Isometrical Terrain
Isometrical terrain is the basis of most maps. While some maps use a lot of rectangular terrain, it quite simply doesn't look good. But what is isometrical terrain? It's a long word for something easy to use. Basically, the terrain Blizzard uses in their maps is isometrical. It is a 'puzzle' of 2 dimensional pictures, or 'puzzle pieces', that looks 3 dimensional when the puzzle is put together. The easiest way to see what the puzzle looks like is to open your map editor, and go to the terrain. Click the type you want, and paint it onto the map. It's that simple!
How it works: StarCraft has a bunch of image files known as tiles. These are the 'puzzle pieces'. Starcraft puts these pieces together to make a 3D-looking image using a preset system.
What makes StarCraft terrain look so good, and not just a repetitive bland terrain, is because most tiles have several pairs, which look almost the same. However, although they are different, they can both go into the same place without looking out of place. So Starcraft randomly chooses one of the pairs each time there is a slot that it wants a piece for. You can see all the possible pairs for a particular tile by going on terrain, and changing the dropdown box on the right to Subtile, and Ctrl-click a tile(i think, someone check this, no scmdraft on this comp). We'll have more about the Subtile tool later.
Another thing about Isometrical terrain is that it doesn't let you do everything. For example, you can't put cliff terrain on top of more cliffs and stack it. Instead, it will just replace it with more cliff. Try it and see for yourself! To get around that, you can use extended terrain.
Credits
Thanks to Jack for writing this in September 2009.