Rule #7 Depth
This rule is a favorite among outdoor and landscape photographers. Adding depth to a 2D image is not always easy but there are a few tricks to it.
Layers: By breaking up the image with hard defined lines; like the crest of a hill or mountain ridge, into foreground, middle ground, and background. You help the eye understand easier that there is distance in between them. Take the title image above. The tree in the foreground is on top of a hill which drops off and creates a hard line. Next you see the row of mountains in the middle ground and they are crested with a hard line of deep red sand. Lastly the distance is really felt when you see that blue haze of the mountains in the background. So the three defined layers give the image depth.
Scale: Another way to help show depth is to have objects that would all be equal size in the foreground, middle ground, and background. Like a row of trees or maybe oil pumping rigs... In the photograph they appear different sizes because the ones in the background are so small and the foreground so large. Your brain will calculate this as depth. It is the same as a road going off into the distance, your brain knows the road is equal width the whole way so it recognizes it as depth.