1. A method of determining line feed error in an image forming apparatus for printing a reference image on a print medium, said image forming apparatus comprising a print head having a head sensor configured to sense information from images printed on the print medium, said method comprising the steps of: printing a first swath of a halftone image of the reference image on the print medium, said first swath having a dot pattern oriented in a direction perpendicular to a direction of linefeed of the image forming apparatus; repositioning the print head relative to the print medium by at least one line feed distance; determining a line feed error based upon information sensed by the head sensor from the printed first swath; and printing, using the determined line feed error, a second swath of the halftone image of the reference image on the print medium, wherein said second swath: (i) at least partially overlaps the first swath; and (ii) has a dot pattern configured to at least partly neutralise, in an overlap area of the first swath and the second swath, the orientation of the first swath dot pattern in the halftone image.