Abstract: An apparatus for tracing the locations of a line segment moving across a planar tablet is disclosed in the present invention, which includes a microprocessor, a positioning coil driving circuit, a plurality of positioning coils, a planar sampling circuit, a coordinate acquisition circuit and a counting circuit for the purpose of determining the location of a line segment relative to the planar tablet at a given time instant. A location of the line segment is determined by energizing a pair of positioning coils, each representing an end of the line segment, alternately and thereby achieving positional coordinates of these two ends. Repeating of such positioning operation results in a trace of the trajectory of movement of the line segment. A simple and effective position compensation method for offsetting the positioning error when locations of a physically installed or logically perceptible line object, instead of the line segment pointed by the two positioning coils, are to be traced.