We start out just the same way you would make a map on Earth; we start by making a base map, specifically with photos from the Mars Reconnaissance Orbiter. The MRO has a camera on it called HIRISE, the High-Resolution Science Experiment. HIRISE takes pictures at roughly 25 centimeters square, about the size of a dinner plate. So if you put a dinner plate on Mars, it could take a picture of it, or at least it would show up as a bright dot. When we take pictures of the ground we can take them at different angles, and when we do we can create an elevation model. With that information we can see the shape of craters and rocky areas and figure out where it's safe to land, where it's safe to drive the rover and, just as important, where do we want to go to do the science?