sf dissolve/remove inner borders of POLYGON

Question

This code creates the desired outline of two combined shapes

square.df <- data.frame("x"=c(0,0,1,1,0),
           "y"=c(0,1,1,0,0),
           "ID"=rep("square",5))
square <- st_cast(st_combine(st_as_sf(square.df,coords = c("x", "y"))),"POLYGON")
plot(square)

triangle.df <- data.frame("x"=c(0,1,0.5,0),
                        "y"=c(1,1,1.5,1),
                        "ID"=rep("triangle",4))
triangle <- st_cast(st_combine(st_as_sf(triangle.df,coords = c("x", "y"))),"POLYGON")
plot(triangle)

plot(st_union(triangle,square))

But what if my data frame has multiple shapes, like this?

shapes.df <- rbind(square.df,triangle.df)
shapes <- st_cast(st_combine(st_as_sf(shapes.df,coords = c("x", "y"))),"POLYGON")
plot(shapes)

What sf object do I create and how do I get the outline of the combined polygons?

Jindra Lacko · Accepted Answer · 2021-07-23 17:12:25Z

5

This is an interesting problem; the immediate answer would be to consider sf::st_union() - but your shapes object is not valid; you have to repair it first by calling sf::st_make_valid().

Then (and only then) will the st_union work as expected.

st_is_valid(shapes)
[1] FALSE

house <- shapes %>% 
  st_make_valid() %>% 
  st_union()

plot(house, col = "red")

answered Jul 23, 2021 at 17:12

Jindra Lacko

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1 Comment

Mark R Over a year ago

Thanks. Much more efficient that my loop hack!

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