Using a google map you can allow your users to set locations by simply dragging a marker on the map.
Latitude: Longitude:
Drag the marker on the map to set the coordinates:
Two fields - one for longitue and one for latitude - are updated as the marker is dragged on the map. These fields could be hidden as well. Putting them into a form and adding a submit button is all you need to allow users to update locations.
The javascript code for this can be found here