Public utility systems are often run underground; some by the very nature of their function, others for convenience or aesthetics. Before digging, excavators are required to contact the local One Call System (811) to notify the center and its members of the proposed excavation activity and location such that the existing underground systems’ locations can be determined and marked.
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. For metal pipes and cables, this is often done with electromagnetic equipment consisting of a transmitter and a receiver. For other types of pipe, such as plastic or concrete, 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 if disturbed.
Utility color codes are used to identify existing underground utilities in construction areas, to protect them from damage during excavation. Colored lines, flags, or both are used to mark the location and denote the type of underground utility. A special type of spray paint, which works when the can is upside-down, is used to mark lines, often in a fluorescent color. On flags, a logo often identifies the company or municipal utility which the lines belong to.
Flags may also be an advertisement for a company which has installed an irrigation system for lawns or gardens. In this case, each sprinkler head is usually marked, so that landscaping crews will not cover or bury them with soil or sod, or damage them with tractors or other construction equipment while digging holes for trees, shrubs, or other large plants or fence posts. This is also important because a vehicle (tractor, truck, or otherwise) can break a sprinkler or the hard-PVC pipe or joint it is mounted on, simply by driving over it, particularly on newly moved soil which is uncompacted and therefore unsupportive of such weight.