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Converting a NEXRAD Rainfall Map into a Flood Inundation Map by ...

Converting a NEXRAD Rainfall Map into a Flood Inundation Map by ...

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as GRID models and not as easy to store and manipulate (GRID models can berepresented <strong>by</strong> ASCII files) and the derived terrain features do not have a smoothand natural appearance as do GRID models.A TIN model is normally the first continuous surface generated out of thecontours because all the vertices of the contour lines can be used as mass pointsfor triangulation and because of the ability to include break lines in the modeldefinition to honor physical features like rivers and roads. By using all vertices forbuilding the triangulation, the continuous surface will not only honor the elevationvalues at every known location but also provide a higher level of triangulationover high relief areas (more and closer contours) than over low relief areas (fewand spaced contours). This ability for generating surfaces that honor all pointvalues with various levels of triangulation makes the TIN model more attractiveas the first terrain model to be created from contours. Figure 4.43(a) depicts theTIN model for Rosillo creek generated from all the vertices of the 2-foot contourlines shown above. The extent of the TIN model goes beyond the extent of themain stem floodplain due to triangles generated between border points.Given that the delineation of the flood inundation map is based on rasterproducts (intersection with a grid of water surface elevations), after the TINmodel is generated for the terrain elevation, a GRID model with a 20 foot cell sizewas generated using standard spatial analyst functionalities and clipping thecreated grid to just the extent of the floodplain under analysis. Figure 4.43(b)shows the final GRID model for the ground surface elevation.166

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