Abstract: A robotic work tool system (200) for defining a working area perimeter (105). The robotic work tool system (200) comprises a robotic work tool (100) and a controller (210). The robotic work tool (100) comprises a position unit (175) and a sensor unit (170). The controller (210) is configured to receive, from the sensor unit (170), edge data indicating whether the robotic work tool (100) is located next to a physical edge (430). The controller (210) is further configured to control the robotic work tool (100) to travel along the physical edge (430) while the edge data indicating that the robotic work tool (100) is located next to the physical edge (430) and to receive, from the position unit (175), position data while the robotic work tool (100) is in motion. The controller (210) is configured to determine, based on the edge data and position data, positions representing the physical edge (430) and to define, based on the determined positions, at least a portion of the working area perimeter (105).