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.A Digital Elevation Model (DEM) of the study area is required as input for terrain preprocessing: a DEM is a grid in which each cell is assigned the average elevation on the area represented by the cell.The DEM must be in ESRI GRID format.During the processing, potential problems with the terrain representation can be identified, thus preventing the DEM errors from propagating to the later stages of the analysis.A successful preprocessing is an indication that the underlying DEM does not contain major problems that will prevent further analyses.The initial basin delineation that is performed during the preprocessing has no meaning for later basin processing (except for performance during the extraction stage), since all parameters can be changed.In general, the recommended size for stream threshold definition (which in turn defines the sub basin delineation during preprocessing) is 1% of the overall area.For increased performance on large DEMs (over 20,000,000 cells), the size of the threshold may be reduced.Note: Arc Hydro does not provide specific tools for DEM editing and modification.Standard Spatial Analyst functionality can be used for such purposes.Terrain Preprocessing contains the following functionslData Management: set the tags for the themes used in the Terrain Preprocessing menu.llDEM Reconditioning (AGREE): enforce linear drainage pattern (vector) onto a DEM (grid).Implement AGREE methodology.llFill Sinks: fill sinks for an entire DEM (grid).llFlow Direction: create flow direction grid from a DEM grid.llFlow Accumulation: create flow accumulation grid from a flow direction grid.llStream Definition: create a new grid (stream grid) with cells from a flow accumulation grid that exceed user-defined threshold.l Stream_definitionlStream Segmentation: create a stream link grid from the stream grid (every link between two stream junction gets a unique identifier).llCatchment Grid Delineation: create a catchment grid for segments in the stream link grid.It identifies areas draining into each stream link.llCatchment Polygon Processing: create catchment polygons out of the catchment grid.llDrainage Line Processing: create streamlines out of the stream link grid.llAdjoint Catchment Processing: create adjoint catchment polygon for each catchment in the catchment polygon feature class.Adjoint catchment is total upstream area (if any) draining into a single catchment.llDrainage Point Processing: create a drainage point at the most downstream point in the catchment (center of a grid cell with the largest value in the flow accumulation grid for that catchment).lData Management - Terrain PreprocessingArc Hydro manages the input/output to the tools by using tags that are automatically assigned by the functions to the selected inputs and outputs.A tag may be used as input by one function and as output by another one.For example, the "Flow Direction Grid" tag is an output from Flow Direction, and an input to Flow Accumulation.The Data Management function in the Terrain Preprocessing menu provides a global view of the tags assignments for that menu in the active Map/Data Frame.The function also allows assigning, reassigning or resetting the tags.A tag may be reset by selecting "Null" as the corresponding layer.When a reset tag is used as output, the function presents the user with the default layer name associated to the tag.This default name is defined in the XML file and may be modified (see XML Manager).How the function workslThe pull down menu next to each tag offers for selection only those themes in the active data frame that match the required theme type (line, point, polygon, grid).llA "Null" option is available in the list.Selecting "Null" remove the theme assigned to that role.llA layer may be assigned several tags.When a layer is used with an additional tag, the user is warned that at least one other tag has already been assigned to this theme [ Pobierz całość w formacie PDF ]