Hello, given the set of grey points (corners of squares, see visual)... something like this: r.789695.n4.nabble.com/file/n4363505/Slide2.png how do I calculate the average value of the variable z, that was measured at the corners of the rectangles (represented by the black points in the picture above). #here is some data (dat) dat <- structure(list(z = c(0L, 0L, 0L, -1L, 0L, -1L, 0L, -1L, -1L, + 0L, 0L, 0L, -1L, -1L, 0L, 0L, -1L, -1L, 0L, 0L), x = c(-0.6, + 0, 0, 0, 0.4, 0.6, 1, 1, 1, 2, 2, 2, 2, 2, 3, 3, 3, 3, 3.8, 4 + ), y = c(1, 0, 2, 0.8, 3, 1.6, 0.19, 1, 2.6, 1, 4, 3.7, 2, 3, + 1.6, 3.8, 2.4, 3, 3, 1.9)), .Names = c("z", "x", "y"), class "data.frame", row.names = c(12L, + 1L, 11L, 13L, 10L, 20L, 2L, 14L, 19L, 3L, 8L, 9L, 15L, 18L, 4L, + 7L, 16L, 17L, 6L, 5L)) attach(dat) library(akima) # interpolating to square grid data.interp <- interp(x,y,z) I would like the "black dots" (rectangle corner averages) from data.interp[z] anybody know how? -- View this message in context: r.789695.n4.nabble.com/3d-average-values-tp4363505p4363505.html Sent from the R help mailing list archive at Nabble.com.