Revision as of 16:54, 28 November 2023 view source Pavlina Monhartova (talk \| contribs) Bureaucrats, Administrators 5,395 edits →When should I not use a combination of SGS and Quadtree? ← Older edit		Revision as of 17:03, 28 November 2023 view source Pavlina Monhartova (talk \| contribs) Bureaucrats, Administrators 5,395 edits No edit summary Newer edit →
Line 11: =When should I use SGS?= Almost every time to take advantage of the higher accuracy. There is a threshold beyond which SGS isn’t required. The benefit of SGS is focused on cells with variety of elevations within each cell. For flat cells, where all SGS sample points have almost the same elevation, there won’t be much of a difference in comparison without SGS, as there is no detailed sub-cell terrain to sample.<br> <br> Line 17: The main advantages of using Quadtree are shorter run times, lower GPU RAM footprint and smaller size of output files. Shorter run times are achieved with considerable cell count reduction compared with using a HPC grid, some models with cell count reduction of only around 30% may even run slower with Quadtree than the original HPC model.<br> Reasons for using Quadtree depend on modelling objectives and include: Large models that require higher resolution in the focus area (e.g. the town is the focus but the model due to boundaries, etc goes much further afield)~~. The combination of SGS and Quadtree is particularly powerful as this allows much larger cells away from the focus area without affecting the results~~. Where high resolution velocity-based outputs (e.g. velocity, hazard, bed shear stress) are required. SGS is great for enabling the use of larger cells with good results convergence, but it still only produces one (average) velocity per cell. Quadtree will produce higher velocity output resolution (noting run times might become slower due to smaller cell sizes). *Often popular for urban models where the streets, which take the bulk of the flow, are modelled at a higher resolution than properties, parks, etc. Where buildings and other obstructions are in an urban flowpath a higher resolution along the flowpath may be utilised. Line 23: =When should I use a combination of SGS and Quadtree?= Based on the benchmarking so far there seems to be little reason not to use SGS with Quadtree. The combination of SGS and Quadtree is particularly powerful for large models that require higher resolution in the focus area as this allows much larger cells away from the focus area without affecting the results.<br> Based on the benchmarking so far there seems to be little reason not to use SGS with Quadtree. The one exception is if the underlying resolution of the DEM is of similar (or coarser) resolution to the 2D cells, then there is little reason to use SGS as there is no detailed sub-cell terrain to sample. SGS was not made the default in the 2020-01 release because for some models, especially those with coarse cell sizes over highly variable terrain, ~~you~~substantially ~~can~~different ~~see~~results ~~substantially~~can ~~different~~be ~~results~~seen due to SGS picking up a lot more detail of the terrain within a 2D cell. Using Quadtree with SGS, and by using coarser cells, much better results and good results convergence can be achieved. It's highly likely that SGS will be made the default in a future release once there has been extensive industry benchmarking demonstrating using SGS to be consistently superior to not using it.<br> <br>

Quadtree and Sub-Grid Sampling FAQ: Difference between revisions