This is a hell of an idea and very important. I have a question though - by enabling location sharing on device, is the location not being leaked to Apple and Google regardless?
Either way, awesome idea and love to see what you’re doing.
My current understanding is that location data is not leaked to Google or Apple by just enabling location services (I'm always happy to be proven wrong :-) ).
In the case of Google/Android, they make it very easy to unknowingly opt-in to sharing your data with them, but it's not too hard to double check that and disable+delete the data it if was on [1].
I know there has been much news about Google providing police with a list of devices near the time and location of a crime, and I believe that data is coming from the Location History feature of Google accounts. But that's something that can be turned off.
Apple more explicitly requests the data via app permissions on your iPhone, so it basically comes down to what Apple apps to which you've given location permission [3].
"To use features such as these, you must enable Location Services on your iPhone and give your permission to each app or website before it can use your location data."
"If Location Services is on, your iPhone will periodically send the geo-tagged locations of nearby Wi-Fi hotspots and cell towers (where supported by a device) in an anonymous and encrypted form to Apple"
"By enabling Location Services for your devices, you agree and consent to the transmission, collection, maintenance, processing, and use of your location data and location search queries by Apple and its partners and licensees to provide and improve location-based and road traffic-based products and services."
It is, to get back coordinates you have to use Google's location API, which tells Google where you are. That's why the actual app doesn't matter to me (a privacy advocate) because no matter how private your app is, Google will always have my location.
Nowadays I just keep the GPS off unless I need to use Maps, hopefully that does something.
Either way, awesome idea and love to see what you’re doing.