Assign polygon duplicate area (overlap) to only one polygon

Question

I have a shapefile (which can be downloaded here http://www.car.gov.br/publico/municipios/downloads?sigla=MA (any would do)) where each polygon represent a rural private property, as the owner submited it. However, the polygons overlap. For the analysis I need to run, an area cannot be accounted twice (ie. be in two properties ). So, on the areas that do overlap, I wanted to assign this duplicate area to the polygon with the smallest area and erase it from the other one.

I've looked at other questions such as this one. But none do really adress this.

Skittle

Answer 1

You can do this by subtracting the smaller polygon from the larger using rgeos::gDifference.

An example:

First we make some dummy polygons to demonstrate on

library(sp)
library(rgeos)
p1 <- Polygon(matrix(c(
  100, 100,
  100, 500,
  900, 500,
  900, 100,
  100, 100), ncol=2, byrow = T))
p2 <- Polygon(matrix(c(
  50, 400,
  50, 600,
  800, 600,
  800, 400,
  50, 400), ncol=2, byrow = T))
p1 <- SpatialPolygons(list(Polygons(list(p1), "p1")))
p2 <- SpatialPolygons(list(Polygons(list(p2), "p2")))
plot(p1)
plot(p2, add=T)

We can use gArea to find which is smaller

a1 = gArea(p1)
## [1] 320000
a2 = gArea(p2)
## [1] 150000

Now we can remove the intersection from the larger polygon with like this:

if (a1>a2){
  p3 = gDifference(p1,p2)
  p4 = p2
} else {
  p3 = gDifference(p2,p1)
  p4 = p1
}
plot(p3)

The two polygons together look like this

plot(p4, add=T)

Airdrome