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.