concepts described in the previous process, but conflates the synthetic streams to the NHD streams rather than NHD to synthetic.
While this process is semi-automated and uses a variation of the AML used in the previously described process, it also requires the analyst to perform edits. In essence, once the conflation aml has been run and a relate field has been created linking the NHD and synthetic streams, the NHD is edited until its attribute table includes the NHD attributes associated with corresponding synthetic streams. This includes one-to-one, one-to-many, and many-to-one arc relationships between NHD and synthetic streams.
Once editing is completed, the NHD attribute table is copied to a new table, which can be linked to the conflated synthetic stream coverage. This table is included with the synthetic stream coverage as a link or relate table. It should be noted that any modifications to the NHD are for creating this table only. The original NHD data, resident in the NHD data base, is not modified. Figures 3 illustrates the case where many NHD reaches correspond to one synthetic stream.
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.
The Stage 3 flow chart illustrates the processing steps involved in creating a hydrologically correct DEM and conflating NHD attributes to the corrected synthetic streams. These steps are followed after mismatches have been identified (and flagged) between the NHD streams and the original NED synthetic streams. Topographic information, in the form of Digital Raster Graphics (DRG), are then used to verify the correct stream path.
Once the problems have been identified and verified, the next step involves the editing of the DEM. Edits are made to individual cells or groups of cells to produce subsequent flow characteristics (and ultimately stream lines) that more closely match those shown by the NHD. When all edits are completed, new flow direction, flow accumulation, and stream lines are generated and compared to the NHD prior to continuing to the conflation stage of processing. In order to keep a record of the edits made to the DEM, a difference grid (original, filled DEM minus the edited, filled DEM) is created. In addition to this grid, a vector coverage is created from this grid. Attributes are included in this coverage that describe the reason for making the DEM edit.
One the hydrologically correct DEM has been created, conflation of the NHD attributes (e.g. reach code, name, reach date, etc.) can take place.
EDNA Stage 3 Processing Steps (Conflation)
Conflation can be described as a process by which two separate coverages are processed so that they can share common attributes. Two different processing approaches have been investigated thus far within the EDNA project.
The first process is automated and carried out through the implementation of an AML (Arc-Macro Language) script. This AML uses arc midpoints and incremental nearness tolerances to transfer attributes from the NHD to the synthetic streams. The AML also identifies those arcs that meet certain criteria, which identifies them as candidates for possible errors. Error coverages are created within the AML, which helps the analyst locate and fix errors.
Within this process the synthetic stream lines are segmented, where appropriate, in order to best match the NHD stream arcs on a one-to-one basis. Figure 1 illustrates the results of this conflation process. Figure 2 shows an example of the error coverages generated by the conflation aml.
A second process for conflation has also been developed that does not segment the original stage 1 synthetic streams. This process uses the same concepts described in the previous process, but conflates the synthetic streams to the NHD streams rather than NHD to synthetic.
While this process is semi-automated and uses a variation of the AML used in the previously described process, it also requires the analyst to perform edits. In essence, once the conflation aml has been run and a relate field has been created linking the NHD and synthetic streams, the NHD is edited until its attribute table includes the NHD attributes associated with corresponding synthetic streams. This includes one-to-one, one-to-many, and many-to-one arc relationships between NHD and synthetic streams.
Once editing is completed, the NHD attribute table is copied to a new table, which can be linked to the conflated synthetic stream coverage. This table is included with the synthetic stream coverage as a link or relate table. It should be noted that any modifications to the NHD are for creating this table only. The original NHD data, resident in the NHD data base, is not modified. Figures 3 illustrates the case where many NHD reaches correspond to one synthetic stream.