Iv&#225;n Higuera Mendieta - 1 year ago 168
R Question

# Remove intersecting points from polygon

I am trying to create a new set of

`SpatialPoints`
from the
`gDifference`
of two
`sp`
geometries. Suppose the following:

You have two
`SpatialPolygons`
:

``````library(rgeos)
library(sp)

#Create SpatialPlygons objects
polygon1 <- readWKT("POLYGON((-190 -50, -200 -10, -110 20, -190 -50))")
#polygon 1
polygon2 <- readWKT("POLYGON((-180 -20, -140 55, 10 0, -140 -60, -180 -20))") #polygon 2

#Plot both polygons
par(mfrow = c(1,2)) #in separate windows
plot(polygon1, main = "Polygon1") #window 1
plot(polygon2, main = "Polygon2") #window 2
``````

Now, you want to get the
`gDifference`
between them:

``````polygon_set <- readWKT(paste("POLYGON((-180 -20, -140 55, 10 0, -140 -60, -180 -20),",
"(-190 -50, -200 -10, -110 20, -190 -50))"))

par(mfrow = c(1,1)) #now, simultaneously
plot(polygon_set, main = "Polygon1 & Polygon2")

clip <- gDifference(polygon2, polygon1, byid = TRUE, drop_lower_td = T) #clip polygon 2 with polygon 1
plot(clip, col = "red", add = T)
``````

How can I get a
`sp`
geometry with only the non-intersecting points of the
`polygon2`
?(red points in the following image):

I think the comparison of the coords of `clip` and `polygon2` gives you the non-intersecting points.

``````library(ggplot2)                         # as @shayaa commented, ggplot2::fortify is useful.

clip_coords <- fortify(clip)[,1:2]          # or, clip@polygons[[1]]@Polygons[[1]]@coords
polygon2_coords <- fortify(polygon2)[,1:2]  # or, polygon2@polygons[[1]]@Polygons[[1]]@coords
duplicated_coords <- merge(clip_coords, polygon2_coords)
# duplicated_coords is the non-intersecting points of the polygon2
res <- SpatialPoints(duplicated_coords)

plot(clip)
``````

### [ Additional code : ver. intersecting points (suppose above code has run) ]

``````## an independent method
library(ggplot2); library(dplyr)

clip2 <- gIntersection(polygon2, polygon1, byid = TRUE, drop_lower_td = T)
res2.1 <- fortify(clip2)[,1:2] %>% setdiff(polygon2_coords) %>%   # not_duplicated_coords
SpatialPoints()

## a method usign a gDifference.sp.coords
res2.2 <- fortify(clip2)[,1:2] %>% merge(clip_coords) %>%
distinct() %>% SpatialPoints()    # res2.2 is equal to res2.1 in elements.

plot(polygon_set, main = "Polygon1 & Polygon2")