A method is described for expanding the radio-map of a WiFi system. A hotspot (HS4) of which the position is unknown and to be estimated is “seen” in a number of fingerprints (FP1, FP2, FP3), each taken at a different position. Several hotspots with a known position (HS1, HS2 and HS3) are also “seen” in the fingerprints (FP1, FP2, FP3). The geo-positions of the measurement locations, where the fingerprints (FP1, FP2, FP3) were taken, are estimated based on the signals from the hotspots (HS1, HS2, HS3) with known geo-position seen in the fingerprints and the known geo-positions of those hotspots. The position of the hotspot (HS4) is determined based on the signals from that hotspot (HS4) seen in the fingerprints and the geo-positions of the measurement locations where the fingerprints were taken, estimated in the previous step.