since the sprayers have a configurable spray width I think you could get pretty far with some sort of wide-angle sensor (a few ultrasonic range finders, or mayb

since the sprayers have a configurable spray width I think you could get pretty far with some sort of wide-angle sensor (a few ultrasonic range finders, or maybe a Kinect?) placed at one long-edge end of the lawn and use that to trigger sprayers laid out perpendicular to the camera.