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I am trying to count how many times a polyline with a specific attribute intersects a polygon in QGIS. For example, I have a polyline with a feature class called "size", this contains a text value i.e. small, medium & large. I have also made a grid of 100m x 100m squares.

I would like to be able to count how many "large" polylines intersect each grid square polygon, and then repeat this for "medium" etc.

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In database/DB manager/Virtual Layers/Qgis layers you can try the following query :

select count(*), line_id, grid_id from line_layer, grid_layer
where st_intersects(line.geometry, grid_geometry)
and line.width='large'
group by grid_id

You'll have to adapt the attributes according to your data structure.

  • Thanks. I would also like to populate a new attribute field in the polygon grid shapefile with a count for each large, medium and small. For example, the "count_large" field would contain the value of how many lines with "size" attribute "large" intersect each grid polygon. – G Fry Aug 29 '18 at 11:02
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    The previous query almost does the job. It gives u your count_large and your grid_id. All you have to do is joining the result to your original grid table. You will have to base the join on "grid_id". After the join is made you can save the final result to create a new table containing the new columns you added. – snaileater Aug 29 '18 at 11:37

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