[R-sig-Geo] Counting overlapping polygons in a given area

[Roger Bivand]

On Thu, 17 Apr 2014, Andrew Bernj wrote:

> Hi,
>
> I am used to work with ArcGIS but I am new to R spatial functionalities.
> So, I have been searching in the archives of this list, but did not find a
> previous question exaclty on what I have to accomplish.

On lists in this community, it is usual to follow the instructions, in 
particular to provide a toy example illustrating your problem. If you 
don't, nobody knows whether the suggested resolution matches your problem. 
You also posted HTML-mail; please use plain text only, so that included 
code may be pasted straight into an R console.

>
> It is the following. I have two shapefiles, one with US city and town
> divisions, where each polygon is a city/town (UScities.shp) and another
> with some US religious groups, where polygons represent the area where
> there are significant followers of a given religion (USrelig.shp).
>
> So, in UScities.shp of course the polygons do not overlap. While in
> USrelig.shp polygons do overlap, since followers from different religions
> can be present in the same place.

library(sp)
library(rgdal)
dsn <- system.file("vectors", package = "rgdal")[1]
scot_BNG <- readOGR(dsn=dsn, layer="scot_BNG")
plot(scot_BNG, axes=TRUE)

gives something like UScities.shp.

set.seed(1)
pts <- spsample(scot_BNG, n=200, type="random")
plot(pts, add=TRUE, col="red")
library(rgeos)
polys <- gBuffer(pts, width=50000, byid=TRUE)
plot(polys, add=TRUE, border="red")

gives something like USrelig.shp.

library(rgeos)
gO <- gOverlaps(scot_BNG, polys, byid=c(TRUE, TRUE))
dim(gO)

makes a logical matrix with TRUE where scot_BNG[i,] overlaps with 
polys[j].

count_by_city <- apply(gO, 2, sum)

The output vector is named using row.names(scot_BNG), so if you need 
recognizable names in the output, change the input row.names to something 
informative.

This is OK if the number of cities times the number of religions is 
moderate. If the product is large, try the development version of rgeos 
from:

https://r-forge.r-project.org/R/?group_id=602

and use:

gO <- gOverlaps(scot_BNG, polys, byid=c(TRUE, TRUE), returnDense=FALSE)
length(gO)
count_by_city1 <- sapply(gO, length)

and in this case we see that they are equivalent:

all.equal(count_by_city, count_by_city1)

Hope this helps,

Roger

>
> What I want do is to count how many religious groups according to
> USrelig.shp there are in each city present in UScities.shp. It means, to
> count how many polygons overlap in USrelig.shp for each polygon division
> given by UScities.shp.
>
>
> Is there a way to do that in R? Thanks in advance for any tip you may have
> on this task.
>
> Bernj
>
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>
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; fax +47 55 95 91 00
e-mail: Roger.Bivand at nhh.no