This is the third post in a series looking at how much data is in Google Earth. We have already looked at how much data per unit area 3D imagery requires and how much data different types of 2D imagery require. Today we are looking at historical imagery.
Our first test was to pick a location that we know has a lot of historical imagery. We chose Rio de Janeiro, Brazil. We prepared the view we were interested in and cleared the Google Earth cache then allowed the default layer to load. Just the default layer, without moving the view at all, filled the cache to 4 MB. We then switched to historical imagery and cycled through all the historical imagery, again without moving the view. The cache was now at 580 MB. The reason for the enormous size is that Rio has approximately 220 unique images for the location we chose. On average, each image added about 2.6 MB to the cache. This is less than the default view takes, because many of the historical images do not cover the full area in the view, with some of them being only barely visible at the edge of the screen.
The area we chose in Rio de Janeiro, Brazil.
So how much historical imagery is there globally? We have tried to answer that question before with our historical imagery density map. Since then, we have created a higher resolution version, although it is not quite complete.
In the standard heat map, we used a logarithmic scale to help make the patterns in the historical imagery stand out. To get a better idea of just how much historical imagery there is overall, we have a simplified version shown below. It shows areas with 10 to 24 images in green, areas with 25 to 49 images in yellow and areas with 50 and over images in red. Most of the world has less than 10 images in a given location and in fact most of the oceans and much of the polar regions and deserts have no historical imagery whatsoever. There is also a bug in the way Google Earth shows historical imagery on the timeline that causes it to incorrectly report the number of images along coastlines or other regions that have no historical imagery, but are near to areas with a lot of historical imagery. This is what causes all the large patches around the edges of the continents in the screenshot below. This makes it impossible to directly do calculations on the data. However, by rough approximation we believe that historical imagery, if uniformly distributed, could cover the worlds land masses at least 5 times over.
To see the historical imagery density map in Google Earth download this KML file.