# Intersecting points and overlapping polygons

May 27, 2018
By

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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"))

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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