EM Tracing

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 absolute position of the target.  The value of EM tracing is that it is a non-destructive method that usually provides an accurate indication of position at a fraction of the cost of excavation by safe means.

When using EM locating equipment the manufacturer's stated horizontal accuracy is +/- 0.30m (although we usually do better than that) and the vertical accuracy is +/- 0.60m.  The latter is not good enough to rely on for excavation purposes and is used primarily as an indicator of change in target depth.  Keep in mind that surface grades may change, particularly in construction situations, and alter relative depths.  For these reasons locate marks – paint, flags or stakes – indicate horizontal positions only.