OK I assume you know how to tile terrains it's just getting them to line up. 1. Select 2 terrains that are side by side (it helps to do this as you are tiling). 2. In top view press the + key to zoom in as far as you can go. 3. Select the terrain you will move. Alt-[arrow key] until the edges meet. There will still be a slight gap if you render it you'll see it. 4. Go into the attributes and add/subtract .04 more in the direction you were moving the terrain. Now they should exactly match.