The ultimate goal of flow modeling is computing indices that correctly estimate real phenomena. There is a large body of GIS literature concerned with issues involved in flow modeling on terrains. The focus of TerraFlow are the computational aspects of the indices rather than their suitability to the real phenomena. The accuracy of flow accumulation depends on the accuracy of the flow directions, and in particular on the accuracy of assigning flow directions on flat areas. As we have mentioned above, TerraFlow gives two choices for the computation of flow directions, SFD and MFD. From the computational point of view MFD and SFD do not make a (big) difference. In practice, it looks like MFD are natural on slopes, while SFD give better results on flat areas. MFD produces unconnected, divergent flow lines, while SFD produces convergent flow.
Consider the sample DEM, its plateaus (as computed by TerraFlow) and the flow accumulation computed by ArcInfo.
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Below we illustrate the results of running the different combinations of FILL and FLOW on the sample dataset. We show only the flow accumulation grids. The meaning of the combinations can be found in Features.
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In addition to these the user can obtain even more flow accumulation results by experimenting with different -c thresholds.
| Total running time of Terraflow at different main memory sizes. Data size is in million elements and running time is in hours. | ||
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| Kaweah DEM | MFD-MFD flow | MFD-SFD flow |
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| PuertoRico DEM | PuertoRico MFD-MFD flow | PuertoRico MFD-SFD flow |
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| Sierra DEM | Sierra MFD-MFD flow | Sierra MFD-SFD flow |
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| Hawaii DEM | Hawaii MFD-MFD flow | Hawaii MFD-SFD flow |
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| Cumberlands DEM | Cumberlands MFD-MFD flow | Cumberlands MFD-SFD flow |
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| LowerNE DEM | LowerNE MFD-MFD flow | LowerNE MFD-SFD flow |
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| Usadem6 DEM | Usadem6 MFD-MFD flow | Usadem6 MFD-SFD flow |