Find which grid cells contain a point

I have a dataframe with x,y coordinates. I've sampled 10 x,y coordinates from my dataframe:

y <-c(24.637032,21.569813,21.687,19.677183,24.72438,21.433136,24.637032,22.21065,21.443762,25.273882)

x <-c(-156.02597,-112.00287,-112.00287,-105.27803,-159.47324,-157.858,-156.026,-109.68626,-112.12681,-112.15393)

I've converted these points to a SpatialPoints objects:

library(sp)
myPoints <- data.frame(x,y)
coordinates(myPoints) <- c('x', 'y')
proj4string(myPoints) <- CRS("+proj=longlat +ellps=WGS84 +datum=WGS84")

I've used these points to make a SpatialGridobject:

bb <- bbox(myPoints)
myPointsGrid <- GridTopology(cellcentre.offset =  bb[, 1] + (cs/2), cellsize = c(1, 1), cells.dim = ceiling(diff(t(bb))/cs))
myPointsGrid <- SpatialGrid(myPointsGrid, proj4string = "+proj=longlat +ellps=WGS84 +datum=WGS84")

I need two things 1. find which grid cells contain a point 2. Find the number of points in each grid cell. I tried...

over(myPointsGrid, myPoints)

...but this has thrown an error. How can I complete this task?

• What kind of error do you get? And waht is 'cs' exactly? Cause when I run your code I get an error there. Feb 21 '13 at 12:51

Assuming that your code is correct (I have given the value 1 for the cs variable), the error it gives is:

Error in function (classes, fdef, mtable)  :  unable to find an inherited method for function ‘over’ for signature ‘"SpatialGrid", "SpatialPoints"’

According to the help file, this is what the function is waiting as input:

x: geometry (locations) of the queries
y: layer from which the geometries or attributes are queried

It means that you are giving the wrong order of inputs, first is the point location followed by the layer. This is what you want:

> over(myPoints,myPointsGrid)
  59 268 213 330   1 222  59 215 268  48

with cs=1