1. An apparatus for combining two wire mesh models comprising: a model store configured to store model data representative of three dimensional co-ordinates of a first and second wire mesh model of a surface; an overlap identifier operable to identify portions of wire mesh models stored in said model store representative of the same portions of a surface; and a processing module operable to: delete from first and second wire mesh models stored in said model store model data representative of vertices of the boundary of one said models where triangles including said vertices are completely represented in part of said other model; classify vertices at the boundary of a model on the basis of whether said vertices define part of one or more triangles and the number of edges the triangle defines by said vertices overlap; generate new co-ordinates for shared points on the boundary between said models on the basis of the classification of said vertices; and delete said classified vertices, wherein said processing module is operable to generate new co-ordinates only when a co-ordinate is classified as defining a vertex of one or more triangles where the edges of individual triangles only overlap a single edge of triangles defined in said other model.