Geospatial Technologies Aid in Locating Underground Utilities
Integrating GNSS with advanced technologies for underground asset location improves accuracy and boosts productivity.
As the U.S. works to improve and rebuild its infrastructure, a major concern is to gain accurate knowledge on the locations and conditions of existing assets. In addition to identifying and planning needed repairs, critical assets must be protected against accidental damage. One of the most important needs is the location of underground natural gas pipelines.
According to the U.S. Pipeline and Hazardous Materials Safety Administration, America has more than 1.8 million miles of gas distribution mains and service pipelines. In addition, roughly 300,000 miles of large collection and transmission lines carry gas from production fields to distribution center.
Some of the most vulnerable pipelines are in developed and municipal areas including commercial, residential and industrial settings. In these regions, miles of gas pipelines are not located with any degree of accuracy, or the locations are not recorded in an easily retrievable and shareable format. For example, outdated paper maps often are not tied to accurate coordinate systems. They rely on “tribal knowledge” that comes from an aging workforce—workers nearing retirement age may be the only ones who know the (often approximate) location of the lines.
There are enormous hazards related to not knowing where and how deep these gas pipelines are located. Without accurate, readily-available location data, pipeline operators, construction companies, farmers, land owners and other stakeholders will continue to face the risk of accidental and potentially catastrophic damage to a buried gas pipeline.
The 2016 PIPES Act passed by Congress calls for increased use of data and technology to improve pipeline safety. Supported by industry players such as the American Petroleum Institute and the Interstate Natural Gas Association, the PIPES Act includes efforts to prevent damage by third parties, i.e., accidental contact with or damage to a buried line.
The problem is being addressed from two directions. First, a new technology has emerged that can accurately detect the location depth of buried pipelines. Second, locating technologies can couple with geospatial solutions to produce accurate position information that is tied to known coordinate systems. The resulting survey-quality data forms the basis for GIS-based approaches for planning, asset management, operations and emergency response.
An Integrated Solution
In 2015, surveyors from Woolpert, Inc., a major U.S. architecture, engineering and geospatial (AEG) firm, used a SPAR 300 subsurface utility surveying system on a pilot project in northern Ohio. According to Dave Kuxhausen, Woolpert discipline leader for surveying and geomatics, the work was done under a contract to locate underground utility lines on a client’s property. “We used the SPAR unit coupled with Trimble R10 GNSS receivers to detect those lines,” Kuxhausen said. “It wasn’t a large project, but it established our relationship with them and showed that we had the capabilities to perform this type of work.”
The SPAR 300 uses magnetic field sensors to determine the distance to a buried pipe or other asset that is capable of carrying an electric current. (The system induces a current into the pipe or tracer wire via connection at a valve or other exposed component.) The SPAR sensors can be integrated with Trimble GNSS receivers or total stations that are connected to Trimble Access software running on a Trimble TSC3 or other field controller.
In operation, field crews use the SPAR 300 to locate buried pipes in three dimensions. The Trimble Access display indicates when the SPAR has located a pipe and aids the crew in following the pipe. The system provides horizontal and vertical offsets from the sensor to the pipe while the GNSS receiver supplies precise geographic positioning. When the crew wants to capture a measurement, Trimble Access automatically combines the data from the SPAR and GNSS sensors and stores the resulting positions into its database. In addition to a 3D coordinate on the pipeline itself, the solution also computes coordinates for points on the surface directly above the pipe.
In a single pass, the survey crew can detect and mark the pipe as well as capture survey-grade positions. The resulting locations approach the accuracy of Level A excavations. (See: Four Levels of Accuracy and Cost.) Kuxhausen said that, depending on the integrity of the tracer wire, the system enables his crews to capture pipe depths accurate to roughly 0.25 ft (8 cm) while working with the speed and flexibility associated with Level B electromagnetic sensors.
The field data is transferred to Trimble Business Center software (TBC). “We run the data through a QA/QC process in TBC,” Kuxhausen explained. “Then we export the data into shape files (SHP format) and look at them in an Esri type environment to check for gaps or overlaps. We take advantage of the fact that the data comes with a depth and a surface elevation. In many instances, we’ll turn it into a profile view to make sure that the depths look consistent down the line and there are no spikes or obvious issues. Then that data is delivered to the client.”
New Business from Underground
Kuxhausen said that clients are quick to recognize the value of the integrated technologies. In addition to mapping mandated by the PIPES Act, he is hearing from people both at large and small construction companies. In many cases when they are designing or upgrading roadways, engineers don’t have a lot of leeway on the depth of construction. A utility line may be only three feet underground and the design subgrade needs to maintain a specified clearance above the line.
“It’s very, very important for them to know depths when they’re reconstructing these roadways,” Kuxhausen said. “In a lot of places, clients are forced to do Level A excavation. They may need data every 10 feet (3 m) along the road, which makes for a lot of potholing. It can be very costly. The idea behind this technology is wonderful for them. We are able to use the SPAR system to reduce the frequency of the Level A excavations. We could make a Level A excavation, say, every 50 feet (15 m) and then use the SPAR information in between those Level A excavations. It’s a faster and more cost-efficient approach.”
In addition to road construction, Kuxhausen sees potential clients in a variety of industries. He cites airports as a good example, where operators need accurate data on the complex web of underground pipes, wires and conduits. “We repeatedly receive requests to perform mapping and subsurface utility engineering (SUE) services to support the re-design or relocation of navigation equipment” he said. “For instance, we’ve completed work for airports where they might be deconstructing a control tower or some other site. Our crews will go out and locate the existing utilities, both active and decommissioned so that no lines are damaged when the deconstruction takes place.”
Kuxhausen said additional business comes from highway departments that like to see all the existing utilities in their transportation corridors for design mapping purposes. There has been some specialized work as well. “We’ve also performed locations prior to setting geodetic control marks for the National Geodetic Survey (NGS) or the Federal Aviation Administration (FAA). For a Class C monument, we need to dig a four-foot hole, and a Class B monument is a stainless steel rod driven to refusal. So, the utilities are required to be designated on those sites.” He noted that “Call Before You Dig” location services often might not be responsive on airports or private property, especially for design mapping surveys when construction is not imminent. As a result, it’s important that Woolpert be able to provide SUE services.
At the end of the day, Kuxhausen is looking for productivity and the ability to meet the needs of Woolpert’s clients. “It comes down to how we can streamline our processes,” he said. “We need to ask how can we quickly, safely and accurately locate utility data and have accurate data that could be implemented into the larger infrastructure mass. I think this is a solution for that. Taking all the information that comes out of the SPAR/GNSS system and coupling it with the GIS database and additional attribution could be invaluable for any client that has a large inventory of underground assets.”
There are four generally accepted classification levels of a buried pipeline’s location. The levels reflect decreasing degrees of accuracy and reliability. The most accurate, Level A, requires the pipe to be physically exposed, typically via digging or hydro or pneumatic excavation (a process known as “potholing”). Survey teams can then use GNSS or total stations to measure the 3D position of the exposed pipe accurately.
Techniques for Level B location include using electromagnetic sensors to detect a pipe; the position is marked on the ground using paint or small pin flags that can then be measured by surveyors. Conventional electromagnetic detectors are fast and can reveal a pipe’s location horizontally but they offer little information on its depth. Additionally, surveyors must coordinate with the pipeline location crew so that markings can be measured before they fade or are removed. It’s not unusual for survey crews to make multiple visits to a site to capture needed information.
Level C location is based on visible evidence of underground assets such as manholes, meters, valve covers or utility pedestals. Surveyors can capture accurate location on these features, but the approach provides no data on what is out of sight below ground. Level D locations are based on existing utility plans, as-built drawings or the “tribal knowledge” of someone familiar with the pipeline While this is a common approach, it’s very difficult to assess if the information on older maps is accurate or complete. In particular, data on the depth of a pipeline is subject to question and may be affected by surface changes such as grading, cultivation or paving.
The different location classifications provide a striking illustration of the costs associated with accurate information. While Level A offers the best accuracy, the cost to find and measure pipelines is high, especially when seeking pipelines beneath paved surfaces. Level D location data is essentially free, but comes with low confidence.
Despite the cost and challenges, demand for accurate locations is strong and extends across multiple types of assets. It has opened the door for technological solutions that combine the speed and flexibility of electromagnetic detection with instruments and software for precise positioning and mapping. Companies are putting the technology to work—and keeping it busy.