Assign polygon duplicate area (overlap) to only one polygon
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.
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)