How Accurate is Cell Phone Location Data?

It depends.  Unless you’re still there all it can do is place you within a wide area of the cell tower location. Even that depends on whoever is trying to locate you having an exact location of the cell tower, i.e., exact GPS coordinates.  They would also need to know the size of the area that the cell tower covers because you could be anywhere within that area.

Elementary plane geometry from the 10th grade is what’s involved in knowing your exact location within that area, and so we know that to pinpoint your exact location it’s necessary for 3 different towers to be accessing your phone.  Then by triangulation it may be possible to determine the exact point where you are.  But only if you sit tight will triangulation produce any relevant data. The minute you start moving a track line may be established by yet more sophisticated data gathering.  If that is not begun before you’re gone from the area, no data will ever show your exact location at any particular time.  It will only be possible to show you were somewhere within a wide area during a particular time period, and the size of the area will depend on how large an area is covered by the cell tower you were hitting on.  If you were hitting on a tower in the middle of the Kansas prairie the area could be hundred’s of square miles.

if you’ve been watching a lot of cop shows where cell tower data is obtained that purports to pinpoint a suspect’s exact location at a particular time of day several days or weeks ago, you’ve been hoodwinked because that’s not possible. This false impression has been used to convict innocent people of crimes, according to the Economist:

SOMEONE strangled a prostitute in Portland, Oregon in 2002. The police arrested Lisa Roberts, the victim’s ex-lover, who spent more than two years in custody awaiting trial. Shortly before the trial the prosecutor told Ms Roberts, via her lawyer, that tower data collected by Verizon, her mobile-telephone network, showed precisely where she was at the time of the murder. As her lawyer recalled, the prosecutor said Ms Roberts could be “pinpointed” in a park shortly before the victim’s naked and sexually assaulted corpse was found there. She was told she faced 25 years to life in prison. She accepted a deal to plead guilty and serve 15 years.

But the high-tech evidence against her was bunk. Routinely collected tower data can place a mobile phone in a broad area, but it cannot “pinpoint” it. That would require a special three-tower “triangulation”, which cannot reveal past locations. It took a decade for Ms Roberts’s guilty plea to be thrown out. On May 28th she left prison, her criminal record clean, after nearly 12 years in custody.

I don’t know why she accepted the plea deal, though.  Doesn’t seem an innocent person would do that.  See also, “Turns Out Cell Phone Location Data Is Not Even Close To Accurate, But Everyone Falls For It,” at techdirt.


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