A method of sensing coded data on a surface is disclosed. Images of the coded data on the surface are captured using a sensor of a sensing device. The coded data is formed from tags, with each tag encoding a unique identity of a region of the surface and the position of the tag within the region. A perspective distortion of the coded data is estimated and compensated for when detecting data elements in the tags. The unique identity of the region from the images is determined, and the position of the sensing device within the region from the position of at least one tag in at least one image captured by the sensor is determined.