Function to create a biorthogonal grid using the thinplate solution;
the grid object created can be used with the draw
function to plot the grid.
1 
start 
coordinates of the starting point to create the grid  usually near the center of region 
xmat 
n x 2 matrix containing xy coordinates of locations 
coef 
coefficients of the thinplate solution, for instance as returned by

xlimt 
vector of 4 elements, representing x and yranges for the grid to
be created; if not provided, the ranges of 
iter.limit 
limit of iterations 
perpc 
parameter to control spacing of the grid 
A list with following named components:
grid 
coordinates of points in the grid 
ngrid 
number of points in the grid 
fldmag 
the “gradient” index (see Sampson and Guttorp, 1992, for details) 
nn 
internally set for the maximum number of grid points allowed 
Sampson P.D., and Guttorp, P., "Nonparametric estimation of nonstationary spatial covariance structure", in Journal of the American Statistical Association, vol 87, pp 108119, 1992.
draw
, sinterp
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