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 Authority’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 conjunction 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 terminated just south of the City of Lockhart (See Figure 1).
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 vegetation triggered concerns that pockets of topography data could be obscured, potentially leaving out an important level of detail needed to design the pipeline. Therefore, an approach utilizing an aerial survey in tandem with limited ground survey was crafted to optimize the surveying 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:
- Ground survey established control points and set aerial targets – illustrated in figure below
- Aerial survey performed photogrammetric and LiDAR flights
- Ground survey collected limited topography of obscured areas
- The data collected from both aerial and ground survey was compiled and processed
- Final surveying deliverable was provided to design team
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 between 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 topographic data accurate within an inch, with the exception of a few obscured areas, which were corrected by limited ground survey to give the full picture of the surface. As you can see from the figure below, when comparing final data to readily available online LiDAR data for the area, discrepancies of more than seven feet are apparent, with a clear level of definition missing at creeks. For critical areas such as the Plum Creek crossing, a 52-mile stream that flows southeast through Caldwell County, this level of detail from the aerial survey allowed for a drastically improved design and analysis to select the best construction methods.
An additional benefit of using aerial survey for the Segment A project was the added flexibility for the designer. Due to the wider swath of survey area than would typically be scoped out for a ground survey, design features could be shifted significantly without additional field work. In all pipeline projects, the horizontal pipe alignment is typically adjusted as the design progresses. Being able to make these changes without the headache and schedule delay of additional survey work can be critical to keeping a project on track.
With an aerial survey, it is even possible to obtain data outside the scope of the original survey width without getting more field work. In the aerial flight, a much wider area is measured and collected but is not processed due to longer production times and distractions of extraneous data. However, a designer can request the aerial surveyor to process additional areas of data with minimal effort if the area is within the limits of the original flight. For the Segment A project, more than 500 linear feet of the alignment was adjusted outside of the original 400-foot wide survey corridor. LAN coordinated the additional area needed with the surveyor and receive an updated survey file within 24 hours of the request. The additional area was processed at the same resolution as the original data with no added field work. A similar field work effort by a ground surveyor would have taken at least two more weeks and come at an additional cost to the owner.
Challenges and Limitations
As with any technology, there are some limitations to aerial survey. Aerial survey is at its best with clear, sunny days on relatively flat ground with sparse vegetation. Cloudy weather, abrupt changes in topography, and thick vegetation can all attribute to obscured areas in the data.
Also, depending on scale and other site characteristics, aerial survey is not going to be the most cost-effective approach for every project. For a relatively small project site that a traditional survey could complete in short order, aerial survey may not be cost-effective. Similarly, a site mostly covered in large, dense tree canopies may cause significant obscurity in the data, which would need to be corrected by ground survey, resulting in additional costs to the owner.
Aerial surveying can provide great value for design projects, achieve cost savings and offer schedule relief without compromising the quality of surveying data. Collaboration between surveyors and the design team during the planning phase of projects can lead to an optimal solution to identify the best methods and scope for the particular site and project. For a long linear project such as ARWA’s Segment A, aerial surveying has proven to be a great fit.
Hunter B. Hanson, P.E., is a hydraulic engineer at Lockwood, Andrews & Newnam, Inc. (LAN), a national planning, engineering and program management firm. He can be reached at HBHanson@lan-inc.com.