Z-Axis

The depth dimension pointing into the body, describing how deep the field reaches. Readings are taken on horizontal planes at set depths along it, and every one of those heights is measured from the same starting point. That starting point is z=0: the face of a disc, the top surface of a mat, and the center of the tubing on a loop. A plane at 5 cm sits 5 cm above z=0, not 5 cm above the plane below it.