Question

I’m looking for some feedback, blog links, etc that offer some tips and tricks for iPhone GPS development. I’ve read and understand the API, I have my demo app up and running, etc but I have run into some ‘quirks’ with gps on iphone. For example, it seems that you you will receive a lot less location updates when your iphone enters ‘power save’ mode. I’ve read that you can combat this by playing music in your application [not sure if this is true].

It also seems that, terms of making a reasonable pedometer, you should filter out any reading with horizontal accuracies less than 20 meters. Does this sound right?

Is there any kind of checklist out on the web for iphone gps gotchas?

Thanks in advance.

Answer 1

I don’t know of any specific list, but here are a few of things you should keep in mind:

• You’re not doing GPS lookups. You’re doing Core Location lookups. Core Location might or might not be getting its information from GPS. Lots of people still use pre-3g iPhones that don’t have GPS, and even on a 3g phone GPS may be unavailable in many cases (if the person is indoors, for example). In those cases the phone will attempt to triangulate based on cell phone towers, and Core Location will return the result. It’ll be much less precise than GPS.
• Core Location will cache data. The first reading it provides to your app is likely to be an old reading, which might or might not be accurate, depending on whether the phone has moved. Make sure to check the timestamp of any location and see if it’s from before your app started.
• I don’t know what you mean by ‘power save’ mode, but if you’re thinking of when the screen is locked/off, that does not stop Core Location from running if your app is still running. On the contrary it’s easy to run down your phone’s battery much more quickly than you’d expect if you lock the phone while an app that uses Core Location is running, because the phone will continue to update the app as new location data is available. You could avoid this in your application by listening for UIApplicationWillResignActiveNotification to detect the screen locking, and UIApplicationDidBecomeActiveNotification to detect unlock.
• Higher precision results will take more time to acquire, because the longer you wait, the better the results, up to a limit. If you decide you need to be within N meters, consider how long the user might have to stand there waiting for the phone to home in on its location.

Whether 20 meters is accurate enough is something only you can answer, based on how you expect your application to be used. Test the app and see if it works the way you want it to work.