######################################
# Set up the points at which to determine the values
######################################
f.create.grid <- function(data, numb.xy.pts, size = {}) {
   if (length(size) == 4) {
      x.min <- size[1]
      x.max <- size[2]
      y.min <- size[3]
      y.max <- size[4]
   } else {
      x.min <- floor(min(data[,1]))
      x.max <- ceiling(max(data[,1]))
      y.min <- floor(min(data[,2]))
      y.max <- ceiling(max(data[,2]))
   }
   xp <- seq(x.min, x.max, (x.max - x.min)/numb.xy.pts[1])
   yp <- seq(y.min, y.max, (y.max - y.min)/numb.xy.pts[2])
   zp <- expand.grid(xp, yp)
   list (xp = xp, yp = yp, zp = zp)
}