Sample Roughness
================

Overview
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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.

.. image:: ../../img/115-Sample-Roughness-Data/img2.png

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.

.. image:: ../../img/115-Sample-Roughness-Data/img3.png

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
------------------------------

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

.. image:: ../../img/115-Sample-Roughness-Data/img4.png

Roughness Point Sample
----------------------

3. 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.

.. image:: ../../img/115-Sample-Roughness-Data/img5.png

Roughness Update
----------------

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

.. image:: ../../img/115-Sample-Roughness-Data/img6.png

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

.. image:: ../../img/115-Sample-Roughness-Data/img7.png

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

.. image:: ../../img/115-Sample-Roughness-Data/img8.png

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

.. image:: ../../img/115-Sample-Roughness-Data/img9.png

Troubleshooting
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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.