Starcraft 2 Galaxy editor mirroring tools?
Highlight the terrain, copy and paste it, but don't place the copied terrain yet.
Under the Edit menu, use the Flip and Rotate options to get it symmetric.
Check
Map
-> Symmetry
if you set it up before drawing on your map, all your changes to the terrain, decorations, units (e.g. minerals) will be automatically replicated on your map.