Iván Higuera Mendieta - 1 year ago 106

R Question

I am trying to create a new set of

`SpatialPoints`

`gDifference`

`sp`

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`

`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`

`polygon2`

Thanks in advance!

Answer Source

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)
plot(res, col="red", pch=19, add=T)
```

```
## 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")
plot(clip2, col="red", add=T)
plot(res2.1, col="blue", pch=19, add=T)
```