Evaluate trend surface over a grid. Syntax: trmat(obj, xl, xu, yl, yu, n) Input: obj object returned by `surf.ls' or `surf.gls' xl,xu,yl,yu limits 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.ls', `surf.gls' Example: topo.kr <- surf.ls(2,topo) trsurf <- trmat(topo.kr, 0, 6.5, 0, 6.5, 50)