Remove intersecting points from polygon

Question

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 (i.e., the red points in the following image)?

Thanks in advance!

turn them into individual lines and test for intersection then pick the ones that don't? — hrbrmstr, Jul 31 '16 at 19:25
I have tried that, but the whole `polygon2` is returned. I assume that it only consider intersection when the whole line is "contained" — topcat, Jul 31 '16 at 19:32
After you [determine the points of intersection](http://stackoverflow.com/questions/20519431/finding-point-of-intersection-in-r), you need to determine the ordering that produces the desired polygon. The `ggplot2::fortify` function will give a nice data.frame for your use in plotting, after you adjust the ordering. — shayaa, Jul 31 '16 at 19:32

cuttlefish44 · Accepted Answer · 2016-08-03T02:41:54.490

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)

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

I used `tidy` instead of `fortify`, the outcomes remains equal. I have been trying the same code to get the `gIntersection` **and not the `gDifference`**, but the resulting `sp` object is clip and not the `SpatialPoints` of the polygon. `merge(clip_coords, polygons2_coords, all.x = T)` do not work. — topcat, Aug 02 '16 at 17:43

Remove intersecting points from polygon

1 Answers1

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