grid list {fields}R Documentation

Grid list format for describing equally spaced grids


The object grid.list refers to a list that contains information for evaluating a function on a 2-dimensional grid of points. If a function has more than two independent variables then one also needs to specify the constant levels for the variables that are not being varied. This format is used in several places in fields for functions that evaluate function estimates and plot surfaces.


parse.grid.list( grid.list, order.variables="xy"),nx=80, ny=80, xy=c(1,2))


grid.list No surprises here – a grid list!
order.variables If "xy" the x variable will be subsequently plotted as the horizontal variable. If "yx" the x variable will be on the vertical axis.
nx Number of grid points for x variable.
ny Number of grid points for y variable.
xy The column positions that locate the x and y variables for the grid.


The form of a grid.list is list( var.name1= what1 , var.name2=what2 , ... var.nameN=what3) Here var.names are the names of the independent variables. The what options describe what should be done with this variable when generating the grid. These should either an increasing sequence of points or a single vaules. Obviously there should be only be two variables with sequences to define a grid for a surface. Most of time the gridding sequences are equally spaced and are easily generated using the seq function.

The variables in the list components are assumed to be in the same order as they appear in the data matrix.

Some fields internal functions that support interpreting grid list format are: Takes an "x" matrix of locations or independent variables and creates a reasonable grid list. This is used to evaluate predicted surfaces when a grid list is not explicited given to predict.surface. The variables (i.e. columns of x) that are not part of the grid are set to the median values. The x grid values are nx equally spaced points in the range x[, xy[1]]. The y grid values are ny equally spaced points in the range x[, xy[2]].

parse.grid.list: Takes a grid list and returns the information in a more expanded list form that is easy to use. This is used, for example, by predict.surface to figure out what to do!

See Also

as.surface, predict.surface, plot.surface, surface, expand.grid


#Given below are some examples of grid.list objects and the results  
#when they are used with make.surface.grid. Note that  
#make.surface.grid returns a matrix that retains the grid.list  
#information as an attribute. 

grid.l<- list( 1:3, 2:5) 

grid.l <- list( 1:3, 10, 1:3) 

#The next  example shows how the grid.list can be used to  
#control surface plotting and evaluation of an estimated function. 
# first create a test function  

set.seed( 124)

X<- 2*cbind( runif(30), runif(30), runif(30)) -1
dimnames( X)<- list(NULL, c("X1","X2","X3")) 
y<- X[,1]**2 + X[,2]**2 + exp(X[,3])   

# fit an  interpolating thin plate spline  
out<- Tps( X,y) 

grid.l<- list( X1= seq( 0,1,,20), X2=.5, X3=seq(0,1,,25)) 
surface( out, grid.list=grid.l) 
#  surface plot based on a 20X25 grid in X1 an X3  
#                       over the square [0,2] and [0,2]   
#                       holding X2 equal to 1.0. 

[Package fields version 3.3.1 Index]