In the past, Google would publish a network linked KML file that showed the outlines of imagery releases, including all historical releases going back to 2009. However, early this year they discontinued it and instead have been publishing a map on the Maps Gallery that only shows the latest release. Also, other than taking a screenshot of the maps, there was no way to save them for later viewing.
Now Google have released a map on the Maps Gallery that once again shows us historical releases. Note that it does not currently include the two latest updates from November 3rd, 2014 and November 8th, 2014. It must also be noted that the dates given are not the dates that the imagery was captured, but rather the dates that the imagery was added to the Google Earth database. In many cases the imagery being added is older than previously existing imagery and gets added to historical imagery rather than the default layer.
There is still no way to extract the KML, which makes further analysis difficult. It would have been fun to do heat maps showing which areas received multiple updates.
If we look at all updates from 2009 to October 2014 we see the map below:
We can see a number of interesting patterns. There is poor coverage over tropical forests, deserts, and northern regions. Tropical forests tend to have cloud cover most of the time, making it difficult to capture imagery with clear skies. Similarly, the northern regions often have snow cover, making it difficult to capture good imagery. It is possible that capturing imagery over deserts is also difficult, or it may simply be that their low populations make them less interesting to imagery providers. Iraq and Afghanistan have received very few updates, presumably for security reasons.
[Update: As pointed out by GEB readers Maarten, Chris and an anonymous email, the ‘Latest Google Earth Imagery Updates’ map has been updated to November 19th 2014.]