Nick Crouch Nick Crouch - 3 months ago 20
R Question

Counting polygons in shapefile

So I have multiple species ranges which look like the following (colored blue for example) This one runs east to west across Africa:

Example species range

I can get the total area by using

gArea
in the
rgeos
package. What I want to know is how many individual polygons make up this file - i.e. how many distinct regions are there to the total range (this could be islands, or just separated populations) and what the ranges of those polygons are. I have been using the following code:

#Load example shapefile
shp <- readShapeSpatial("species1.shp")

#How many polygon slots are there?
length(shp@polygons)

>2

#How many polygons are in each slot
length(shp@polygons[[1]]@Polygons
length(shp@polygons[[2]]@Polygons


and to get the area of a particular one:

shp@polygons[[1]]@Polygons[[1]]@area


Is this correct? I'm worried that a lake in the middle of the range might constitute a polygon on its own? I want to end up with a list that is roughly like:

Species A Species B
Polygon 1 12 11
Polygon 2 13 10
Polygon 2 14 NA


If I wanted to compile a list for every species of how many polygons and their individual ranges would be pretty straightforward to pass to a loop if the above code is correct.

Thanks

Answer

This is a very un-glamorous solution, but it gets the job done at the moment.

for(i in 1:length(shpfiles)){

shp <- shpfiles[[i]]

#1) Create master list of all polygon files within a shapefile

#How many lists of polygons are there within the shpfile
num.polygon.lists <- length(shp@polygons)


#Get all polygon files
master <- vector(mode="list")
for(i in 1:num.polygon.lists){
m <- shp@polygons[[i]]@Polygons

master[[i]] <- m
}

#Combine polygon files into a single list
len <- length(master)

if(len > 1) {

root <- master[[1]]
for(i in 2:length(master)){
root <- c(master[[i]], root)}

} else {root <- master[[1]]}

#Rename
polygon.files <- root

#2) Count number of polygon files that are not holes

#Create a matrix for the total number of polygon slots that are going to be counted
res <- matrix(NA,ncol=1 , nrow=length(polygon.files))

#Loop through polygons returning whether slot "hole" is TRUE/FALSE
for(i in 1:length(polygon.files)){

r <- isTRUE(polygon.files[[i]]@hole)

res[[i,1]] <- r
}

#Count number times "FALSE" appears - means polygon is not a hole
p.count <- table(res)["FALSE"]
p.count <- as.numeric(p.count)

print(p.count)

}

It's a start