Utility location is the process of identifying and labeling public utility mains that are underground. These mains may include lines for telecommunication, electricity distribution, natural gas, cable television, fiber optics, traffic lights, street lights, storm drains, water mains, and wastewater pipes. In some locations, major oil and gas pipelines, national defense communication lines, mass transit, rail, and road tunnels also compete for space underground. Description Public utility systems are often run underground—some by the very nature of their function, others for convenience or aesthetics. In the United States, it is estimated that subsurface lines amount to over 20 million miles in total. Before digging, local governments often require that the underground systems' locations be denoted and approved, if it is to be in the public right-of-way. Technology and equipment Because of the many different types of materials that go into manufacturing each of the different types of underground lines, different detection and location methods must be used. The two general methods are called passive locating and active locating. Passive locating uses signals that occur naturally on some utility lines. This method often fails to locate 60% or so of utility lines, and misidentifying utilities is an associated risk. Active locating, on the other hand, is more accurate. It makes use of signals that the locators generate through a transmitter. Then, with the help of a receiver, locators can trace as well as identify what type of utility line lies underground. The active method is the most suitable for locating conductive metal pipes and cables. This process often involves using a piece of electromagnetic equipment, consisting of a transmitter and a receiver. For other types of pipe such as plastic or concrete, a metallic tracer wire or conductive foil (detectable tape) may be buried alongside or immediately above the pipe, to aid in locating it. In the absence of such a conductive tracer, other types of radiolocation or modern ground-penetrating radar must be used. Location by these technical means is necessary because maps often lack the pinpoint precision needed to ensure proper clearance. In older cities, it is especially a problem since maps may be very inaccurate, or may be missing entirely. A few utilities are permanently marked with short posts or bollards, mainly for lines carrying petroleum products. This may be done because of venting requirements, and also serves to indicate the location of underground facilities that are especially hazardous