Model-Based Deep Line Utility Survey

By John W. Jaeger, Senior Project Manager, Binkley and Barfield Inc. | April 2014, Vol. 69 No. 4

During field investigations, BBI found substantial utilities at varying depths, crossing and running parallel to the gas line. Direct connection via a test station was successful and the transmitted signal could be picked up through an area of interest but stopped at an electric tower. Direct connection to a test station on the other side of the project was successful but the signal again stopped at the same electric tower (which was now between the test station and the area of interest). Unlike the other projects, this forced the use of a single direction in performing the designation.
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During data collection, it was noted that the Dual Spar was showing that many locations did not have a good confidence level due to presence of other utilities in the same right-of-way. This required an adjustment to the locations of both the Base and Rover Spars, and the separation between spars. Other challenges involved the existing highway overpass shadowing the GNSS satellite signals from the Spar units, which require GPS time synchronization in Dual-Spar mode. This was quickly solved by placing “Spar points” with a total robotic station and pole mounted prism and entering them into the Trimble TSC3 Data Collector. All the collected data was processed to render an output in the coordinate system used by the overpass designers and the smart pig. The resulting line very closely followed the smart pig and showed that the 50-foot test hole had only missed exposing the gas line by a few feet. From mobilization to the site, collection of and processing data and issuing a report was three working days.

Remote sensing has always been seen as the safest, lowest cost, and most convenient way to find utilities. Unfortunately reliable elevation data is not attainable with conventional tools. The Dual Spar reports geospatial elevations of the targeted utility, and renders vertical accuracy which has not been previously realized by existing electromagnetic locating equipment. Should this be considered a replacement for performing test holes, which can cost thousands of dollars (or much more, depending on depth)? Where some variation in the location is acceptable, the answer is yes. For those occasions where absolute location is necessary, the answer is perhaps not.