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Quadtree and Sub-Grid Sampling FAQ: Difference between revisions

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# Read the 2d_zsh merge polygon into the model using the "Create TIN Zpts Write TIN" command in the tgc file.
=My SGS Sample Target Distance is set to 1 metre, but the model output resolution is larger than 1 metre. Why?=
The SGS Sample Target Distance sets the spacing at which elevations are sampled within each 2D cell, not the model output resolution. SGS works by sampling the terrain at a smaller scale inside each computational cell to improve how topography is represented, especially across cell faces. The hydraulic behaviour across the cell is determined using these smaller elevation samples, without changing the size of the 2D cells themselves.<br>
For example, using an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each face. This results in 25 elevation points being used to define the volume versus elevation relationship within the 2D cell, and the same points are used along each face to define the conveyance versus elevation relationship. This helps better capture more details from input topography while maintaining the chosen input grid size.<br>▼
 
If a smaller output resolution (such as for velocity or depthvelocity related outputs like hazards) is required, this is controlled by the  2D cell size and/or the nesting level of the Quadtree polygon, not by the SGS sampling settings. SGS improves the internal hydraulic calculations within each cell but does not alter the 2D grid or the output resolution.▼
SGS (Sub Grid Sampling) works by sampling the terrain at a smaller scale inside each computational cell to improve how topography is represented, especially across cell faces. The hydraulic behaviour across the cell is determined using these smaller elevation samples, without changing the size of the 2D cells themselves.
 
▲For example, using an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each face. This results in 25 elevation points being used to define the volume versus elevation relationship within the 2D cell, and the same points are used along each face to define the conveyance versus elevation relationship. This helps better capture more details from input topography while maintaining the chosen input grid size.
 
▲If a smaller output resolution (such as for velocity or depth outputs) is required, this is controlled by the  2D cell size and the nesting level of the Quadtree polygon, not by the SGS sampling settings. SGS improves the internal hydraulic calculations within each cell but does not alter the 2D grid or the output resolution.
 
Note SGS provides the most benefit when the input DEM resolution is significantly smaller than the 2D cell size. If the DEM is similar to or coarser than the 2D cell size, SGS offers little improvement.
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