T-Mobile's new coverage map is based on customers' real life experiences
The map reveals exactly where T-Mobile customers will find 4G LTE, 4G and 3G coverage. Certain parts of the map show a "Verified Coverage" icon that points out areas of the U.S. where the majority of the data provided comes from T-Mobile subscribers. These customers are relaying their real-life experience using the mobile operator's wireless service.
The carrier updates the map twice a month and shows the results from "trusted" third party apps that measure the data speed experienced by T-Mobile subscribers. Data from the last 90 days is included in the map.
It's just another innovation from T-Mobile that we probably will see other carriers add to their websites in the near future.
source: T-Mobile via AndroidCentral