Feature: By Air and Ground
In the rugged mountains of Maine, aerial mapping supplements ground surveying to maximize profits on wind farm projects.
By Patrick Graham
As the demand for renewable energy increases worldwide, wind energy companies are challenged to develop clean, reliable sources of wind power with maximum profitability. Developers recognize the enormous opportunity that exists in the northeastern United States, particularly in the state of Maine, which leads New England in new wind farm construction. Maine’s mountains offer abundant wind resources, proximity to transmission lines, and low population density.
But they also comprise some of the toughest terrain east of the Mississippi. Steep slopes, rocky soil, wetlands, and extremes of weather create unique survey, design, and construction challenges that can profoundly affect project cost. Wind project experience in these mountains, however, shows that accurate aerial mapping reduces the amount of field survey required and streamlines the civil design process, saving time, effort, and money. Given the complexity of wind projects in general, with multiple phases, teams of contractors, and long permitting lead times, investment in aerial photography of the site can prove critical to a project’s overall profitability.
Two scenarios for wind projects undertaken in Maine’s mountains give compelling evidence of the value of flying accurate aerial photography early in the development lifecycle. For the first project that runs along the ridgeline of Kibby Mountain and Kibby Range in Kibby Township, engineering consultant James W. Sewall Company
used existing aerial photography to map and design the site. For the second project, on Stewart Mountain and nearby ridgelines in Highland Plantation, Sewall flew new photography at 1”=600’ scale for developing 2-foot topographic contour mapping of the site and transmission line. Still under development, both projects hold promise for success. The acquisition of new aerial photography for the second one, however, has significantly increased its profit potential.
Kibby Mountain Project: Old Photography
Sewall began work on the Kibby Mountain Wind Power Project in early 2008, providing mapping and civil design services for the prime contractor Reed & Reed
of Woolwich, Maine and the project developer TransCanada
of Calgary, Alberta. The $320-million, 132-megawatt project includes 44 Vestas V90 3.0 MW wind turbines being erected atop two ridgelines. The first 22 turbines are on schedule to begin producing power in October 2009 with the remaining scheduled for erection in 2010. The project also includes design and construction of over 36 miles of access and ridgeline roads.
Aggressive final design and permitting schedule requirements for the project drove the decision to map the extensive project development area using existing aerial photography from Sewall archives. Flown for the landowner in 2005 under leaf-on conditions, the photography was captured for cover type mapping of tree canopy. As a result, the photography was not optimal for use in civil design projects, which generally require 1”=100’ scale mapping with 2-foot contours. Typical of photography for cover type, an airborne GPS was used with no on-ground survey control, and the photo scale (1”=1320’) allowed production of 5-foot topographic contours at best, not meeting National Map Accuracy Standards (NMAS).
To address the challenges of using the existing photography, Sewall worked with Reed & Reed and TransCanada to develop a plan for additional on-site control that would increase map accuracy for producing the requisite 2-foot contours. The additional on-site control consisted of using on-the-ground survey with GPS and conventional survey equipment to establish and collect elevation data along the centerlines of the proposed ridgeline roads. Additionally, the surveyors collected elevations at the proposed turbine pad locations and at spot locations along the edges of the roads. This intense field survey effort, completed over a period of two-and-a-half months, required five survey field crews provided by Sackett & Brake Survey
of Madison, Maine. The field survey conditions, consisting of rough, overgrown, and unharvested timberland above 2,700 feet in elevation, were harsh and difficult, slowing the process.
The field survey data, collected and provided to Sewall on a weekly basis, was used to update the accuracy of the mapping. As a result, several updates were made to the existing-conditions topographic base map during the course of the civil design process. The benefit of this process was a continually refined map that provided for a more accurate civil design. Refinements to the base map required several additional design iterations, however, to accommodate upgrades in the mapping, again adding time and cost.
Highland Plantation Wind Project: New Photography
Sewall initiated mapping and preliminary civil design work on the $150-200-million, 120-140-MW Highland Plantation Wind Project in 2008, working with the project developer Independence Wind of Brunswick, Maine. The development of accurate topographic mapping was crucial to civil and electrical design along the ridgelines and transmission line corridor, which would interconnect with the regional grid.
Typical of wind energy development, the Highland Plantation project had aggressive and compressed design and permitting schedules. In contrast to the Kibby Mountain project, however, the Highland Plantation aerial photography was planned early and executed specifically to provide an accurate topographic and planimetric base map of existing conditions for use in civil design. As a result, the flying height and photographic scale were selected to provide 1”=100’ scale mapping with 2-foot contours. In addition, the aerial photography was captured during leaf-off conditions in fall 2008 using airborne GPS, an inertial measurement unit (IMU), and ground control consisting of a survey network with targets placed and captured in the aerial photography.
The resulting aerial photography for the Highland Plantation project was ideally suited for photogrammetric mapping. The established ground control, airborne GPS, and IMU data provided sufficient control to meet NMAS as well as the project mapping scale and contour interval accuracy requirements for design without the need for costly additional on-site control. Although the acquisition of new photography required an additional investment upfront, as opposed to the use of existing photography, the resulting topographic mapping was completed quickly with no need for refinements, resulting in lower photogrammetry cost and fewer civil design iterations. Overall, the Highland Plantation mapping costs were one-half those for the Kibby Mountain Project.
Benefits of New Aerial Mapping
The use of aerial mapping on wind energy projects proves particularly beneficial due to the aerial scale of these projects and the typical elevations in the rugged terrain of Maine and the Northeast. Such conditions affect the efficiency and effectiveness of field survey team operations. When properly planned and executed, the acquisition of new aerial mapping reduces overall project survey costs, provides civil designers with accurate and usable data in a shorter timeframe, and provides a more accurate base map, reducing the number of civil engineering design iterations. Ultimately, the gain to the project developer is lower overall mapping, survey, and engineering costs.
The key to a successful aerial mapping effort for wind projects, as with most large-scale and high-budget projects, is project planning and scheduling. In many parts of the country, including the northeastern United States, specific flying windows in the spring and fall dictate the time to capture leaf-off aerial photography with no snow cover on the ground. As a result, it is important to work with the aerial mapping consultant to schedule aerial photography and subsequent mapping work to coincide with optimum photographic conditions as well as the overall project schedule.
An additional key component of planning aerial mapping is specifying the flying height, photographic scale, and type of photography (e.g., black and white, color, false-color infrared) to match the ultimate use of the base map. Further, with proper planning and placement of survey control targets, the use of airborne GPS and IMU can reduce the scope of the field survey control effort and increase the overall accuracy of the mapping. Finally, planning and coordination of the aerial mapping with the overall project design and permitting efforts can add value, such as using color orthophotography for site feasibility simulations or color or false-color infrared imagery for wetlands delineation, habitat identification, and environmental permitting.
A licensed professional engineer, Patrick Graham is director of project development for renewable energy services at James W. Sewall Company, headquartered in Old Town, Maine. He has worked the past three years as a project manager for civil engineering and GIS consulting projects related to commercial-scale wind farm development.
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