The fifth decimal place is worth up to 1.1 m: it distinguish trees from each other. Accuracy to this level with commercial GPS units can only be achieved with differential correction.
The sixth decimal place is worth up to 0.11 m: you can use this for laying out structures in detail, for designing landscapes, building roads. It should be more than good enough for tracking movements of glaciers and rivers. This can be achieved by taking painstaking measures with GPS, such as differentially corrected GPS.
If a cellphone GPS system can't get a specific read on a device, the system will return a general location. For Sprint users in North Las Vegas, the general location returned is the home of Wayne Dobson, and over the last couple years, several people have knocked on his door looking for their phone with the Find My Phone feature. These situations are generally diffused when Dobson calls the police, and he's even invited people in to wait for the police to show up.
It's a more serious issue, though, because 911 dispatchers have also sent police to his house for domestic disputes when an address isn't given and his address shows up via GPS. Chilling to think of what would have happened to Dobson if he had confronted officers with a gun.
About two weeks later he was awakened at 4 a.m. by a person prowling along the side of his house. Dobson followed a flashlight beam to his bathroom window. When he looked out, the person flashed the light in his face.
"I screamed at him, 'Who are you? Get out of my yard!'?" Dobson said. "And he said, 'We're the police, open the door.'?"
North Las Vegas cops had received a 911 call from a woman on a cellphone who was arguing with a man. The argument was escalating, but dispatchers weren't able to get a location from the woman.
They looked at the location of the phone and sent officers, who arrived minutes later at Dobson's house. He was taken outside to his front yard and searched. When officers realized the mistake, they apologized.
Dobson said he is grateful that he didn't confront the officers with a weapon.
"I would have been on the losing end, and it would have been because of that issue," he said.
When we were up in Vermont earlier this month, we rode the single chair to the top of the mountain at Mad River Glen and then hiked down. Before we left, we installed iTrail on Meg's phone. iTrail uses the iPhone's GPS capability to track your progress along a trail, jogging path, etc. The reviews at the iTunes Store aren't glowing but we found that it worked pretty well for us. Here are a couple of graphs generated by iTrail of our hike:
iTrail also allows data export to a Google Docs speadsheet. From there, you can import that data into Google Maps, like so:
It's not perfect (we weren't doing 8.2 mph at the beginning of the hike) and GPS mapping apps are hardly new, but I've never done this before and it feels like living in the future.