I use aerial imagery, though i do have some familiarity with the region. I usually add ridges, peaks, and cliffs first. The USGS 3D Elevation Program is good for this. and USGS Topo for finding elevations. The sand/shingle washes are good to add early on since they tend to divide the land uses. For adding bare rock, I use Esri Clarity, as it shows it quite clearly. The FastDraw plugin in JOSM can make this process much faster. For determining whether is a heath or scrub i use the pictures on their wiki pages as reference. I might map other areas around Vegas in the future.