Evaluate Kriging surface over a grid.

Syntax:
         prmat(obj, xl, xu, yl, yu, n)

Input:
obj           object returned by `surf.gls'
xl,xu,yl,yu   imits of the rectangle for grid
n             use `n' x `n' grid within the rectangle

Output:
list with components `x', `y' and `z' suitable for `contour'
and   `image'.   

Se also:
`surf.gls',  `trmat',  `semat'  

Example:
topo.kr  <-surf.gls(2, expcov, topo, d=0.7) 
prsurf <- prmat(topo.kr, 0,6.5, 0, 6.5, 50) 
contour(prsurf, levels=seq(700, 925, 25))