design the pipeline. Therefore, an approach utilizing an aerial survey in tandem with limited ground survey was crafted to optimize the survey- ing process without compromising data quality. A fixed-wing aircraft was selected to perform photogrammetric and LiDAR flights. The surveying data specifications are listed below: The overall workflow for the surveying approach was as follows:
Just Wing It – Aerial Surveying for Pipeline
Projects By Hunter Hanson, P.E.
Quality site survey data is an integral component of all pipeline design projects, which can become quite costly and time consuming as the scale of the project increases. For Alliance Regional Water Author- ity’s (ARWA) Phase 1B Program, which includes 95 miles of water transmission lines serving several communities in central Texas, aerial surveying helped achieve cost savings and reduce schedule risk. Lockwood, Andrews & Newnam, Inc. (LAN), partnering with Dallas Aerial Surveys (DAS), performed a fixed-wing aerial survey in con- junction with traditional surveying methods on one of the program’s pipeline segments to optimize efficiency. The pipeline segment, called Segment A, involved 17 miles of 48-inch water line and began at the proposed water treatment plant in eastern Caldwell County and termi- nated just south of the City of Lockhart (See Figure 1).
1. Ground survey established control points and set aerial targets – illustrated in figure below 2. Aerial survey performed photogrammetric and LiDAR flights 3. Ground survey collected limited topography of obscured areas 4. The data collected from both aerial and ground survey was compiled and processed 5. Final surveying deliverable was provided to design team
Figure 2 - Project Control & Aerial Target Layout, Image by DAS
Quick Delivery with Enhanced Accuracy A major benefit of an aerial survey is a much quicker turnaround than could be expected from a traditional field crew. For linear projects, a good rule of thumb is that an aerial survey can cover roughly four times as much linear footage per day than a ground crew. For projects across rural, rugged terrain, that production discrepancy can be even more. The Segment A pipeline alignment is almost completely cross-country, in easements on private property across forty parcels. Acquiring rights-of-entry using a traditional approach would have taken at least an additional month. Implementing an aerial survey helped start the process earlier due to reduced right-of-entry. In fact, with a properly designed control plan, an aerial survey can often allow for no right-of- entry needs. As a result, the project survey was delivered to the design team six weeks ahead of schedule. In addition, it saved approximately $120,000. The schedule relief also allowed the design to advance more quickly, reducing overall schedule risk to the owner. Even with such a speedy delivery, accuracy and data quality is not compromised with an aerial survey. Accuracy is approximately be- tween 1:5,000 to 1:8,000 of the height from which the flight is taken. For the relatively narrow Segment A corridor, this translated to topo-
Figure 1 - Phase 1B Program Map, Image by LAN
Project Approach From the conception of the Segment A project, there were multiple challenges with the surveying component. The long, linear pipeline segment is in a rural area and traverses heavily wooded areas. This location, coupled with the rolling hill terrain of central Texas, lead the design team to pursue an aerial survey to collect the design data needed. However, the frequent changes in topography and thick veg- etation triggered concerns that pockets of topography data could be obscured, potentially leaving out an important level of detail needed to
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csengineermag.com
november 2020
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