A robot lawnmower (10, 11, 12, 13, 14, 15, 16, 17, 18, 19) includes a drive system (400) carried by a body (100) and configured to maneuver the robot (10, 11, 12, 13, 14, 15, 16, 17, 18, 19) across a lawn (20, 1020). The robot (10, 11, 12, 13, 14, 15, 16, 17, 18, 19) includes a grass cutter (200, 200A, 200B, 2000A, 2050, 2100, 2200) carried by the body (100) and a swath edge detector (310A, 310B, 310C, 310D, 310E, 310F, 310G, 310H, 310I) carried by the body (100) and configured to detect a swath edge (26) between cut grass (24) and uncut grass (22) while the drive system (400) maneuvers the robot (10, 11, 12, 13, 14, 15, 16, 17, 18, 19) across the lawn (20, 1020) while following a detected swath edge (26). The swath edge detector (310, 310A, 310B, 310C, 310D, 310E, 310F, 310G, 310H, 310I) includes a calibrator (320, 320A, 320B) that monitors uncut grass (22) adjacent the swath edge (26) for calibration of the swath edge detector (310, 310A, 310B, 310C, 310D, 310E, 310F, 310G, 310H, 310I).
