QGIS – How to List Names of Overlapping Polygons in QGIS

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In QGIS, I would like to assign polygons in one layer (ZIP codes) to polygons in another layer (counties). Specifically, I would like to know, for each ZIP code, in which county it is located. In case a ZIP code overlaps with multiple counties, I would like to pick the county for a ZIP code that contains the largest share of the ZIP codes' area.

I tried the "Overlap Analysis" tool, which already gets me very close: In the resulting attribute table I find, for each ZIP Code, the overlapping area between the ZIP code and the county as well as the percentage of the ZIP code's area that overlaps with the county. Crucially, however, I am missing the name of the county. What am I doing wrong?

Best Answer

On the ZIP layer, use this expression to get the name of the county polygon with the largest intersection:

array_max (
    array_foreach (
        overlay_intersects('county', $id),
        if (
            area (
                intersection (
                    $geometry, 
                    geometry (get_feature_by_id ('county', @element))
                )
            ) = 
            array_max (
                array_foreach (
                    overlay_intersects('county', $id),
                    area (
                        intersection (
                            $geometry, 
                            geometry (get_feature_by_id ('county', @element))
                        )
                    )
                )
            ),
            attribute (get_feature_by_id ('county', @element),'name'),
            ''
        )
    )
)

Blue label is generated as a dynamic label with the expression from above:

Related Solutions

[GIS] R- Polygons overlapping Polygons

What I'd like to do is basically create a dummy variable called CBSA with a 1 for CBSA counties (red in the map) and a 0 for those not.

I'm no spatial expert, but maybe you tweak something based on this:

library(rgeos)
# shrink the cbsa polygons a bit to play it safe:
cbsa_b <- gBuffer(cbsa, byid=TRUE, width=-0.05)
# get intersection matrix:
m <- gIntersects(county, cbsa_b, byid=TRUE)
# which county polygons intersect any cbsa?
in_cbsa <- colSums(m)>0

in_cbsa is the dummy variable CBSA with TRUE (or 1 if you make it in_cbsa+0L) for CBSA counties:

head(in_cbsa)
#     0     1     2     3     4     5 
# FALSE FALSE FALSE  TRUE  TRUE  TRUE 
sum(in_cbsa)
# [1] 1868

# plot to verify:
library(ggplot2)
county_f <- fortify(county)
cbsa_f <- fortify(cbsa)
ggplot() + 
  geom_polygon(aes(long,lat,group=group), county_f, fill=NA, color="black") + 
  coord_quickmap(xlim=c(-128,-75), ylim=c(25,55)) +
  geom_polygon(aes(long,lat,group=group), cbsa_f, fill="red", color=NA) +
  geom_polygon(aes(long,lat,group=group), county_f[county_f$id %in% names(which(!in_cbsa)), ], fill="blue", color=NA, alpha = .3)

[GIS] Merging overlapping polygons with original polygons in QGIS

The SAGA Polygon self-intersection would have created new ID field to store the combination of overlapped polygons. Please keep it when you export it as Polygon C.

Example output of Polygon self-intersection

With a bit of editing, this can be used as id field to put together with Polygon A (or Polygon B).

For instance, if we want to use second number from ID, expression like:

substr( "ID", strpos("ID", '|')+1, 1)

will return below.

(Please modify it to fit with your data. If you want to use the first one, it will be substr( "ID", strpos("ID", '|')-1, 1). )

After merging them, Collect geometries (QGIS 3.0) or Singleparts to multipart using this field (overlap in the above example) as Unique ID will do the job.

Best Answer

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[GIS] R- Polygons overlapping Polygons

[GIS] Merging overlapping polygons with original polygons in QGIS

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