1. A method of generating recognition data, comprising: receiving lattice data defining a lattice structure comprising a plurality of nodes connected by links, said links being associated with data identifying units; determining for at least part of the lattice structure defined by said lattice data a plurality of areas having edges and vertices defined by the nodes and links of at least part of said lattice structure; generating for said areas, data identifying the number of links bounding each area and data identifying the location of said areas within at least part of said lattice structure; and outputting as recognition data: data identifying said units associated with links and said data defining areas.