An apparatus obtains results of measurements of a mobile device on radio signals transmitted by a radio transmitter at a site. The results include at least an identifier of the radio transmitter and an indication of a received signal strength of the radio signals. A plurality of radio transmitters are distributed at the site. The apparatus compares the received signal strength with a threshold. If the received signal strength exceeds the threshold, the apparatus causes a notification of a user of the mobile device, obtains an indication of a location based on a user input identifying a location on a map of the site, the map presented on a display of the mobile device, and causes storage of the identifier of the radio transmitter and the indication of the location as approximate location of the radio transmitter.