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Wombat
Wombat

Reputation: 138

R remove duplicate polygons from a SpatialPolygonsDataFrame

I need to remove duplicate polygons from a SpatialPolygonsDataFrame in R. There is a method for points but not for polygons.

I need to do that for creating a tool to similar reshape() for spatial data.

Upvotes: 1

Views: 4165

Answers (3)

Robert Hijmans
Robert Hijmans

Reputation: 47146

The answer would depend a bit on what you want to do with the attributes.

Example data:

library(raster)   
p <- shapefile(system.file("external/lux.shp", package="raster"))
p <- p[1:2, ]
pp <- bind(p,p)
data.frame(pp)

#  ID_1   NAME_1 ID_2   NAME_2 AREA
#1    1 Diekirch    1 Clervaux  312
#2    1 Diekirch    2 Diekirch  218
#3    1 Diekirch    1 Clervaux  312
#4    1 Diekirch    2 Diekirch  218

If you want to combine polygons that have the same attributes (but perhaps different geometries), you can use aggregate:

a <- aggregate(pp, names(pp))
data.frame(a)

#  ID_1   NAME_1 ID_2   NAME_2 AREA
#1    1 Diekirch    1 Clervaux  312
#2    1 Diekirch    2 Diekirch  218

If you know they are duplicates in geometry and attributes, you can use @DJack's solution:

b <- pp[!duplicated(data.frame(pp)),]

If it is just the areas you care about, you could do

d <- union(pp)

And you could reconstitute the attributes with the info in

data.frame(d)
#     ID.1 ID.2 ID.3 ID.4 count
#1    1    0    1    0     2
#2    0    1    0    1     2

But in that case, it would be simpler to do:

g <- geom(pp)
x <- split(g[,c(2, 4:6)], g[,1])
dd <- duplicated(x)
dd
[1] FALSE FALSE  TRUE  TRUE

Followed by

z <- pp[!dd, ]

Package terra has a unique method for SpatVector objects:

library(terra)
v <- vect(pp)
u <- unique(pp)
u

# class       : SpatVector 
# geometry    : polygons 
# dimensions  : 2, 5  (geometries, attributes)
# extent      : 5.826232, 6.315773, 49.78479, 50.18162  (xmin, xmax, ymin, ymax)
# coord. ref. : +proj=longlat +datum=WGS84 +no_defs 
# names       :  ID_1   NAME_1  ID_2   NAME_2  AREA
# type        : <num>    <chr> <num>    <chr> <num>
# values      :     1 Diekirch     1 Diekirch   312
#                   1 Diekirch     2 Diekirch   218

Upvotes: 3

Edzer Pebesma
Edzer Pebesma

Reputation: 4121

Try using rgeos::gEquals(x, byid = TRUE) with x the object with the polygons.

Upvotes: 1

DJack
DJack

Reputation: 4940

If by duplicated polygons, you mean polygons with identical data, you can use this code: 

shp <- shapefile("shape.shp")
shp <- shp[!duplicated(shp@data),]

EDIT

If by duplicated polygons, you mean polygons with identical coordinates (@coords slot), you can use this code:

shp <- shapefile("shape.shp")
coord_poly <- lapply(shp@polygons, function(x){lapply(x@Polygons, function(x){coordinates(x)})}) 
shp <- shp[!duplicated(coord_poly),]

Upvotes: 5

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