A method and apparatus for identifying visibility of targets. A first function is selected to indicate whether locations in an environment are at a surface for objects in the environment, outside the surface, or inside the surface. First volumes are formed for the environment. Each volume in the first volumes has a size selected such that a difference between first interpolated values for each volume and first values generated using the first function for each volume is within a threshold. Second volumes are formed for the environment. Each of the second volumes has a size selected such that a difference between second interpolated values for each volume and second values generated using a second function for each volume is within a threshold. The second values are minimum values along lines from each volume in the second number of volumes to a target in the environment.