As we have discussed before Google Earth shows different imagery depending on the zoom level and whether or not you are in ‘historical imagery’ mode. We have also talked about how the ocean bathymetry data changes in ‘historical imagery’.
When you are looking at the whole globe in ‘default’ mode, the imagery used for the land was cleverly created by taking Landsat imagery captured over a period of time and automatically removing cloud and snow cover by combining multiple images of each spot. As you zoom in, this transitions to higher resolution satellite or aerial imagery. For much of the world, the individual satellite images or patches of aerial imagery can be identified by means of the ‘historical imagery feature’. However, there are some ‘background’ sets of imagery that are not at first obvious.
If you view the whole globe in ‘historical imagery’ and move the time slider all the way to the left, the image you will see, looks very similar to the Landsat imagery seen in the zoomed out ‘default’ mode. However, this image is a different one and the attribution shows NASA as the provider. It is most likely MODIS data, which comes from Nasa’s Terra and Aqua spacecraft. Now, still in ‘historical imagery’, if you move the timeslider all the way to the right, all the satellite and aerial imagery found in ‘historical imagery’ gets superimposed on top of the background image. However, because the background image is such low resolution it is fairly easy to see where it shows through and there is no other satellite or aerial imagery available to date. If you find one such location, zoom right in, and then switch back to ‘default’ mode instead of the NASA imagery, for most parts of the world, Google Earth shows imagery attributed to CNES/Spot image. This is imagery that has been obtained via an agreement with Spot Image that we first reported on in 2007 when the imagery was first added in parts of Europe. Since then, most of the rest of the world has obtained similar imagery.
None of these ‘background image’ sets have any dates associated with them, nor, as far as we are aware, are they noted on Google’s Imagery update maps when they receive updates.
|Left: Landsat imagery as seen in ‘default mode’ when zoomed out.||Right: NASA imagery (probably MODIS data) as seen when in ‘historical imagery’ mode.|