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 ob- scurity in the data, which would need to be corrected by ground survey, resulting in additional costs to the owner. Conclusion Aerial surveying can provide great value for design projects, achieve cost savings and offer schedule relief without compromising the qual- ity of surveying data. Collaboration between surveyors and the design team during the planning phase of projects can lead to an optimal solu- tion 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.
Figure 3 - Plum Creek Crossing, Image by LAN
graphic data accurate within an inch, with the exception of a few ob- scured 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. Flexibility 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
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.
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november 2020
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