[R-sig-Geo] calculate fraction of a polygon that is intersected by another polygon

[satishr]

Robert,
   Thanks for all the detailed information. I am able to work around it with
the help of your email and Roger Bivand's reply from archives. For others
benefit, I am putting the final version code here.
Thanks,
Satish

	rm(list=ls())

	library(maptools);library(raster);library(rgeos);library(rgdal);

	grid=readOGR("C:/test","Gaussian_Poylgon_ExtendedCONUS")

	climdiv <- readOGR("C:/test","US344ClimateDivisions1928Polys")

	# climdiv has several polygons with the same attributes, need to do be
"aggregated" first

	climagdiv <- aggregate(climdiv, 'PopupInfo')

	iclimdiv=256 # say, we are interested in this polygon

	int = gIntersects(climagdiv[iclimdiv,],grid,byid=TRUE)

	vec = vector(mode="list", length=dim(int)[2]) 

	for (i in seq(along=vec)) vec[[i]] =
try(gIntersection(climagdiv[iclimdiv,],grid[int[,i],], byid=TRUE))

	out = do.call("rbind", vec) 

	rn = row.names(out) 

	nrn = do.call("rbind", strsplit(rn, " ")) # nrn[,2] --> grid index

	gridindex=as.numeric(nrn[,2])+1	# gridindex, need to add "1" because row
names start from "0"

	fracgridarea=sapply(slot(out,"polygons"), slot, "area")	# fraction of a
grid in a polygon

# "gridindex" corresponds to grid number whereas  "fractgridarea"
corresponds to fraction of a grid in a polygon/climate division

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