Sample Roughness

Sample Roughness#

Overview#

In this task, the spatially variable manning’s roughness is calculated from a polygon shapefile. The polygons represent roughness associated with different LandUse categories such as building, street, grass, desert brush and many others.

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The Plugin has 4 methods for calculating roughness. Use the Sampling Manning’s tool to access the calculator.

  1. Click the Sample Manning’s button from polygon layers.

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This layer requires a polygon shapefile with roughness data or digitized data assigned to the Roughness User Layer. The tool will calculate Manning’s roughness values with three different processes.

Roughness Polygon Intersection#

  1. To calculate a weighted average of manning’s polygons to grid element polygons, use the Source Layer and Intersect cell rectangle option.

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Roughness Point Sample#

  1. To calculate a point sample from the centroid of the grid element on the manning’s polygons, use the Source Layer and Intersect cell centroid option.

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Roughness Update#

  1. This option will only update cells whose centroid lies within the Roughness Layer polygons and leave all other values as is.

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  1. Once the sample is complete, the following window will appear. Click OK to close the window.

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The roughness values are assigned to the Grid layer in the Schematized Layers group.

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  1. If it is necessary to update or change a small selection of elements, use the Roughness layer in the User Layers group.

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Troubleshooting#

  1. The Roughness layer must be a polygon layer. It is usually a shapefile.

  2. The layer CRS must match the project CRS.

  3. The polygon geometry must be valid to process the intersection area average. Try Check Geometries if the calculation fails.

  4. If a Python error appears during the sampling process, it may indicate that attribute table is missing. Save and reload the project into QGIS and try again.

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