1. A method of generating data representing open sky, the method comprising: obtaining range data associated with a given position; using the range data associated with the given position to generate data at a processor, the data representing visibility of open sky at the given position; and storing into data storage the data representing the visibility of open sky at the given position, wherein the data representing the visibility of open sky at the given position comprises data indicating one or more ranges of angles at which the open sky is visible from the given position and the visibility data includes a partial visibility rating associated with the one or more angles at which the open sky is partially visible from the given position.