overlay spatialgrid on spatial polygons data frame

On Thu, 20 May 2010, Agustin Lobo wrote:

I have an SpPolDF and a coarse SpatialGrid, i.e.,
srdf=SpatialPolygonsDataFrame(sr, data.frame(cbind(1:3,5:3),
row.names=c("r1","r2","r3")))
#the one used in help(overlay);

srdf at data

?X1 X2
r1 ?1 ?5
r2 ?2 ?4
r3 ?3 ?3

bbox(srdf)

? ?min ? ?max
x 178544 181477
y 329665 333676

test <- SpatialGrid(GridTopology(cellcentre.offset=c(178440,329600),
cellsize=c(500,500), cells.dim=c(8,10)))
plot(srdf)
plot(test,add=T)

My goal is to "overlay" test over srdf and tranfer the data in
srdf at data to a data frame associated
to test (=> test must become a SpatialGridDF) so that each cell has
the average value of the overlaid srdf at data polygon(s) weighted
by area: in case 25% of a given cell in test would overlay polygon r1
and 75% polygon r2, the value of X1 for that cell would be
0.25*1 + 0.75*2

This involves a topology operation on two sets of polygons, and is not
supported. You can approximate it by creating a finer raster and using that
subsequently aggregate, for example using the polygons in ?overlay to form
srdf, then:

test2 <- SpatialGrid(GridTopology(cellcentre.offset=c(178440,329600),
?cellsize=c(50,50), cells.dim=c(80,100)))
o <- overlay(test2, srdf)
o1 <- SpatialPointsDataFrame(coordinates(test2), data=data.frame(o=o))
o2 <- o1[!is.na(o1$o),]
o2$X1 <- srdf$X1[o2$o]
o2$X2 <- srdf$X2[o2$o]
u <- overlay(test, o2)
uu <- aggregate(slot(o2, "data"), list(u), mean)
X1 <- rep(as.numeric(NA), nrow(coordinates(test)))
X2 <- rep(as.numeric(NA), nrow(coordinates(test)))
X1[uu$Group.1] <- uu$X1
X2[uu$Group.1] <- uu$X2
testdf <-
SpatialGridDataFrame(GridTopology(cellcentre.offset=c(178440,329600),
cellsize=c(500,500), cells.dim=c(8,10)), data=data.frame(X1, X2))
spl <- list("sp.lines", as(srdf, "SpatialLines"), col="grey30")
spplot(testdf, c("X1", "X2"), col.regions=bpy.colors(20), sp.layout=spl)

The finer resolution should be fine enough to capture the relative areas of
the polygons adequately.

Roger

According to help(overlay),
"Value
a numerical array of indices of x on locations of y, or a data.frame
with (possibly aggregate) properties of x in units of y"

I've tried this:

overlay(srdf, test, fn=mean, na.rm)

? ?X1 X2
NA ? NA NA
NA.1 NA NA

Cannot go beyond, perhaps overlay() is not suited for this: the
definition of overlay in its help page
"overlay combines points (or grids) and polygons by performing
point-in-polygon operation on all point-polygons combinations"
is a bit confusing, the first part of the sentence gives me some hope
but the second one seems to exclude what I'm trying to do.

Any help appreciated (or perhaps pointing to more doc?)


Agus

--
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no