as I am developing for iPhone, I've just bought an iPhone 4 to test my application which needs to measure the coordinates of my location. I don't have any Internet (3GS, GPRS or whatever...) on my iPhone and the problem is:
1) Without internet I get a 1744m horizontal accuracy, and that's very bad. (I've also tested the accuracy in other applications too, and it is always as bad or worse)
2) With WiFi-Internet I get a 80m horizontal accuracy.
Is that normal? What can I do to improve my coordinates-measurement accuracy?
Thanks in advance for any help.
From my experience you need cellular data reception (3G or Edge) to get an accurate location on any iPhone. With that and a clear view of the sky you should be able to get within a few meters of your actual location.
Yes, this is normal. To improve accuracy, you can move somewhere with a clear view of the sky.
First thing I'd advise is make sure you have a clear view of the sky to get a good satellite signal.
I think that it is becoming somewhat "accepted" that the iPhone's GPS accuracy is somewhat lacking (in comparison to other handhelds)... I had to search through my history, but I remembered reading about this very issue on Hacker News - http://news.ycombinator.com/item?id=1526664.
If you don't want to follow the thread, here is the article directly - http://rnr.davidlokshin.com/post/825290568 .
I learned recently that the phone needs to download a batch of coordinate files in order to make any real sense of the GPS data it picks up. So without an internet connection, GPS service is very poor or possibly completely unavailable.
I'd bet your poor accuracy with WiFi is because you were indoors... That's my guess anyway.
来源：https://stackoverflow.com/questions/3515158/iphone-gps-accuracy