As far as I can tell PD use very high resolution Satellite Images to get the textures around the track making them 100% accurate in style and colour at the time of the picture was taken. It allows for the differing colours on the ground you don't see in other games making GT's renditions more accurate. Just look at Willow Springs and Silverstone for example. They look loads better because of the Satellite Image. They only use proper texture-work close to the track itself. I imagine doing this makes them closer to the real things and keeps the file size down as to make it work on the PS3.
In my opinion. One massive Satellite Image looks better than one or 2 grass textures because of the accuracy it creates even if it's slightly lower resolution on the boundaries. Besides, you won't see these lower resolution textures when you're actually driving around the track, which if I'm not mistaken, is what is intended to be done with the tracks.