Use this option to update elevations for the records in your database.
Only if missing - Only update the elevation if missing. That is, if the elevation has not already been updated. As elevations can be zero, the trigger for knowing if an elevation has been updated is the Resolution field. If the resolution field is blank, then the elevation has never been updated.
Always - Update the elevation regardless of the status of the resolution field.
To speed up this process, GSAK accumulates all elevations in a local database (see this link for more information). When the update is complete you will get a summary similar to the following:
Local - Updated from the local database (these updates will be *very* quick)
GSAK server - No entry found in the local database, but we do have an entry on the GSAK server (this just means the latest updates haven't propagated to the local version yet)
External - Neither the local or online database has this entry, this one will be the slowest as the information had to be fetched from an external source Mapquest, Google, etc.
Notes:
1. "user" (without the quotes, and is not case sensitive) entered in the resolution field will prevent the elevation from being updated regardless of update option selected
2. When correcting coordinates, the resolution field is blanked out (this will force an elevation update the next time this option is taken)
3. Elevation updates can also be started from the GPX load dialog (see the "Elevation" box)
4. If the elevation is not correct (could be the result of an outdated look up) you can force GSAK to fetch again from the external server in the hope of getting a more current/accurate value. To do this you must use the "right mouse click" menu for the waypoint and then select Force elevation refresh
4. You can view a summary and "Heat map" of the online GSAK database of elevations here