A robotic work tool system (200) comprising a robotic work tool (100), said robotic work tool (100) comprising a position determining device (190) for determining a current position, such as through receiving satellite signals, and a navigation device (195) arranged to at least provide a compass heading, the robotic work tool (100) being configured to: determine a compass heading (CH) obtained through the navigation device (195)； compare the compass heading to a true heading (TH) obtained through the position determining device (190)； determine an error (e) between the true heading (TH) and the compass heading (CH)； determine a robot position (XR； YR)； and store the error (e) for said robot position (XR； YR) thereby generating a magnetic correction matrix.
