TPE on iOS supports offline use, with some limitations. To get the most out of the app when offline, you should follow a few guidelines:
- Save the locations you will need to use before going offline. This ensures that TPE has stored the timezone and elevation above sea level (these are required to provide the most accurate data).
- Select one of the offline map options (OpenCycleMap Topographic, OpenStreetMap – these options can be selected via the map type controls on the map, but you will need to have Google Maps set as the selected map technology in the app’s Settings screen)
- Pan and zoom the map to cover the areas you will need to view when offline. Doing this will automatically save the map tiles to your device for use when offline.
If you are offline and didn’t view the required maps areas in advance, it’s possible the map image tiles will still be in your device’s local cache, but you cannot rely on it. Use one of the offline map types, and view the map tiles you need in advance, for reliable map access when offline.
Any map tiles you view using Open Street Map or Open Cycle Map are automatically saved to your device for one year, subject to available storage. On iOS, the tiles are saved as data to the application folder. They are not saved to a cache folder, as the cache folder can be cleaned automatically by the operating system when your device runs low on storage. (Note, TPE for Android works differently - map tiles are saved to cache.)
Note: map tiles cannot be loaded into the app from external sources or other apps.
Warning: do not rely on offline maps for navigation
You should not rely on offline maps being available in the app for navigation purposes. These maps may be unavailable if your device runs low on storage space, and are deleted automatically after one year. Ensure you have a reliable, accurate map with you at all times for navigation purposes.
Time zone and elevation while offline
TPE will look up the local time zone for wherever you are looking on the planet, so you don’t have to second guess. However, it needs an internet connection to do that. If there isn’t one available, you will be prompted to specify a time zone manually. However, if you select a saved location, the timezone will be set from that data.
Finally, TPE also looks up elevation above sea level for both primary (red) and secondary (grey) map marker locations. Elevation above sea level affects the actual time of rise/set and apparent position of the sun/moon in the sky, due to atmospheric refraction. If there’s no internet connection, TPE will assume that the location is at sea level (which is what very many sunrise/set calculators do all the time), and the data will be very slightly less accurate as a result.
Geodetics is unavailable while offline
For the secondary map marker and geodetics functions, knowing the elevation above sea level is essential – this is how you determine the angle of elevation to a nearby mountain-top, for example. With no internet connection, the geodetics information cannot be determined and is unavailable.