An instrument for use in minimally invasive hip surgery, the instrument being for use in forming and using a portal incision in a hip arthroplasty procedure to implant an acetabular implant. A novel means is presented herein by which error magnitudes can be calculated directly without requiring the generation of an error value polynomial. The heater section heats the sensor section to activate it. The bladder is mounted on the elongate body over the outlet opening 44, surrounding the elongate body and sealed to the elongate body at opposite ends of the bladder. A support structure implementation can include a scanner device and a rotary device that respectively control scanning and rotating movement of a movable support structure or of a sensor. The method comprises the steps of: operatively positioning an optical reader relative to the surface and capturing an image of a portion of the coding pattern; sampling and decoding local tag data contained in the imaged portion; and determining a position of the reader.