do local police have access to this data though? Maybe I've missed something, but the cell phone data has been through PRISM, and that local police doesn't have access to this sort of information.
https://www.aclu.org/national-security/police-requests-cellp...
The type of surveillance NSA can do is quickly trickling down to FBI, DHS, DEA and even the police. Even this wasn't too long ago:
http://news.cnet.com/8301-1023_3-57471570-93/facebook-scans-...
Some police departments are also working on face detection systems, or even "pre-crime" in New York:
http://www.activistpost.com/2012/08/microsoft-and-bloomberg-...
http://www.thenewamerican.com/usnews/item/16942-dhs-creates-...
http://www.democracynow.org/blog/2013/5/22/dissent_or_terror...
http://www.csmonitor.com/USA/Latest-News-Wires/2013/0718/Lic...
And these are just examples that I remembered off the top of my head, and did some quick searches on them now.
Not being involved in this myself I don't know the extent or the checks in place but police do have access to cell phone location data since about ten years ago? (if you call 911 on a cell phone they use it to find you)
Local police have access to license plate scanners (both at fixed locations and mounted on police vehicles). The data is not as fine-grained as GPS data from a person's phone, but it can also yield information about the patterns of a person's movements over time (e.g., they pass these ten intersections every weekend night between 2:30 and 3:00am). Of course, this can only track people with cars (but in some cities, that's pretty much everyone).
Facial recognition software can track people who walk, but I don't know how widely deployed it is at this time. As soon as it gets attached to all of the millions of surveillance cameras that are out there (some of which are operated by police departments, like in NYC), tracking pedestrians will become easier.
And, of course, the local police can get a warrant to attach a GPS device to somebody's car or to pull cell tower data if they have probable cause.
That's hardly an argument. For one, they collaborate frequently with the FBI. As for local police, well, give it time.
Yet?