In a roof window with a frame structure and an openable, pane supporting sash structure, the top, bottom and side members of the frame and sash structures are for the major part made as wood profiles, which on the outwards facing sides are covered by covering members of weather-shielding material connected with the wood profiles by means of engagement and securing means. The location of transponders is determined by a location system. The reception buffer time setting unit 208 calculates a reception buffer time using the maximum transmission delay time, a transmission delay time of the data transmission by the reception unit 113, and a prescribed reception buffer time. The acquired plurality of echoes from different echo times is used to create a chemical shift corrected calibration map.