This should be the correct procedure for creating invisible terrain for Space Maps. The basic premise of this is that because objects cannot spawn below ground the terrain must stay at the bottom of the map. However in BF2/2142 the terrain texture and mesh (collision) are saved in different places so as such they can be placed at different heights. For space we want objects to spawn so they must be above the col mesh but we don't want to see it so we place the terrain texture infinitely higher than the collision mesh. This ensures you will never fly high enough above it that you can see the texture.
1. Open the heightmap in PS
2. Select all and set the color to pure white
3. Save the The heightmap
4. Set the TerrainHeight to something ridiculously huge (Tweakerbar>[Map Name]>Level Settings>TerrainHeight)
4. Open the map in the editor then save the compiled terrain ONLY
5. Open the heightmap in PS
6. Select all and set the color to pure black
7. Save the The heightmap
8. Open the map in the editor then save the terrain ONLY