If you find yourself confused by some of the words being used in the field of utility locating. Or you are asking yourself what the different utility flag colors represent, you are not alone. So, we have put together a practical guide to help provide some clarity about utility locating.

What Is Utility Locating?

Utility Locating

This is the process of finding and marking utility lines that are buried below ground. There are different types of equipment and methods used to locate utility lines.

Utility Lines

Locating services, whether public or private, are used to mark the following types of utility lines:

  • Water pipes,
  • Sewer lines,
  • Gas lines,
  • Electric lines,
  • Ethernet and fiberoptic cables,
  • Telephone or communication cables,
  • Metal and concrete pipes that serve various purposes.

Public Utility Locator

Companies that are charged with marking public utility lines – those that run from the street to the meter – are considered public utility locators. Public utility marking can be requested from any state using the coordinated 811 “Call Before You Dig” number.

Private Utility Locator

Any underground utilities that are not the responsibility of public locators can be located by private utility locator companies. This includes those on private property.

Find out When to Call a Private Utility Locator.

Subsurface Utility Engineering

SUE is a process that combines civil engineering, surveying, and geophysics in order to manage risks related to utility design and mapping, assessing the condition of lines, estimating the cost of relocating utilities, and communicating with relevant parties.

Passive Utility Locating

This refers to utility locating methods used to map unknown lines, broken pipes and cables, as well as plastic and concrete pipes that don’t conduct electricity. Passive utility locating can be carried out with ground penetrating radar.

Active Utility Locating

This refers to utility locating procedures used to map an unbroken, metal utility line underground using an electromagnetic frequency that is detected by equipment on the surface of the ground. Active utility locating uses electromagnetic (EM) location.

Utility Markings

Once underground pipes and cables are located, they can be identified on the surface using chalk, spray paint, tape and/or flags. These markings serve as a guide for excavating and digging projects. The American Public Works Association has an official color chart regulating the color of marking that should be used for each type of utility line. But what do the different colors mean?

Utility flags and other markings of various colors alert to the presence of different underground services and potential hazards. This color-coded flagging system ensures safe and informed processes for construction and excavation projects.

  • Red utility flags and/or markings signify electric power lines, conduits, cables, and lighting cables. These demarcate areas of residential electrical systems beneath the surface.
  • Yellow utility flags and/or markings warn of the presence of gas, oil, steam, petroleum, and other gaseous or flammable materials, indicating caution is needed while working in these zones.
  • Orange utility flags and/or markings denote communication systems like cable TV, alarm or signal lines, cables, conduits, as well as fiber optics, demonstrating the presence of technologies running underneath.
  • Blue utility flags and/or markings are signals for drinkable water lines, while green flags correlate with storm, sanitary sewers and other drainage facilities. These help to avoid water-related disruptions and potential contamination.
  • Purple utility flags and/or markings reveals the existence of reclaimed water pathways, irrigation systems and slurry lines underground.
  • White utility flags and/or markings outline proposed excavation areas. They indicate both the outer boundaries of the proposed excavation and the centerline and width of future buried facilities.
  • Pink utility flags and/or markings function as temporary survey markers, or to denote unknown or unidentified facilities.

Utility Mapping

Utility maps are made using various underground location equipment to depict the location of buried pipes and cables.

What Types of Equipment Are Used in Utility Locating?

Electro-Magnetic/Radio Frequency Detection Devices

EM/RF technology transmits electronic signals through a metal pipe or cable. The path of an underground pipe or cable can then be determined using a receiver that tracks the signals from above the surface. EM/RF locating instruments measure the depth and size of the line, plus they provide information about its physical features and conditions of the ground materials surrounding it.

Ground Penetrating Radar

A GPR transmitter sends microwaves into the soil and a receiving antenna records the waves as they bounce off objects underground. As the locator scans the site, software is used to make a map of the structures detected below the surface.

Laser Scanning

Mobile laser scanners and optical cameras are used to collect data that is used to locate and create a 3D Utility maps of objects buried underground.

Infrared Thermography

Thermal imaging detects leaks in underground pipes by identifying variations in wavelengths transmitted into the ground.

Geographic Information System

GIS tools gather, manage, and analyze data collected during locator surveys so that it can be rendered useful in mapping underground utilities and making technical decisions about projects.

Vacuum & Hydro Excavation

Vacuum and hydro excavation are highly accurate methods for excavating ground materials, enabling utility locators to find the exact location and position of lines and uncover them to carry out repairs, modifications, or relocation.


Is a minimally invasive excavation method to verify the location of subsurface lines. Potholing requires vacuum excavating and making a series of holes in the ground.

What Else Do We Need to Know?

Subsurface Object

Utility locating equipment can be used to trace structures made of metal, wire, concrete, and plastic materials that are buried below soil.


Different materials have different capacities to transmit electricity. This variation in conductivity determines how underground structures must be located and what data can be collected about them.

Electromagnetic Field

When an electrical current runs through a line, it creates an electromagnetic field that is used to understand its exact location via an electromagnetic locator.

Utility Easement

Easements provide public utility companies the right to access private property in order to repair, install, and maintain utility lines.

Tolerance Zone

The tolerance zone is the set horizontal distance measured from the outer edge of a subsurface utility line or pipe that serves as a warning to excavators that they are nearing the line and need to dig carefully.

Clarity and Care When It Comes to Utility Location Services

For more information on how our crew can assist your excavation or construction project, contact SoftDig.


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