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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.

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  • 1
    Is there a specific software you're using or desire a solution in?
    – Jon
    Aug 16, 2018 at 17:58
  • Yes, I am using QGIS.
    – Laura
    Aug 16, 2018 at 18:08

2 Answers 2

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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"

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

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  • Hi csk, thanks for your response. I tried both methods and for some reason my wetland_percent column ended up with numbers that were way over 100%... I have numbers like 115, 139 etc. Anything I may have done wrong?
    – Laura
    Aug 20, 2018 at 14:13
  • The only difference between the two methods is that the first method allows you to check your work. Check the values at each stage and make sure they make sense. Also make sure that the intersection layer doesn't have overlapping polygons. If it does, first dissolve it, then split multiparts to single parts, then re-calculate wetland_area.
    – csk
    Aug 21, 2018 at 17:12
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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!

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  • Thank you for the response. Unfortunately I have not really worked with Raster layers before, so this is a bit confusing for me. Would you be able to provide more detailed steps? And, is this something I could do with QGIS?
    – Laura
    Aug 16, 2018 at 19:52
  • Just want to acknowledge that I've read your comment. Give me a few minutes to play around in QGIS and I'll edit my response to have more detailed steps!
    – Shelby G.
    Aug 16, 2018 at 19:54

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