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The deliverables provided by Sewall will form a basis for our planning, engineering, and environmental support for future projects to increase the safety and reliability of our electric transmission corridors in the Greater Rhode Island area.
David J. Aho, PLS, LLS, Land Survey Coordinator--New England
Land Surveying Department, National Grid




Digital Aerial Acquisition and Mapping of Electric Transmission Line
National Grid

In fall 2009, National Grid commissioned Sewall to fly and map a portion of its electric transmission line corridor in Rhode Island and Massachusetts. The firm, an international investor-owned gas and electric utility that delivers electricity to 3.3 million customers in Massachusetts, New Hampshire, New York, and Rhode Island, needed to acquire new photography for 1"=40' scale planimetric and 2-foot topographic mapping, plus 0.25-foot pixel resolution orthophotography, of the 30-mile corridor from Jepson Substation to Brayton Point. The deliverables--aerial imagery of the entire route and mapping and orthos for five out of six substations along the route--were key to National Grid's planning and engineering of future electric transmission projects in the region.

Sewall, a prequalified vendor with National Grid for aerial photography and ground surveys, was the consultant of choice based on previous successful projects with National Grid and our ability to deliver a broad range of services to National Grid's service territory.

At project launch, Sewall captured digital imagery of the entire project area at 7.5 centimeter GSD resolution, using our recently purchased DiMAC WiDE large format digital aerial camera. Sewall also established control to support the 1"=40' mapping and, as requested, developed the mapping and orthophotography for the corridor and for the Brayton Point, Somerset, Bell Rock, Dexter Street, and Jepson substations.

Use of the Sewall's large format digital camera produced several benefits. First, the digital camera acquired brilliant, crisp imagery with excellent detail in the shadows as well as bright areas. Second, the clarity of the imagery was especially beneficial during the compilation of planimetric features, specifically of transmission line wires and catenaries. Third, without the film processing or scanning requirements of an analog solution, delivery of the project as a whole was expedited.

National Grid is now using the deliverables to assess, maintain, and increase the safety and reliability of the electric transmission corridor in the project area.



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