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I work for a local authority and I'm measuring the surface area of river catchments inside our local authority.

I have a polygon layer showing me the catchments but a few of them go over our boundary into other local authorities - so just measuring the polygon's surface area would be no good.

Is there any way I can measure how much of the polygons 'sit' inside our boundary (which I also have as a separate layer)? I'm using QGIS 3.

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Use the Overlap analysis tool:

This algorithm calculates the area and percentage cover by which features from an input layer are overlapped by features from a selection of overlay layers. New attributes are added to the output layer reporting the total area of overlap and percentage of the input feature overlapped by each of the selected overlay layers.

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    Thank you! Exactly what I was looking for. The only porblem I'm getting is I don't know what unit the area is calculated in and I can't seem to find a way to set it to kilometres squared. Oct 26 at 14:40
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    It is in the unit of your coordinate system. If it is meters, use Field Calculator to alter it, or to add and calculate a new field. If it is in degrees it gets a little bit more complicated, but you can still use field calculator.
    – BERA
    Oct 26 at 16:09
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You can clip the river layer with the boundary layer. Create a field like area_sqkm and get the area via round(($area/1000000),2).

Then join that layer's field to the unclipped river layer and rename the field appropriately.

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