Exclude parts of terrain from lightmapping?

Hello,

I have a square terrain, of course, but the playable area of the map is only about 50% of the area, in a more realistic fashion, as seen in the image.

Lightmapping for this causes problems – 112 lightmaps, (56 near, 56 far), which causes Unity to crash when I do it manually or automatically. Plus it seems to be taking 1GB of space just for the light maps!

Is there a way to exclude the non-visible area from being light mapped?

Ifyou could make the terrain into two seperate gameObjects, than you could exclude one. However, I guess this would only be possible if you were to create the terrain directly via your 3d graphics editing program.