A method of using a mobile device to determine a first relative rotation of coded data on a print medium, the print medium configured to be printed on by the mobile device in a print direction, the mobile device including a printer, a first sensor and processing means, the print medium comprising a substrate defining first and second opposite faces, the first face bearing coded data, the method comprising the steps of: (a) using the first sensor to capture a first image of at least some of the coded data when the print medium is at a first position; (b) using the processing means to determine, from a known physical orientation of the print medium and the first image, a first relative rotation of the coded data with respect to the print medium.