Run length encoded data derived from a raster scanner (30) at an arbitrary angle to the horizontal direction of a rectilinear symbol (25) such as a two-dimensional PDF code is examined to detect the skew angle between the horizontal direction (Xs) of the symbol (25) and horizontal direction (Xr1) of the raster (36). Using the detected skew angle, a computer (70) generates a set of virtual scan lines overlaid on the real scan lines. The computer (70) partitions the virtual scan lines and the real scan lines into associated real and virtual line segments having common intersection points. The locations of mark to space or space to mark transitions are mapped from the real line segments onto the virtual line segments. The system can handle run length encoded data with low computational overhead.