Finding overlay percentage between two polygon shapefiles

Question

I have two layers that I am currently working with: 1. Parcels/property boundaries and 2. Wetlands

I am trying to determine the parcels that have 0 wetlands and then for the parcels that do have wetlands, what the percentage of wetland coverage is for EACH parcel?

Is there a simple way to do this? Parcels are in orange, wetlands are in the brighter blue.

Answer 1

All fields are created using the Field Calculator. Install the refFunctions plugin if you don't already have it. This plugin enables the intersecting_geom_sum() function.

1. Intersect the parcel and wetland layers, creating a layer called Intersection.
2. Add a field called "wetland_area" to the wetland layer with the expression $area
3. Add a field called "parcel_area" to the parcel layer with the expression $area
4. Add a field called "wetland_area" to the parcel layer with this expression:

   intersecting_geom_sum('Intersection', 'wetland_area')

5. Add a field called "wetland_percent" to the parcel area with this expression:

   100 * "wetland_area" / "parcel_area"

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Or if you don't want to create so many new fields, you can skip steps 3 to 5 and instead use this expression to calculate "wetland_percent"

100 * intersecting_geom_sum('Intersection', 'wetland_area') / $area

Answer 2

Personally, I would convert the wetland into a raster (Feature to Raster, information on this can be found here), and then perform a Zonal Statistics analysis (more info here) to determine how much of the raster overlaps with each polygon. You can then join the the statistics table back to the polygon shapefile (the parcels) to see what the pixel count is.

To put it as as simply as possible, if you convert the wetland polygons into a raster file, it will represent the wetlands as a collection of individual pixels in a grid, rather than polygons. The zonal statistics tool can then count how many pixels of the value "wetland", how many pixels of the value "not wetland", and how many pixels total are located within each parcel polygon. From these numbers, you could calculated the percentage of wetland within each parcel polygon.

Please let me know if there's anything else you need me to expand upon!