1. In a touch system having a touch screen with a touch surface on which an image is displayed, a method of automatically mapping a touch screen co-ordinate system to the display co-ordinate system comprising the steps of: including in said displayed image at least one reference mark, the location of the at least one reference mark in said display co-ordinate system being known; acquiring an image of said touch surface including said displayed image; determining the location of said at least one reference mark in said acquired image relative to said touch screen co-ordinate system; and automatically mapping said touch screen co-ordinate system to said display co-ordinate system using said determined location and the location of the at least one reference mark in said display co-ordinate system.