Why do maps sometimes switch to satellite and roads?

Problem: When you zoom in or out the maps automatically switch to another theme or obviously the "wrong" map type is being displayed.
Background: Not all map sources are available at all resolutions and in all areas of the world. So if the resolutions range of geographical coverage for a map source is exceeded, TrailRunner will display the best logical match it can find. For high resolutions this is satellite and for low resolutions this is roads.