Google's got a big map
Along with Verizon, ATt&T, any anyone else who hosts large webs of hot spots.
Both cell providers and large corporations.
And it is updated all the time.
Therefor the NSA has a big map. And they didn't even have to pay for it, at least not much. Just the tech fee for the data handover.
They may not know the exact location, since the ongoing signal strength data isn't continuous, but mere blips can pretty much track all of us due to the number of sensors pretty much anywhere, which then can be used to find the center point during most time periods.
