Technology has impacted everything we do every day. One of the more overlooked impacts of technology in the T&D industry is how transmission lines are routed today. We used to put graphic tape on aerial photographs or U.S. Geological Survey (USGS) quad maps, but now we have powerful and ever-evolving geographic information systems (GIS) and other mapping technologies.

The science of transmission line routing is used to identify feasible routes that are economical, and to provide a reasonable balance between impacts on the social and natural environment. The first step in identifying a transmission line route is the data-collection phase, where existing constraints are documented and mapped. Using the collected information, a variety of routes may then be identified that avoid or minimize impacts to the constraints. Generally, the routes are presented to the public to provide an opportunity for stakeholders to see what the project would look like and to provide comments and concerns about the project. Once public input is received, the utility typically evaluates all alternatives and then identifies a preferred route, which may then be presented to a siting council or utility commission for approval.

ESRI's ArcGIS has made connecting a global positioning system (GPS) to a GIS map a relatively simple process. The program has an integrated toolbar (called GPS appropriately enough) that provides the necessary functionality to connect the GPS, project your “live” location on the map, and record your path of travel so there is a record of where you have been. The toolbar also allows the user to automatically scroll and zoom the screen to the live location with the click of a button and to record specific destination points.

The ArcGIS function for recording destination points does not provide the detail needed to record the many different features and constraints that may exist within a project area. As a result, Burns & McDonnell GIS specialists created a toolbar, called the Field Feature Attributor, to allow quick entry of typical constraints and structures into a GIS system. The toolbar allows the user to click anywhere on a map and quickly identify the type of feature, and then provide any descriptive notes associated with the feature. This toolbar significantly increases efficiency over using the ArcGIS Editor interface, which requires several different steps to record the same information and does not automatically save the data. Minimizing steps is crucial when conducting field surveys, especially when flying the routes.

The advantages of using this system are many. The GPS allows you to keep constant track of where you are in a study area, which can shorten field time by minimizing turnarounds. It is especially useful when flying a route, because the route can be easily followed even when there are no identifying features on the ground. Using the GPS toolbar and Field Feature Attributor simplifies the data-entry process in many ways. The data is immediately input into the system while in the field. The need for cumbersome, large-scale maps is eliminated. The results are more accurate and readable than pen marks on a map. There is a permanent record of where you have been for future reference. And, the system can be used just about anywhere there is satellite coverage.

The disadvantages to using this system are relatively few. A laptop, GPS unit, batteries or power cords that can be plugged into a car, helicopter or plane battery are needed. While this might be considered cumbersome, it is offset by eliminating the need to carry large rolls of maps. Even with these limitations, the benefits of using GIS and GPS systems together far outweigh the disadvantages for most projects.

While graphic tape has almost completely vanished from the routing scene, there are still many utilities and consultants who develop routes manually, using a GIS system to display the routes. In these cases, routes are identified using expert interpretation and judgment. In some cases, this process may be preferred over a more technological alternative, because it is relatively quick and inexpensive, and is not dependent on the availability of digital data. However, the decision process may be more difficult to explain to a utility commission or the public, because there is no standard process and the inherent subjectivity is relatively apparent.

One method being used more frequently to identify transmission line routes is a grid-based GIS program. This program automatically identifies corridors or routes based on a given set of inputs and constraints, and can be used to evaluate differences between routes. Although this process has been around in various forms for about a decade, it is being used more frequently now for transmission line routing. One reason it has not been used is that it is dependent upon extensive digital data that represent the issues or features of concern within each project area. Since the program was developed, federal, state and local jurisdictions have been digitizing more information and making it available online, thus lowering the cost of data input. As digital data becomes more readily available and economical, this grid-based process may become the transmission line routing standard. This grid-based process provides better documentation of the decision-making process, is quantitative and makes subjectivity (which is still needed) less apparent. These features may result in a more defensible process and could help expedite project approvals.

There are several steps involved in this GIS-based routing process. First, as with the traditional routing method, issues of greatest concern for a project area must be identified for inclusion in the model. These issues would be a combination of environmental and social impacts. For example, avoiding houses and sensitive areas, and maximizing lengths along existing transmission lines are issues that may be included in the model. These features must be collected from existing sources or digitized (Fig. 1). Each feature is typically called a “layer” within the model.

The second step is to derive the sensitivity levels to be used for each layer and then calibrate each of them to the same scale. For instance, to derive the impact to residences, a value must be assigned to each pixel around an individual house or cluster of houses to indicate whether the distance from the feature is considered a high, medium or low impact.

For example, a distance of 50 ft (15 m) from a house might be assigned a rating of 5 for the greatest impact, while a distance greater than 500 ft (152 m) might be assigned a rating of 1 for the least impact. A similar process would be completed for the remaining layers in the model, assigning each pixel around features a value between 1 and 5, or whatever scale is selected. Figure 1 shows the raw digital data for the houses, roads and sensitive areas; each layer's uncalibrated sensitivity grid; and the sensitivities for each layer calibrated to an identical scale. Involving the public to help define these values can be critical to receiving public acceptance and future approval.

The third step is to combine each layer's sensitivity grid into an average cost map, the cost being the relative merit for locating the line at any location within the project area given the identified constraints. Figure 2 depicts the average “cost” for each grid within the area defined by the houses, sensitive areas, and roads layers digitized and calibrated in Fig. 1. Each layer is assigned a relative weighting that defines the overall importance or impact that layer may have to the proposed route.

For instance, proximity to houses may be considered more important than avoiding a sensitive area. In this case, the weight on the layer depicting residential impacts would be greater than the layer depicting the distance from sensitive areas.