I am writing this because I am curious about how developers find a GPS fix in iOS - do they have to use a standard Apple algorithm or are developers free to create their own? The reason why I ask is that I use a GPS map program to keep track of how much I walk for health & fitness. Yesterday I took a walk in rural Scotland and lost the GPS fix in a forest. However, once I had reached a clearing, the GPS was still searching for a fix. My location was shown as being at a phone mast nearly a mile away, and the circle indicating the uncertainty in my possible position did not include my actual position. Twenty minutes later the GPS was still searching. I put my phone into airplane mode (eliminating the phone mast as a potential position) and restarted my iPhone. Within 2 minutes or so the GOS fixed my location accurately. I presume whatever algorithm the app was using tried the phone mast location as the initial seed and then starting searching within the circle surrounding the mast for a location compatible with the GPS signals being received. I found all of this surprising: the a priori probability of teleporting a mile to a phone mast when I had been walking on a steady heading at 3mph is pretty much zero. I was getting ready to blast the app for this issue, but realized that the it might arise from Apple's GPS routines. I just want to be fair in my review of the app, so if you know whether Apple constrains the GPS routines, then please post that fact here.