For those wondering how the Google VR Photographers and editers add the ground to their Equirectangular panoramas, check out the feature called Viewpoint Correction, as seen in software like PTGui:
ptgui.com/excamples/vptutorial.html
(Note that this is NOT the software used by Google)
If you take a closer look at the ground in street view, you see that the stitching seems streched, and sometimes it even overlaps with information from the viewpoint next to the current one. (With that I mean that you can see something in one place, and suddenly that same feature is shown as the ground in the next place, revealing the technique used for the ground stitching).