Traditional electromagnetic induction utility locating equipment require utilities to be conductive. They are ineffective for locating plastic conduits or concrete storm and sanitary sewers. Because GPR can detect variations in dielectric properties in the subsurface, it is highly effective for locating non-conductive utilities.
The common sense approach for locating is to use all of the tools available. Understanding where and when a particular approach is most cost-effective comes from experience, business practice and local construction.