grid list {fields}  R Documentation 
The object grid.list refers to a list that contains information for evaluating a function on a 2dimensional 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") fields.x.to.grid(x,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. 
x 

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:
fields.x.to.grid
:
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!
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) make.surface.grid(grid.l) grid.l < list( 1:3, 10, 1:3) make.surface.grid(grid.l) #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. #