Quadtree and Sub-Grid Sampling FAQ: Difference between revisions

Turning SGS on can increase run times by 20-30% for a model that is well designed with appropriate cell sizes. However, in cases where the model resolution is too coarse for the terrain, the improvement in model stability and hydraulic performance by using SGS can actually cause a reduction in run times. For example, for a direct rainfall model of the Johnstone River catchment SGS vastly reduced the choking of narrow flow paths caused by one elevation per cell face in the steeper part of the catchment, thereby reducing high depth and high velocity areas that allowed the simulation to progress at much larger timesteps reducing the simulation time from 26 hours to 4(!). Using SGS may allow you to use larger cell size(s) without adversely changing results so that runs can be performed much faster - this is of great benefit when developing or calibrating a model when you wish to have a high turnover of simulations - if you can carry out this phase of the modelling using coarser cell sizes by using SGS without greatly changing results your workflow efficiency can be greatly enhanced.<br>

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=I'm using SGS and my water level extent is larger than depth extent. Why?=