Pre-Processing:Constructing a Grid

Once a boundary feature polygon has been defined by one of the methods described above, a 2-D grid can be created in WMS using the Create Grid command in the Feature Objects menu. WMS will then prompt you for the grid size and fill the interior of the rectangle that just bounds the watershed. Each grid cell is flagged as being inside the watershed (active) or outside the watershed (inactive) according to the location of the centroid of each grid cell. The results of a typical grid generation are shown in Figure 2.

Figure 2. The finite difference grid covers the extents of the watershed. Each grid cell is flagged to indicate whether it is inside or outside the watershed. Elevations for the cells inside the watershed are then interpolated from the background DEM

For more information on how to use this command, please see the WMS Help File. This grid is the heart of the GSSHA model and defines the watershed mask (grid cells within the watershed) and elevation map required to run GSSHA.