You must have created a new map to follow this example.
Having sufficiently beaten the terrain system to death through explanation, let's get on with what we are most interested in, which is creating our own heightmap to use for our game:
As discussed in the previous example, you should now see a flat plane of terrain slightly submerged beneath the ocean.
At the top of the Sandbox interface in the main toolbar, you will find a menu selection called Terrain; open this. The following screenshot shows the options available in the Terrain menu.
As we want to adjust the terrain, we will select the Edit Terrain option. This will open the Terrain Editor window, which is shown in the following screenshot:
You can zoom in and pan this window to further inspect areas within the map. Click-and-drag using the right mouse button to pan the view and use the mouse wheel to zoom in and zoom out.
The Terrain Editor window has a multitude of options, which can be used...