There is a much more detailed post about this here: http://caseorganic.com/e9

I started carrying around a GPS with me starting on 12/28/2009. With the exception of Japan, I’ve been logging pretty much everywhere I’ve been. My friend Aaron Parecki has been logging GPS data for over two years.

GPS is useful for a number of things. For instance, proximal messaging reduces the need for outgoing text messages. Instead of needing to send a text message like "I'm late!" or "stuck in traffic", I can simply look at Aaron's GPS any time to see if he'll be late for a meeting, if he's having trouble finding a parking spot, or if he's left the office.

But that method of co-location negotiation still requires user action. In an effort to reduce that, Parecki set up what has proven to be my favorite part of the entire system: proximity notification.

Now, instead of having to look at Parecki's GPS map, the system detects when we are within a certain distance of each other. I usually know when Parecki is near when I get a text message that says "you are 0.4 miles from aaronpk". When I get a message that says "you are 0.1 miles from aaronpk" I know that he's arrived, and I can go meet him.

This structure reduces the need for two common co-location drags: the message "on my way", and the message "here". Both messages require user action and imputs. These actions can be costly, especially when struggling to split concentration between driving and texting, or the sheer inability to text while on bike.
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Aaron Parecki developed this. You can learn more about his GPS experiments here: aaronparecki.com/GPS_Visualization
And he can be followed on Twitter at: twitter.com/aaronpk