"fields.x.to.grid" <-
function (x,nx=80, ny=80, xy=c(1,2))
{
      M<- ncol( x)
      grid.list<- as.list( 1:M)

# add columns names 
  names(grid.list)<- dimnames( x)[[2]]

#     cruise through x dimensions and find medians.
     for( k in 1:M){
          grid.list[[k]]<- median( x[,k]) }
# 
#
# overwrite with sequences for the two variables of surface  
     xr<- range( x[,xy[1]])
     yr<- range( x[,xy[2]])
     grid.list[[xy[1]]]<- seq( xr[1], xr[2],,nx)
     grid.list[[xy[2]]]<- seq( yr[1], yr[2],,ny)
grid.list
}