How accurate is the GPS on the iPhone's Find My iPhone app?
I used Find My iPhone app to track a friend. The first 3 stops were dead on but when it refreshed it showed them to be several streets over from actual location. What could cause that?
The generic answer you can find at Apple support website
About location precision or accuracy
Depending on your device and available services, Location Services uses a combination of cellular, Wi-Fi, Bluetooth, and GPS to determine your location. If you're not within a clear line of sight to GPS satellites, your device can determine your location using crowd-sourced Wi-Fi and cell tower locations or iBeacons.
If you want to have specific numbers, in meters, approximately they would be next (according to the 2009 study):
In 2011 Dr. Zandbergen tested several Android smart phones. Here he found the accuracies to be slightly better than the 2009 study. They ranged from 5-8 meters. It is likely that the iPhone 4S/5 is within this range as well. It can also be assumed that iPads and other Android tablets will be comparable.
It's the same accuracy as the GPS accuracy of the device. If the GPS on the iPhone can't obtain a good signal, it may use Wi-Fi triangulation which will reduce the accuracy.
GPS accuracy may be decreased depending on the phone's environment (i.e. a tunnel would not have great GPS, but standing in an open field would).
iPhone fell off my belt riding a motorbike across an enormous field at a model airplane contest. Got a friend with a laptop with a hotspot, linked my iPad to it, and got the location of the phone with the Find My iPhone app. Once we got close, started calling the phone. I’m pretty hard of hearing so the volume was all the way up. Found it in about 15 minutes. I’m very pleased!
I don't know about the positioning between cell towers and satellites, but I was having problems with getting an accurate reading/position of my sons iPhone (1-2 mile variance).
I took off the plastic cell phone case he had on it, and what do you know: it dialed right in to his exact position (1-2 yards). So try changing phone cases to see if it makes a difference, it did in my case.
The Find my iPhone location can be off by up to 600m. In my case, this was because my phone was in a steel and concrete structure (high rise). If you look at your location on a GPS-enabled application (other than Google Maps) and zoom in, you will see your location is distorted.
Location in phones is a combination of:
Even though it might seem like Wifi should be more accurate, GPS is best. However, it is supplemented by the other two when the GPS signal is weak.
Why is GPS better? GPS satellites use an atomic clock to send timestamped signals that are insanely accurate. If your phone is in position to receive 4+ strong satellite signals, most phones GPS should be accurate within 20 feet. A weak signal can quickly reduce that to 100+ feet or even 1,000+ feet!
Now Broadcomm is said to be launching dozen of new satellites with much higher precision but it requires a new chip. The combination of those satellites and that cheap will bring precision to about a 1 foot. Source
However, most of us will be using a more rudimentary GPS chip for a while. In that case, you'll continue to see wider variation, especially when signals are weak.
Things that hurt accuracy:
Not enough strong GPS satellite signals (in a tunnel, under a storm, etc)
Not near wifi
Weak cell signal
Now your phone actually knows how accurate it is. Some apps increase the radius circle to reflect the strength/weakness of the combination of signal strengths. The Apple API tells iOS apps about signal strength using a 1-5 rating.
Important note: Wifi assisted GPS is worth looking up. My understanding is that it's not about networks you join, just networks your phone picks up on. Your phone sends data back to the maps company, reporting locations of wifi networks detected. In a city with hundreds of networks on a single street, that data starts to form a wifi map that works well when the maps app has no/weak GPS.
Your GPS accuracy can change by the second. Satellites are flying around the Earth at thousands of miles per hour, so at any given second one GPS satellite might be falling out of range and (hopefully) another may be coming into range. So you might think you want privacy, but the next time that Google Maps dot is jumping around think about how nice it would be if satellites were actually following you!
Privacy... just a GPS circle you can't live completely in, but can't live completely outside of!
Now the list of variables just keeps going here. Another example would be your phone's battery. If your battery is low, your phone might start powering down non-essential services or checking-in less to save processing power. Background threads are likely the first to be throttled down in low power mode.
Anyways, hope this helps. It's probably just scratching the surface of why your GPS location might seem a bit erratic.
Sometimes it is way, way off. By a mile or two.
GPS doesn't use Satellites. It is all ground based antenna arrays. Cells use mostly cell tower triangulation.