Finding buried utilities, pipes, cables, wires and other underground objects by detecting the electromagnetic field around them is referred to as EM tracing. The significance being that it is the field around the object not the object itself that is being detected. Sometimes fields can be distorted or on occasion even cancelled out by other fields, meaning that results do not always indicate the correct position of the target. The value of EM tracing is that it is a nondestructive method that usually provides an accurate indication of position at a fraction of the cost of excavation by safe means.
EM locating equipment typically has one or more antennae that combine to predict the position of a target. Each antenna consists of coils wound about an axis and the orientation of this axis determines which direction the antenna will be most effective in. EM Receivers are usually set up to favor horizontal accuracy because this is what is required for excavation locates. Keep in mind that surface grades may change over time, particularly in construction situations, and for this reason locate marks – paint, flags or stakes – indicate horizontal positions only.