How to create a flatland DEM
1. Zoom in (Ctl+) in the Map View window and use the Measuring String to confirm that the Map View covers the approximate area you want the DEM to cover. If you don't, you may end up with most of a planet.
2. Open the DEM Design window. Change your middle elevation slider value if you don't like where it is. Click the Add button. This will allow you to add control points to the Map View window.
3. Return to the Map View and click 4 corners to define your DEM.
4. Click the Build button in DEM Design. This will open the DEM Build window.
5. Deselect the Gradient box and give your DEM a name. Everyone needs a name. If you want more than 1 DEM, say so in the E-W DEMs and N-S DEMs boxes.
6. Click the Grid button to create your DEM (s).
7. If all went well, you will have one color and see the same elevation displayed in the Map Control box message area as you move your mouse around the Map View.
8. The DEM is flat but it won't render flat if you don't change a few things in the Terrain Parameters window. In this example, the Vertical Exag was set to 0, Polygon Smoothing was selected, and HorizDisp was deselected. You may have to play more to get something as flat as you need.
If you forget to scale your Map View before starting, you will probably end up with a flat DEM that is quite large. So large, in fact, that you will have created most of a planet's surface. This could be useful at a future time. It's not what you wanted now.