

   EEvvaalluuaattee TTrreenndd SSuurrffaaccee oovveerr aa GGrriidd

        trmat(obj, xl, xu, yl, yu, n)

   AArrgguummeennttss::

        obj: object returned by `surf.ls' or `surf.gls'

         xl: limits of the rectangle for grid

         xu:

         yl:

         yu:

          n: use `n' x `n' grid within the rectangle

   DDeessccrriippttiioonn::

        Evaluate trend surface over a grid.

   VVaalluuee::

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

   SSeeee AAllssoo::

        `surf.ls', `surf.gls'

   EExxaammpplleess::

        data(topo)
        topo.kr <- surf.ls(2, topo)
        trsurf <- trmat(topo.kr, 0, 6.5, 0, 6.5, 50)
        contour(trsurf$x, trsurf$y, trsurf$z, levels=seq(700, 925, 25))

