already, thank you for your answers
rdb, I’m using the latest version under python35, panda1.10 devel
Stijn, I use the robot model as an example, in reality I would like to load a large 3D environment, like 1000km^2 map size.
I want to load very large map with grass and threes.
But it is impossible to fully load a 1000km^2 map. I would like to load only objects that are near me : the terrain, Grass, trees, rocks and… robots ! 
for terrain i think that i use GeoMipTerrain, it best solution for very large terrain.