Yesterday I spent some time talking with Grant Castle, T-Mobile's VP of Engineering and QA to better understand the way the data in those maps is sourced.

Mr. Castle took my pointed criticism in stride; he's accustomed to it, working as he does for telecom's most outrageous, salty CEO. Mr. Castle admitted up front that the engineering models that his company and other cellular carriers use to determine coverage maps aren't perfect, but says that T-Mobile's goal really is to improve accuracy by incorporating customer data into the matrix as well. The problem with coverage maps is that they just don't tell the whole story. It turns out that when you install the T-Mobile My Account app on your phone, you're given the option of sending diagnostic info (anonymously) to T-Mobile. Castle explains that the app occasionally collects signal strength measurements, which are then transferred to T-Mobile. The app also distinguishes what type of technology was used during that signal strength measurement (LTE versus Wi-Fi Calling, for example). Get an iPhone SE with Mint Mobile service for $30/mo It's that data which T-Mobile is incorporating into the new maps. Anywhere you see a hexagon on the map is a confirmed data point derived directly from that sort of information. Not every bit of customer-derived data is incorporated into the maps, according to Castle. T-Mobile waits until it has what Castle calls "a statistically valid number of samples" before incorporating it into the maps. The problem with coverage maps is that they just don't tell the whole story. Just because an area may have 4G LTE coverage doesn't necessarily mean you can do anything with that coverage. I often see one, even two bars of 4G LTE on my iPhone 6 (connected to T-Mobile's network), but I'll still time out when instant messaging or trying to connect to the Internet. Here's a practical example: I get one bar of 4G LTE coverage from the parking lot of my wife's work, also in the same town we live in. We often share a car, so I'll pick her up at the end of the day and try to let her know I'm there by texting or calling her. Her work, and the commute home, is in the dark magenta coverage area we see on this new T-Mobile map: