1. A method of routing covert flight paths using digital terrain elevation data of a geographical region, comprising the steps of: forming a hideability map from the digital terrain elevation data, said hideability map being indicative of hideable areas within the geographical region; forming a flyability map from the digital terrain elevation data, said flyability map being indicative of flyable areas of the geographical region; creating a costing surface from a weighted combination of said hideability map and said flyability map, where the weighted combination is based on predetermined weighting factors applied to hideability and flyability; and routing a covert flight path through areas of said costing surface which are both hideable and flyable.