Tom Brown along with with other survival experts deem shelter to be the most important initial factor a person must overcome when faced with a survival situation. As with many survival skills they are extremely situationally dependent, it is all very well knowing how to build an igloo but in the desert it serves no purpose. We can can differentiate shelters into 3 main categories.
Desert
Sub-Arctic Forest (Nortern US)
Snow shelters
Desert Shelter: