A Digital Elevation Model (DEM) is a digital representation of real world terrain. DEM files are used to generate plot wide elevations in the game. This is useful for doing real course renditions or fictional courses based on real locations.
How accurate a DEM is depends on its resolution. It is similar to the resolution of an image in that a higher resolution produces sharper detail. Think of the DEM as a grid of measured elevation points. The resolution is measured by how far apart these points are. In a 30 meter (1 arc second) resolution DEM each point on the grid is separated by 30 meters. A 10 meter (1/3 arc second) DEM is a higher resolution because there is only 10 meters between each point on the grid. There is also vertical resolution which indicates to what decimal place elevation measurements are accurate.
Most DEMs are 30 meter (1 arc second) or 10 meter (1/3 arc second) resolution with 0.1 meter vertical resolution. With these resolutions you get good plot scale elevations but it does not do everything for you. You still have to do the fine detail work like green contours, mounding, bunker shaping, etc. If you're lucky enough to have a 1 meter (1/9 arc second) DEM you may not have to do as much of this fine detail work. Resolutions worse then 30 meter (1 arc second) are not very useful unless your area has extreme elevation changes.
Below is a list of sources you can download DEMs from:
This tutorial goes into detail on downloading from the following sources:
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