# cell phones and compass apps and dropping the phone... **URL:** \ **Category:** Factual Questions\ **Created:** [October 14, 2017, 1:51pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816 "2017-10-14T13:51:28Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [October 14, 2017, 1:51pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816/1 "2017-10-14T13:51:28Z")
I think that cell phones must use GPS for the data that informs compass apps. Otherwise, they’d all have a compass built in and that would require a small magnet, too, wouldn’t it? So, my question is this: my compass app (and others that I’ve tried) no longer work. When I turn the phone, the whole compass rose turns with the phone. Wrong! It should remain oriented while the phone turns underneath it. Yet the map app functions fine, and directions work fine with it. I dropped my phone a few weeks ago. Could I have jiggled something loose? How does this thing work? ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 14, 2017, 2:05pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816/2 "2017-10-14T14:05:49Z")
I think they do actually sense the magnetic field of the Earth, but they don’t do it using a moving permanent magnet: Instead, they use the Hall effect, or some other electronic component. And that component can be broken, just like anything else, in which case no app using it will work. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [October 14, 2017, 3:41pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816/3 "2017-10-14T15:41:11Z")
Look for an app called Physics Toolkit. It lets you test and play around with every sensor type on your phone. If the magnetic field detectors are down, it will tell you. (But it won’t tell you which door to choose.) ---
**Author:** ![beowulff](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/beowulff/32/542_2.png) [@beowulff](https://boards.straightdope.com/u/beowulff)\ **Post date:** [October 14, 2017, 5:58pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816/4 "2017-10-14T17:58:03Z")
Yes, the phones use some sort of magnetometer. If you have a magnetic clasp on your phone case, it will mess up the compass. Even trying to take an accurate reading near a car can be a problem. The compass in my iPhone is pretty darn accurate. I used it to align my equatorial tracker for the Eclipse, and just pointing the tracker in the direction that the phone said was North resulted in over 2 hours of in-frame sun tracking. Not all phones have the sensor aligned as well. ---
**Author:** ![CC](https://avatars.discourse-cdn.com/v4/letter/c/f1d935/32.png) [@CC](https://boards.straightdope.com/u/CC)\ **Post date:** [October 14, 2017, 7:32pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816/5 "2017-10-14T19:32:52Z")
> [@Darren\_Garrison](#): > > Look for an app called Physics Toolkit… Yeah, thanks, but that’s another part of the problem. I have (but not for long!) a Windows phone. It was free and worth every penny. Among the many problems inherent with this device, almost no one makes apps for it, so that app you suggest and most others are unavailable. Soon, Mrs. CC will be getting a new iphone and I will be inheriting hers. Soon!!! ---
**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\ **Post date:** [October 14, 2017, 7:32pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816/6 "2017-10-14T19:32:58Z")
I think some of them might also use the GPS position to look up the local offset between magnetic and true north, and correct for that. ---
**Author:** ![Darren\_Garrison](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/darren_garrison/32/92_2.png) [@Darren\_Garrison](https://boards.straightdope.com/u/Darren_Garrison)\ **Post date:** [October 14, 2017, 7:43pm UTC](https://boards.straightdope.com/t/cell-phones-and-compass-apps-and-dropping-the-phone/798816/7 "2017-10-14T19:43:09Z")
I use an app called Mobile Observatory where you aim the phone at any point in the sky and it tells you which star/planet/ect. you are looking at. You either let it use the GPS to find your location or input it manually. After that, as long as you have the compass calibrated it does a surprisingly good job of tracking through 6 degrees of freedom with no input from GPS just using built-in sensors.