Electromagnetic trackers have many favorable characteristics but are notorious for their sensitivity to magnetic field distortions resulting from metal and electronic equipment in the environment. We categorize existing tracker calibration methods and present an improved technique for reducing static position and orientation errors inherent to these devices. A quaternion based formulation provides a simple and fast computational framework for representing orientation errors. Our experimental apparatus consists of a 6DOF mobile platform and an optical position measurement system, allowing collection of full pose data at nearly arbitrary orientations of the receiver. A polynomial correction technique is applied and evaluated using a Polhemus Fastrak resulting in a substantial improvement of tracking accuracy. Finally, we apply advanced visualization algorithms to give new insight into the nature of the magnetic distortion field.