Directional surveys acquired by Measurement While Drilling (MWD) are subject to many errors that are not easily recognized by traditional Quality Control (QC) procedures. This commonly leads to inaccurate wellbore placement and greater positional uncertainty. Common sources of MWD survey error are inaccurate geomagnetic references, localized distortions in the natural magnetic field, poor instrument calibration, random sensor noise, magnetic mud, and human error. Often times, such...