1. A computer-readable storage medium storing instructions which, when executed by one or more processors, cause the one or more processors to perform:receiving, at a server, from two or more mobile devices of a plurality of mobile devices, measurements that indicate qualities of network communication signals within one or more regions of a plurality of regions,
wherein receiving the measurements comprises receiving a first measurement from a first mobile device that subscribes to a wireless network of a first wireless carrier and receiving a second measurement from a second mobile device that subscribes to a wireless network of a second wireless carrier that differs from the first wireless carrier; and
based on the measurements, updating, at the server, stored map data that specifies, for each particular region of the plurality of regions, a quality of a network communication signal within that particular region,
wherein the updating the stored map data comprises using the first measurement to update first map data that pertains to the wireless network of the first wireless carrier, and using the second measurement to update second map data that pertains to the wireless network of the second wireless carrier.