Intersecting points and overlapping polygons

May 27, 2018
By

(This article was first published on R – Insights of a PhD, and kindly contributed to R-bloggers)

I’ve been doing some spatial stuff of late and the next little step will involve intersecting points with possibly many overlapping polygons. The sp package has a function called over which returns the polygons that points intersects with. The catch though, is that it only returns the last (highest numerical value) polygon a point overlaps with. So it’s not so useful if you have many overlapping polygons. A little playing, and I’ve overcome that problem…

Here’s a toy example.

Create a couple of polygons and put them into a SpatialPolygons object.

library(sp)

p1 <- matrix(c(1,1,
 2,1,
 4,2,
 3,2), ncol = 2, byrow = TRUE)
p2 <- matrix(c(2.2,1,
 3,1,
 3,2,
 3,3,
 2.8,3), ncol = 2, byrow = TRUE)
p1s <- Polygons(list(Polygon(p1)), 3)
p2s <- Polygons(list(Polygon(p2)), 4)
sps <- SpatialPolygons(list(p1s, p2s))

Define a few points and put them in a SpatialPoints object

point <- matrix(c(2.5, 1.5,
 3.2, 1.75,
 2,3,
 1.5, 1.25, 
 2.75, 2.5), ncol = 2, byrow = TRUE)
points <- SpatialPoints(point)

We can plot them…(not shown)

plot(sps, border = c("black", "blue"))
plot(points, add = TRUE)

As here we have the issue:

over(points, sps)
 1  2  3  4  5 
 2  1 NA  1  2 

only returning a single “hit” per point (point 1 overlaps with both polygons 1 and 2).

To get around this, we can rip the individual polygons out of the SpatialPolygons object and put them in a list, converting the individual polygons into SpatialPolygons along the way:

sps2 <- lapply([email protected], function(x) SpatialPolygons(list(x)))

Then lapply-ing over sps2 we can see which polygons each point intersects…

lapply(sps2, function(x) over(points, x))
[[1]]
 1  2  3  4  5 
 1  1 NA  1 NA 

[[2]]
 1  2  3  4  5 
 1 NA NA NA  1

And now we see that point one overlaps with both polygons.

Code >>>here<<<

 

 

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