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'''Figure 3 Water depth simulation results with SGS. Left: 2.5/5/10/20m Quadtree model. Right: 10/20m Quadtree model.'''<br><br>
In TUFLOW, the depth output is interpolated from the depths at the nearest cell centre and cell corners surrounding the output grid. Because TUFLOW currently doesn't store the underlying high resolution DEM values atother than those needed for the momentSGS hydraulic computations, the interpolated depth may not perfectly represent the actual depth as illustrated in the figure below. <br>
[[File:Fig4 sgs depth interporation.png|500px]]<br>
'''Figure 4 TIN interpolation used for water depth map output with SGS.'''<br><br>
WhileWhilst we are developing a high resolution SGS map output inoption thefor a future release to address this issue, we have alsoin addedthe ameantime added new functionality into the [[ASC_to_ASC|ASC_to_ASC]] utility to 'remap' a water level grid to a finer DEM grid. This page introduceintroduces how to use the [[ASC_to_ASC|ASC_to_ASC]] remap function, and also discuss thediscusses limitationlimitations of the method.
=RemapRemapping Water Level to a Finer DEM=
The remap function in the [[ASC_to_ASC|ASC_to_ASC]] remaps a TUFLOW water level grid to a high resolution DEM using TIN interpolation. ItThe output outputsis a high resolution water level grid and a high resolution depth grid.<br>
<tt>asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc</tt>
In this commandwhere:<br>
'''"-wl <wl_file>"'''<br>
Setssets the coarser resolution water level grid to remapuse fromfor the remap.<br>
'''"-dem <dem_file>"'''<br>
Setssets the finer resolution DEM.<br><br>
The figurefirst image below overlaysshows the remapped water depth on top of the 10/20m model water level output, and comparedcompares it with the original water depth output sidein bythe second sideimage. As can be seen, much smooth water depthdepths along the gully and the flood fringes isare produced by the post process utility.<br>
[[File:Fig5 D sgs remap zoom.png|700px]]<br>
'''Figure 5 Remapped vs original water depth for 10/20m mesh SGS model.'''<br><br>
The next example presents the water level output and the remapped depth output infrom a river flood model using SGS with the buildings included in the DEM. As can be seen in the first image, large portions of smallthe smaller buildings areappear submerged by the SGS water level output due to the relativecells sizecontaining ofthese thebuildings meshbeing and thepartially buildingswet. However, the remapped depthdepths has(second clearimage) dry/wetclearly borders aroundshows the buildings and produces a much higher resolution flood map.<br>
[[File:Fig6 IR.png|1050px]]<br>
'''Figure 6 Water level output and remapped water depth in a river flood model.'''<br><br>
=RemapRemapping of Other Map Output Grids=
The utility can also remap extraadditional gridmap filesoutput grids (e.g. a hazard output) to the resolution of the DEM file.<br>
<tt>asc_to_asc.exe -remap -wl lowres_h.asc -dem DEM_highres.asc lowers_hazardlowres_hazard.asc</tt>
This command reads in an extraadditional grid 'lowers_hazardlowres_hazard.asc' and remaps it to the finderfiner DEM resolution. The figure below compares the original and the remapped hazard outputs from the 10/20m SGS model.<br>
[[File:fig7 ZAEM1 sgs.png|700px]]<br>
'''Figure 7 Original vs remapped hazard output for 10/20m mesh SGS model.'''<br><br>
It is important to note that, for any output types other than depth, this utility does NOT interpolate the result from the coarser grid to the finer grid, but only extendedextends/reducedreduces the output extent to the dry/wet extent. Therefore, the resolutiondetail of the remapped hazard above remains the same as the original output grid. The interpolation is not conductedcarried out for the following reasons:
* Hazard categories are usually dependeddependent on both water depth and velocity, and it is not straight forwardproblematic to interpolate/extrapolate the acomputed cell averaged velocity to sub-cell scale velocities with varying water depth.
* Should a higher output resolution be required for outputs utilising velocity, a finer mesh in the area should be used.
* Even if this can be done based on empirical relationship between depth and velocity, the obtained velocity is much less reliable than an actual simulation result from a model with finer mesh.
Therefore, we highly recommend refining the mesh size directly if users want to obtain finer velocity or hazard output at certain locations. The figure below shows the hazard output from the 2.5/5/10/20m model, and as can be seen the hazard result is much smoother and more accurate along the gully where the finer cell sizes occur. <br>
[[File:SGS 02-20m ZAEM1.png|400px]]<br>
'''Figure 8 Original hazard output for 2.5/5/10/20m mesh SGS model.'''<br><br>
''Tips: multiple file names or wildcard are allowed for the extra grid files for remapping.''
=Model Mesh Size vs Remap Result=
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