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I'm trying to aggregate a simple features collection using a group variable that is a factor. Below is a simple example.

library(sf)
#> Warning: package 'sf' was built under R version 4.2.3
#> Linking to GEOS 3.9.3, GDAL 3.5.2, PROJ 8.2.1; sf_use_s2() is TRUE

m1 = cbind(c(0, 0, 1, 1, 0), c(0, 1, 1,0, 0))
m2 = cbind(c(1, 2, 2, 1, 1), c(1, 1, 0,0, 1))
m3 = cbind(c(1, 1, 2, 2, 1), c(1, 2, 2,1, 1))
pol = st_sf(geometry = st_sfc(st_polygon(list(m1)), st_polygon(list(m2)), st_polygon(list(m3))))
pol$value = c(1,1,2)
pol
#> Simple feature collection with 3 features and 1 field
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: 0 ymin: 0 xmax: 2 ymax: 2
#> CRS:           NA
#>                         geometry value
#> 1 POLYGON ((0 0, 0 1, 1 1, 1 ...     1
#> 2 POLYGON ((1 1, 2 1, 2 0, 1 ...     1
#> 3 POLYGON ((1 1, 1 2, 2 2, 2 ...     2

With aggregate, I get the spatial features merged as I want, but I also get a duplicate version of by grouping variable.

aggregate(pol, by = list(pol$value), unique, drop = TRUE)
#> Simple feature collection with 2 features and 2 fields
#> Attribute-geometry relationship: 0 constant, 1 aggregate, 1 identity
#> Geometry type: POLYGON
#> Dimension:     XY
#> Bounding box:  xmin: 0 ymin: 0 xmax: 2 ymax: 2
#> CRS:           NA
#>   Group.1 value                       geometry
#> 1       1     1 POLYGON ((0 1, 1 1, 2 1, 2 ...
#> 2       2     2 POLYGON ((1 1, 1 2, 2 2, 2 ...

I have tried naming the items in the list passed to the -by- argument, but this just results in duplicate variable called Value.1 instead of Group.1.

I am aware that I can use group_by and summarise to get the result I want, but I would like to avoid dplyr if possible. I could just drop the unwanted variable, but I feel that I must be missing something obvious.

Created on 2023-08-10 with reprex v2.0.2

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  • I don't think you are missing anything; in fact I believe this is a bug in sf package. Aug 10, 2023 at 16:16
  • This is what aggregate() should return, the additional grouping variable in value is described in ?sf:::aggregate.sf() as well as in ?stats::aggregate() . Perhaps aggregate(pol, by = list(pol$value), unique, drop = TRUE) |> subset(select = -Group.1)
    – margusl
    Aug 11, 2023 at 9:07
  • I was just surprised that there wasn't a way to do this without the extra (subset) step. But either using aggregate with subset or sf_dissolve, as suggested by Jeffrey Evan, both fit the bill.
    – RichPatter
    Aug 14, 2023 at 7:21

1 Answer 1

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If I get your gist, you want to dissolve the geometry of features sharing a common attribute. Correct? This can be done with the sf_dissolve function in spatialEco. To dissolve feature geometry and attributes it is unfortunately not as simple an using dplyr::aggregate (or base).

library(sf)
library(spatialEco)

m1 = cbind(c(0, 0, 1, 1, 0), c(0, 1, 1,0, 0))
m2 = cbind(c(1, 2, 2, 1, 1), c(1, 1, 0,0, 1))
m3 = cbind(c(1, 1, 2, 2, 1), c(1, 2, 2,1, 1))
pol = st_sf(geometry = st_sfc(st_polygon(list(m1)), st_polygon(list(m2)), st_polygon(list(m3))))
pol$value = c(1,1,2) 

( dpol <- sf_dissolve(pol, "value") )
  plot(dpol)

dissolved features

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