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I'm trying to determine population inside Special Flood Hazard Areas. To do this I'm doing a spatial join of census shapefiles with the FEMA National Flood Hazard Layer, using the Boolean field SFHA_TF to identify areas inside the Special Flood Hazard Area. The map shows that the SFHA cuts through tracts, taking pieces of some, all of others. But when I join the two tables, the resulting attribute table contains multiple copies of the entire tract; it does not split the tract into pieces (for example a quarter of the tract if the SFHA occupies 1/4 of the tract). Is there a way in QGIS to do a spatial join between two polygons that will get proportionate shares of the joined polygon? .Adding screenshot to respond to comment -- green lines are census tracts; orange areas are flood zones

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  • A screen shot would sure help me visualize your problem.
    – Stu Smith
    Feb 22, 2020 at 7:20
  • I added an image to answer your comment: Green lines are census tracts, red lines are highways, and orange fill areas are flood zones. Thanks. Feb 23, 2020 at 16:49

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There is a plugin which provides markedly different functionality to the stock standard VECTOR | INTERSECTION (or did you use spatial join??) detailed here and available via the standard plugin menu.

Not too sure what your end product is intended to be...

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