Use your phone as a GPS logger, for example in order to geotag photos.
This app turns on GPS and records your location (once per second) to a log file in standard GPX format.
Although you can use it for other things, such as recording the route when you take a walk, the main use of these files is to add location data to photos that you have taken with a separate camera that doesn't have a built-in GPS.
It works as follows:
1. Use this app to record a GPX file while you take photos.
2. Send the GPX file to your home computer. You can use the built-in 'share' function to send it by email or to Google Drive or similar services, or you can connect the phone to your computer via USB and transfer the file manually.
3. Using suitable software on your PC, such as Adobe Lightroom, load the GPX file and also the photos that you took. Lightroom can then geotag (add location data to) all the photos based on the time they were taken and the corresponding location from the GPX file.
This app is free, contains no ads, and does not require internet access. There is an option to leave a tip if you like the software (via the in-app purchase mechanism), but this is entirely optional - all functions are available for free.
The GPX position log files are kept entirely on your phone and are not sent anywhere (unless you send it yourself)! This app won't violate your privacy.
While logging your location, a clear notification icon is displayed to indicate that logging is ongoing.
* Although the app makes an effort to filter out bad data, it does entirely rely on your phone's GPS. The recorded logs may be inaccurate if your phone's GPS is inaccurate.
* While logging, your phone is not able to enter full sleep mode (it has to keep recording your location). As a result the battery will drain faster than usual. Before using this app for a long period of time, try for an hour or so to get an idea of battery usage.
Neither of these are under the app's control, so please be aware of them now. That said, for me it works fine and I'm using this personally to geotag all my photos.