Roadmap for transmission line siting

Bob Houston, GAI Consultants

The state of practice in transmission line routing has significantly evolved since many state commissions began adopting siting regulations in the late 1970s. Changes are the result not only of the advancement of technology, but also the heightening of public awareness regarding electromagnetic field (EMF) issues and increasing regulatory requirements.

The first and most important factor in the siting process is defining the project team’s mission and primary and secondary objectives to support the mission (see figure).

Study area sensitivity mapping

With the appropriate input factors (regulatory, engineering, economic, system, and societal parameters) and mission established, the study team is positioned to define the study area and inventory resources. The area should be large enough to permit the siting team to route a sufficient number of independent alternative corridors for later evaluation.

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The area resources inventoried are selected based upon criteria established in conjunction with the process input factors (e.g., state utility commission regulations, utility siting policy, issues of public concern). Resources such as designated natural areas, designated scenic areas, historic areas, and airports are usually identified in the state commission regulations as protected resources. Existing HV lines are also resources to be avoided due to potential conflicts with existing power systems or system reliability issues (e.g., having two many strategic lines in the same or a nearby right-of-way). Not only do most commission regulations discourage conflict with existing land use, but current public opinion stemming from EMF and land value concerns make urban areas sensitive locations.

The table presents a partial list of resource constraints that should be included on the study area sensitivity mapping. These resources must be selected on the basis of the ability to reliably identify the location of each resource and the ability to obtain equivalent and comparable levels of data for each resource in this macro-detail phase of the study. The use of geographic information systems (GIS) facilitates the accurate integration of data into the study at various scales and the adoption of a robust project data set.

Alternative corridor siting and evaluation

Alternative corridor siting must be conducted in relation to the study area sensitivity mapping utilizing a concise set of siting criteria which are established through the project input factors. Siting criteria include both siting opportunities (e.g., parallel existing HV lines, rail roads, open terrain) and siting constraints (e.g., state parks, scenic and recreation areas, cultural resources). Siting opportunities are locations representing land use and resources that are compatible with safe, economical, and reliable construction and operation of an HV transmission line. Constraints represent locations where a transmission line might have a potentially adverse impact on sensitive resources or locations where conditions might affect reliable and safe operation or economical construction of the line.

Alternative corridor routing can be accomplished using map overlays or a GIS approach. Siting is often accomplished in discrete segments with multiple interconnections forming a network or web of potential alternatives across the study area. Alternative corridors should be assembled to minimize distances between the corridor end points and to minimize corridor overlap.

The minimization of distances (short, direct routes) is supported by the following assumptions:

“- Longer corridors, in general, remove more land from established or historic use;

“- Normally, more property owners are affected as more land is acquired for a right-of-way;

“- The greater the length of a corridor, the greater the potential for impacting any particular siting constraint;

“- Increased construction, operation and maintenance are required for longer corridors; and

“- Lower power transfer efficiency for longer corridors translates into higher energy costs to the consumer.

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The next siting step is to evaluate or compare the suitability of the alternative corridors. Once again, the project input factors govern the criteria selected to evaluate the alternatives. The evaluation criteria most often utilized are sensitive resources potentially impacted by the transmission line (natural areas, wild and scenic rivers, hiking trails, highway and river crossings, urban areas, historic sites). Many of the evaluation criteria are resources identified on initial constraints mapping. However, with the identification of alternative corridors, project mapping and aerial photography should be performed at scales enabling the project team to inventory data at much greater levels of detail allowing resources such as historic structures, and rare and endangered species habitats to be accurately mapped and added to the evaluation criteria.

Since criteria are not equally important in selecting between corridors, especially as perceived by the public, weights can be set for each criterion. Any siting study must trade off conflicting objectives. These can only be traded off using social values, covering such things as, “It is a net improvement to change the corridor such that we reduce the number of wetland acres impacted, if, in so doing, we increase the number of historic structures within the viewshed?” Those tradeoffs cannot be found in environmental or science text books. They are expressions of social values.

Houston is a 25-year veteran of the transmission line industry and has conducted routing, licensing, and permitting for more than 5,000 miles of line throughout his career. He is the environmental studies and planning department manager at GAI Consultants, Inc., a multi-disciplinary engineering firm with offices throughout the mid-Atlantic United States. He can be contacted directly at 1-800-437-2150 or More information on GAI can be found online at

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