Before we go on with modeling the terrain, let's set up the texture image which you will project onto the terrain.
Open up GIMP and go to File ¦ New.
Type in width 2048 and height 2048 in pixels.
Go to File ¦ Save As and name the image
Map_Master.xcf
.Draw something random on the image using the Paint Brush tool, and then go to File ¦ Save a Copy.
Save a copy of the image in PNG format named
Map_2048.png.
Back in SketchUp, go to File ¦ Import and select png from the drop down list. Select the image you just saved and select Use as Image. Click on Open.
Select the bottom left, and then the top right of the grid rectangle. This inserts your image at the same size and location as your grid.
With the Move tool, click on the image. Hold Ctrl (the Option key on the Mac) and move up along the blue axis and click.