How about this?
require(FNN)
#FOR DEMONSTRATION PURPOSES, GENERATE 2D DATA
set.seed(3242)
X <- matrix(runif(50),ncol=2)
plot(X,pch=16,cex=1.5, asp=1)
#PLOT GRID
grid <- as.matrix(expand.grid(seq(0,1,by=.1),seq(0,1,by=.1)) )
abline(v=unique(grid[,1]),col="#FF000030")
abline(h=unique(grid[,2]),col="#FF000030")
#FIND NEAREST GRID POINT
nearestn <- get.knnx(data=grid,query=X,k=1,algorithm="kd_tree")
#PLOT LINE BETWEEN DATA AND GRID POINT
for(i in 1:nrow(X)){
lines(rbind(X[i,], grid[nearestn$nn.index[i],]), col="red",lwd=2)
}
### IDEA: WRITE FUNCTION THAT OUTPUTS THE SUM OF SQUARED DISTANCES, THEN
MINIMIZE IT
#FUNCTION THAT RETURNS SUM OF SQUARED DISTANCES FOR A GIVEN GRID
one.grid <- function(X,grid){
nearestn <- get.knnx(data=grid,query=X,k=1,algorithm="kd_tree")
dists <- rowSums((grid[nearestn$nn.index,] - X)^2)
return(sum(dists))
}
#FUNCTION THAT CREATES A GRID BASED ON THE OFFSET FROM LOWER-MOST AND
LEFT-MOST POINT
define.grid <- function(X, offset, grid.size){
colmins <- apply(X,2,min)
colmaxs <- apply(X,2,max)
grid <- expand.grid(seq(colmins[1] - offset[1] ,colmaxs[1] + grid.size
,by=grid.size), seq(colmins[2] - offset[2], colmaxs[2] + grid.size,
by=grid.size))
return(grid)
}
#FUNCTION THAT RETURNS THE OPTIMAL GRID (FOUND BY GRID SEARCH, NO PUN
INTENDED)
find.grid <- function(X,grid.size=1){
out <- optim(c(grid.size/2,grid.size/2), fn = function(par){ one.grid(X,
define.grid(X,par, grid.size)) }, lower=c(0,0), upper=c(grid.size,
grid.size), method="L-BFGS-B")
best.grid <- define.grid(X,out$par,grid.size)
return(best.grid)
}
#PLOT OPTIMAL GRID
plot(X,pch=16,cex=1.5, asp=1)
bg <- find.grid(X,.1)
abline(v=unique(bg[,1]),col="#2491B230")
abline(h=unique(bg[,2]),col="#2491B230")
nearestn <- get.knnx(data=bg,query=X,k=1,algorithm="kd_tree")
for(i in 1:nrow(X)){
lines(rbind(X[i,], bg[nearestn$nn.index[i],]), col="#2491B2",lwd=2)
}
-tgs
Thomas G. Stewart <http://www.bios.unc.edu/~tgs>
On Thu, Nov 21, 2013 at 7:47 AM, Gonçalo Ferraz <gferra...@gmail.com> wrote:
> Hi, I have a cloud of randomly distributed points in 2-dimensional space
> and want to set up a grid, with a given grid-cell size, that minimizes the
> distance between my points and the grid nodes. Does anyone know of an R
> function or toolbox that somehow addresses this problem? This is a problem
> of optimizing the location of the grid, not a problem of deciding what
> should be the grid-cell size, because size is already given. Thank you.
> Gonçalo
> ______________________________________________
> R-help@r-project.org mailing list
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> PLEASE do read the posting guide
> http://www.R-project.org/posting-guide.html
> and provide commented, minimal, self-contained, reproducible code.
>
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