A computer implemented method of creating a map using a geographic information system comprising: (i) creating, by analysing data relating to single direction carriageways contained in a section of existing topographical map data showing a road network, a directed graph of edges and nodes representing said carriageways (2.02), (ii) defining paths through the directed graph, each path representing a section of a carriageway (2.04), and each path comprising: one or more edges of the graph, end nodes; and a unique path identifier; (iii) identifying one or more pairs of paths which represent dual carriageway pairs (2.06); (iv) creating centre lines between the paths within dual carriageway pairs; (v) merging the defined dual carriageway paths to respective centre lines which each represent a dual carriageway (2.08); and (vi) connecting the ends of the centre lines to the road network in the topographical map data to create a collapsed dual map (2.10); characterised in that: the identifying comprises inserting one or more connection edges into the directed graph to connect respective end nodes of a pair of paths in dependence on the proximity of the end nodes and the topographic nature of the edges of the paths, and iteratively removing pairs of paths having at least one connection edge from the directed graph, the one or more pairs of paths representative of dual carriageway pairs being identified based on pairs of paths having nodes connected by connection edges; the creating centre lines comprises representing the pairs of paths having connection edges and that have been removed from the directed graph as a centre line; the merging is performed based on the centre lines created for the pairs of paths having connection edges and that have been removed from the directed graph, wherein merging is carried out at points where the distance between paths is below a predefined threshold, the threshold being defined based on the scale of the map to be created; and the connecting of the centre lines creates a new scalable map layer for display at different zoom levels. (i) creating, by analysing data relating to single direction carriageways contained in a section of existing topographical map data showing a road network, a directed graph of edges and nodes representing said carriageways (2.02), (ii) defining paths through the directed graph, each path representing a section of a carriageway (2.04), and each path comprising: one or more edges of the graph, end nodes; and a unique path identifier; one or more edges of the graph, end nodes; and a unique path identifier; (iii) identifying one or more pairs of paths which represent dual carriageway pairs (2.06); (iv) creating centre lines between the paths within dual carriageway pairs; (v) merging the defined dual carriageway paths to respective centre lines which each represent a dual carriageway (2.08); and (vi) connecting the ends of the centre lines to the road network in the topographical map data to create a collapsed dual map (2.10); the identifying comprises inserting one or more connection edges into the directed graph to connect respective end nodes of a pair of paths in dependence on the proximity of the end nodes and the topographic nature of the edges of the paths, and iteratively removing pairs of paths having at least one connection edge from the directed graph, the one or more pairs of paths representative of dual carriageway pairs being identified based on pairs of paths having nodes connected by connection edges; the creating centre lines comprises representing the pairs of paths having connection edges and that have been removed from the directed graph as a centre line; the merging is performed based on the centre lines created for the pairs of paths having connection edges and that have been removed from the directed graph, wherein merging is carried out at points where the distance between paths is below a predefined threshold, the threshold being defined based on the scale of the map to be created; and the connecting of the centre lines creates a new scalable map layer for display at different zoom levels.