Hi all,

I'm trying to get the sum of value z for all points that fall within a grid
square.  I've been trying to use the overlay function on a grid converted to
polygons, however, it returns the sum of the coordinates and the z values,
how can I have it return only the sum of z?
Also, once I have the sum of z how then can I assign that to it's
corresponding grid square?

## Example code

g <- GridTopology(c(-10,-10), c(1,1), c(21,21))
g.poly <- as(g, "SpatialPolygons")

x <- c(1:10,1:10)
y <- c(10:1,10:1)
z <- 1:20

xyz <- SpatialPointsDataFrame(cbind(x,y),data.frame(z))

ol <- overlay(xyz, g.poly, fn=sum)


Thanks in advance.

Andrew.



-- 
Andrew Hoskins
PhD Candidate
School of Life and Environmental Sciences
Deakin University, Australia

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