Open Street Data OSM

Step 1: Download street center ines

Move all Manning’s n Roughness related rasters into the Manning’s Group.

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Open Plugins → Manage and Install Plugins and install QuickOSM (or OSM Downloader) if it is not already enabled.

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Open the tool:
Vector → QuickOSM → QuickOSM

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Use the Map Preset tab and select Urban for Streets and Buildings.
Use the current map canvas extent so the download matches the project domain.
Click Run Preset.

Note

if a timeout error occurs, run the preset again.

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Select the streets that are inside the Watershed Boundary.
Run the Select by location tool.
Fill the fields as shown and click Run.

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Invert the selection.

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Toggle the editor.
Delete the Selected Streets.
Save the edits.
Toggle the editor off.

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Step 2: Re-project and Export Street Layer

The street layer must be in the project coordinate system before buffering. If it remains in WGS84 (EPSG:4326), the buffer distance will be interpreted in degrees and will not produce correct results.

Right-click the Roads layer.
Select Export → Save Features As.

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Set Format to: ESRI Shapefile
Set the Coordinate Reference System (CRS) to: EPSG:2222
Choose an appropriate file name and save location.
Check Selected Features.
Click OK.

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The street layer is now in projected units (feet), and buffer distances will be applied correctly in the next step.

Step 3: Create street layer buffer

Run the Buffer tool from the Processing Toolbox.

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Load the Street Network in the editor and set the buffer distance to Edit Expression.

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Copy code into the Expression Editor to determine the buffer width by street type.

CASE
    WHEN "highway" = 'primary' THEN 40
    WHEN "highway" = 'secondary' THEN 30
    WHEN "highway" = 'tertiary' THEN 25
    WHEN "highway" = 'residential' THEN 20
    WHEN "highway" = 'service' THEN 15
    ELSE 15
END / 2

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