Unfortunately we haven't had the opportunity to visit locations to get primary research for the stones. However we have been using internet sources to get the imagery we need. Also we can photograph similar stones in the local area for texture references.
In regards to the other aspects like grass and environmental obstacles, we have taken huge number of photos of various bits of vegetation etc, both at the park and in gardens.
Not only have we been looking at content for the project itself but also on the technical side side of things. With the project taking a unique approach to perspective we've needed to learn a lot about the CryEngine SDK. The documentation on the Crydev site has been very useful for pointing us in the right direction.
Our next step is to create a whitebox with some 'place holder' assets to set the world scale and help with further asset creation.