Environment detection - How to detect "city" versus "landscape" background environment in computer vision?
Think of what different features the desert has compared to an urban environment. The desert is probably restricted the a limited set of colors/hues. It is limited to mostly smooth textures. It lacks "man-made" geometries such as straight lines from building edges etc.

So you should be able to differentiate between desert and urban environment by looking at the color components (both the number of colors and the variation), textures and the amount of prominent straight lines/edges in the environment.

By googling "image processing classify outdoor environment" I found the paper Automatic Classification of Outdoor Images by Region Matching which can be a good starting point for reading up on environment classification. They seem to base the classification mostly on color and texture features.

These are just general ideas but hopefully they can help you get started. Good luck.