A laser is pivotally attached to a line striping machine. The laser projects a planar line outwardly from the line striping machine. The laser freely rotates about a longitudinal axis, under the influence of gravity, relative to the line striping machine, independent of the orientation of the line striping machine, so as to project a vertically planar line regardless of the vertical orientation of the line striping machine. The subject technology compensates for changes in surface contour by providing a rotating laser that projects a vertically compensated projection plane that causes a vertically compensated reference line projected upon the surface.