Subsurface Modeling – Utilities Can't Be Seen… Can They Be Surveyed?

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The accuracy and detail of above-ground surveying continues to be enhanced by satellite-based positioning, and optical technologies like LiDAR and laser scanning. Software is now available to greatly ease the reduction of large amounts of 3D geospatial data into useful digital models. Planners, engineers, architects, and analysts can then easily manipulate these models on a computer, viewing existing infrastructure in the context of a proposed design or redevelopment. With databases of 3D models increasingly available for infrastructure projects, virtual representations are possible early in the planning stage, presented in a context that is readily understandable to all project stakeholders.

This richness of data has revolutionized the standard of care in our profession. Surveying practices and deliverables acceptable in the days before widespread use of GNSS, robotics, and scanning have been displaced by higher expectations, whether by statute or competition.

Below-ground however, today’s expectations are not much different from times past. Accurate modeling of 3D position data from underground utility assets lags what is available from optical and satellite-based positioning systems, and hence 3D model data for such infrastructure is lacking. Standard utility locating equipment often makes simplifying assumptions that enable easy operation, yet limit the attainable precision of the measurement. For example, electromagnetic (EM) pipe and cable locators expect the user to determine the X-Y position of the utility simply by finding the peak of the emitted signal. Then the depth function of the locating instrument is used to determine the vertical (Z) coordinate of the targeted utility. Unfortunately, use of such techniques can be error prone, due to distortions in the emitted field resulting from signal coupling underground. This simple truth has kept surveyors from incorporating the use of such tools into their work.

Other underground location technologies like ground penetrating radar (GPR), radar tomography (RT), and sonde tracing methods do not fare better. The only sure way to capture the exact 3D coordinates of an underground utility is by physically exposing it. In fact, this is formalized as the highest "Quality Level A" for reporting position of underground utility assets, per ASCE C-I 38-02, Standard Guideline for the Collection and Depiction of Existing Subsurface Utility Data.

From the modeling perspective, the relatively high cost of physically exposing utilities limits the number of measurement points that can be reasonably collected, resulting in sparse position sampling and simplistic models—­especially compared to the millions of individual position estimates that compose a laser-scanned point cloud above-ground which leads to richly detailed structural models.

Survey-grade Mapping
Short of exposing the utility, can any underground location method be considered sufficiently quantitative that surveyors can properly document 3D positions? In the above-ground world, real-time kinematic GNSS equipment achieves sub-centimeter accuracy. Surveyors trust this equipment to alert them when it isn’t operating at the highest precision, since every position update is accompanied by the expected horizontal and vertical "root-meansquare" RMS position error. Over time, these metrics have proven effective at detecting conditions that lead to a poor RTK fix, like multipath, reduced satellite SNR, and poor geometry. Now with few exceptions, when GNSS equipment reports a "FIXED" condition, surveyors trust that the error is bounded within known limits.

Recently, a new underground utility mapping method has been developed that permits rapid collection of underground utility infrastructure positions, and also estimates both horizontal and vertical RMS error on each 3D position. The method relies on the long understood physics of electromagnetic locating, but incorporates additional measurement sensors and a fundamentally new processing algorithm based on optimization (see sidebar). This enables the new underground mapping instrument, called a Spar, to operate similarly to a GNSS receiver by incorporating additional sensors, just as the GNSS receiver makes use of additional satellites. The net result is a more confident result, even if field distortion limits the overall accuracy of the position. Accurate mapping is possible for underground pipes and cables that have some metallic content, including plastic pipes delineated by tracer wire. In addition, the model-based method maps underground sondes (point transmitters) in 3D as they are drawn or pushed through concrete, clay, or plastic pipes, tunnels, and conduits.

3D Data Collection
The system was recently used in a modeling project at the University of Nevada – Las Vegas, located about two miles east of the Strip. Over the years, various campus redevelopment projects have resulted in a complex underground environment, with incomplete as-built geospatial records of the installed utility system. In preparation for planned infrastructure improvements, V4, LLC initiated the project at UNLV as part of the company’s "Virtual Vegas Valley" initiative. The manufacturer of the Spar utility mapping system, Optimal Ranging, Inc., cooperated in the project. The goal is to combine above-ground models based on point cloud data with below-ground infrastructure models derived from 3D geospatial data collected by the Spar.

In addition to improved reliability that results from providing an RMS error on each position, the Spar has other unique advantages for the project. For one, the Spar includes the Ashtech MB100 dual-constellation RTK-GNSS board, allowing deep integration of the electromagnetic and survey-grade satellite-based positioning results. Managed by Carlson Software’s SurvPC data collection software, the 3D geospatial position of the utility centerline is automatically computed in real-time by FieldSens View, which acts as a software funnel for position data from the GNSS and locate subsystems. The estimated offset and depth between the above-ground range pole reference and the below-ground utility are incorporated by the new method. The Spar needs only to be in the approximate vicinity of the target to calculate position at up to five times per second, while providing error statistics on every point.

For even more reliability, or to capture the coordinates of especially deep utilities (often a result a horizontal directional drilling operation) two Spars can collaborate on the job site through a wireless network. Not only does the number of sensors that are focused on the targeted utility double, but a more optimal measurement geometry can be achieved. This has the effect of reducing the Dilution of Precision, increasing confidence, and is directly analogous to GNSS systems having improved satellite geometry. The arbitrary 3D separation distance between the two Spars can be automatically measured using a RTK moving baseline approach, which is subsequently used in the measurement processing.

Model Creation
These improvements in the collection of 3D underground utility asset data enable the generation of detailed models beneath ground level. Error bounds are propagated through the data collection process into the model phase, allowing planners to view areas of high position reliability, as well as those areas that are subject to distortion and thus may need supplemental validation­—possibly using ASCE C-I 38-02 Quality Level A processes. To build the model, the 3D utility position is exported from SurvPC according
to the local coordinate system, combined with attributes like pipeline diameter, and a model extruded in a close approximation to the process used for synthesizing an above-ground model from point cloud information. Areas of poor confidence, often resulting from electromagnetic field distortion at intersections with pipeline laterals, are defined visually in the CAD environment. This provides the downstream user of the modeled environment an immediate sense of data quality, in a very intuitive context.

Virtual Site Planning
The 1980’s brought the wholesale conversion of hardcopy as-built maps to 2D CAD systems. In the last decade, exciting new instruments have enabled the collection of a fantastic amount of 3D data, and recent software tools have enabled this data to be efficiently synthesized into models. Now, using these same software tools, the warehouse of models available to the planner isn’t limited to above-ground infrastructure. The Spar, borrowing heavily from long trusted satellite-based surveying methods, helps to bring a quantitative view of what is underground, and where it is to be found—with a measure of confidence. This dovetails nicely with the surveyor’s professional view that accuracy is only valuable when coupled with a statement of precision.

Les MacFarlane is a licensed surveyor in Nevada, and a principal of V4, LLC, based in Las Vegas. Founded in 2010 as an extension of Holman’s of Nevada, V4 is focused on the migration of 3D CAD data to 3D models. The company’s goal is to stimulate virtual visualization of existing above and belowground infrastructure into early planning stages for new and sustainable developments.

Jim Waite is President of Optimal Ranging, Inc. Founded in 2009 and headquartered in Santa Clara, California, the company designs and manufactures precision positioning equipment for use in utility mapping, serving the needs of water, electric and telecommunications utilities, land surveyors, gas and pipeline transmission companies, and municipal public works.

FieldSens Technology

The Spar’s FieldSens technology is based on the optimization of data from many sensors against a physical model of the magnetic field expected from a utility line. The effects of ground conductivity and field distortions are accommodated in the real-time processing. Using two 3D magnetic field sensors, a triaxial accelerometer and digital compass, FieldSens identifies the offset, depth, current, and yaw angle to the underground utility regardless of its position in the radiated field. FieldSens need only be in the approximate vicinity to calculate position with corresponding confidence bounds. In combination with GNSS, automatic geo-positioning can occur even from the side of the actual utility. Beyond the convenience offered by the continuous measurement of line offset and depth from an arbitrary point in the field, the method effectively deals with field distortion, one of the biggest problems of conventional electromagnetic line location.

A 1.466Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE