1. A method for ascribing coordinates to positions of points within an image performed in a suitably programmed computer and comprising for each point for which a set of coordinates is desired computing a first coordinate value s and a second coordinate value φ defining the position of said point in a coordinate system (s, φ) wherein said coordinate system comprises a set of curves diverging from an apex point lying on a reference line which extends in a reference direction, each curve in said set of curves extending to intersect a base line extending from a first point to a second point, said first coordinate s representing a measure of a geodesic distance from said apex point along one said curve which contains said point, and said second coordinate φ representing an angle made between said curve and said reference line at said apex point.