Literature Review 8 2.1 Background ::::::::::::::::::::::::::::::::::::::: 8 2.1.1 Visibility ::::::::::::::::::::::::::::::::::::: 8 2.1.2 Parallelism ::::::::::::::::::::::::::::::::::: 9 2.1.3 Siting ::::::::::::::::::::::::::::::::::::::: 9 2.1.4 Terrain Elevation Data ::::::::::::::::::::::::::::: 9 2.2 Characteristics of the Problem and its Domain :::::::::::::::::::: 9 2.3 Practical Considerations ::::::::::::::::::::::::::::::::