why after building light ( high ) i got those stranges tiles of shadows ( see attachment ).
the grass is made by 20 squares meshs.
the mesh have 2 uvw and no overlapping lightmap.
You need to avoid building flat surfaces that way—make them a single mesh as much as possible. Lightmass processes the lighting per object and each object can be slightly different. You can try increasing the Lightmass quality setting and lowering the smoothing, but it might not get rid of it completely.