Fits a trend surface by least-squares. Syntax: surf.ls(np, x, y, z) Input: np degree of polynomial surface x x coordinates or a data frame with columns `x', `y', `z' y y coordinates z z coordinates. Will supersede `x$z' Output: list with components the coefficients and others for internal use only. ` See also: trmat', `surf.gls' Examples: topo.kr <- surf.ls(2, topo) trsurf <- trmat(topo.kr, 0, 6.5,0, 6.5, 50) contour(trsurf) points(topo)