1. A method of estimating the orientation of digital ink, the method comprising the steps of: measuring, at a sampling rate during writing by an optically imaging pen on a surface printed with tags, the azimuth of the pen, each tag encoding data on an identity of the surface associated with a digital description of the surface and on the respective location of that tag on the surface, the digital description being stored by a computer system networked with the pen and the digital ink being generated by associating the digital description with the data encoded by the tags optically imaged by the pen during said writing; and estimating, at the computer system, the orientation of the digital ink using the measured azimuth of the pen at the sampled points.