1. A method performed by a computer of forming a two dimensional map of a three dimensional environment, there being a map origin located in the three dimensional environment, a viewing direction vector defined passing through the map origin, and a one-to-one correspondence between map positions in the map and the directions of vectors passing through the map origin; the method comprising the steps of: associating by the computer an environment position in the three dimensional environment with a folded vector that passes through the map origin, the folded vector lying in a plane containing both the viewing direction vector and the environment position and forming an angle with the viewing direction vector that is a predetermined function of the angle between the viewing direction vector and a vector between the map origin and the environment position; associating by the computer an environment position with the map position corresponding to the direction of the folded vector associated with that environment position; and deriving by the computer properties for a map position from the properties of the corresponding environment position; wherein the predetermined function is a multiplication by 0.5.