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=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,. It does not define the model2D outputcell resolution. SGS works by sampling the terrain at a smallerfiner scale insidewithin 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 actual size of the 2D cells themselves.<br>
For example, usingwith an 8 m cell size and a 2 m SGS Sample Target Distance, the DEM is sampled at 5 points across each cell face. This results in 255 elevation points being used to define the volumeflow area versus elevation relationship withinat theeach 2D cellface, and the25 sameelevation points are used along each face to define the conveyancevolume versus elevation relationship. Thiswithin helpsthe better2D capturecell. This allows more detailsdetail from the input topography to be captured while maintaining the chosen inputoriginal grid size.<br>
If a smaller output resolution (such as for velocity or velocity 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.
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However, TUFLOW still stores only one velocity value per face and one depth value per cell. This means the resolution of mesh based outputs (e.g. XMDF) and standard grid outputs (e.g. ASC, FLT, TIF) is not reduced to match the SGS Sample Target Distance.
When SGS is enabled, users can output high resolution water level and depth outputs based on the cell centre water level and the sampled sub-grid elevations (see: [[TUFLOW HR Output]]). However, if higher resolution velocity or velocity derived outputs (such as hazard) are required, this must be achieved by reducing the 2D cell size, not by adjusting the SGS sampling settings.
=Does using SGS increase model runtimes?=
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