PG&E enhances long-term planning by capturing statistically valid tree inventory along 6,800 miles of critical NERC-regulated rights-of-way.
Among utility arborists, there is a popular saying: “If you can't measure it, you can't manage it.” For Pacific Gas & Electric Co.'s (PG&E) vegetation management team, this concept is familiar ground as California's regulations regarding vegetation and power line clearances have historically been very strict compared to other regions throughout North America. So in light of the recent endorsement of additional North American Electric Reliability Corporation (NERC) reliability regulations, it would seem PG&E is ahead of the curve and, for the most part, this is true.
Over the last decade, PG&E has adapted to an annual patrol to maintain compliance with regulations associated with vegetation management. However, maintaining compliance is not always the same as ensuring reliability, especially when considering the volume of potentially incompatible vegetation under 6,800 miles of critical transmission lines crossing varied terrain and cover types. PG&E's team had to get a better idea of how many trees were within the active rights-of-way (ROW), not just the volume of trees that had been worked and inventoried during the annual patrols for compliance.
A wide range of options are available for capturing an inventory of vegetation, ranging from ground patrol to remote-sensing technologies, old and new. In this case, PG&E needed a method that could be safely and consistently implemented in a relatively short period of time, with controlled costs and minimal impact to customers. What the utility decided on was really a blend of several techniques.
In June 2008, PG&E awarded a contract to Davey Resource Group and a partnering remote-sensing firm. The methodology selected for the project was based on four separate initiatives that would ultimately build one final deliverable: a complete inventory of trees in the active ROW categorized by PG&E area and standardized habitat types.
The first initiative resulted in the creation of a geographic information system (GIS) layer of transmission line locations that was corrected to match up with aerial photographs of 1-meter resolution. Second, the active ROW layer was developed using existing easement width information and the new corrected transmission line layer. The third initiative was to use laser range finders to gather a statistical field sample of 190 vegetation plots representing each vegetation type present under PG&E's NERC-critical transmission system.
Upon completion of these three tasks, the remote-sensing firm was able to use the same 1-meter-resolution aerial photographs and its automated feature-extracting GIS tools to capture a statistically valid inventory of the trees within the ROW. Sounds pretty simple and straightforward, right? The truth is, this was a complicated endeavor that required the utility to create a solid set of GIS tools and layers before it could begin. This type of project also required adequate time to complete each task and plenty of communication between the client and vendors along the way.
Using GIS Tools
GIS provides a valuable tool to the utility industry. Combining the deliverables of this project with existing GIS data, PG&E was able to update and improve the accuracy and usability of its data related to NERC-critical transmission lines, towers and ROW. These upgrades where achieved using proven techniques in a new way.
Remote-sensing analysis was used to extract vegetation information from high-resolution imagery, which was applied to transmission line corridors to determine vegetation densities. Contractors were provided 1-meter National Agricultural Imagery Program data. This data has a horizontal accuracy of within 6 meters, meaning that objects seen on the image are within 6 meters of actual on-the-ground location. The imagery was used for the remote-sensing process to extract vegetation boundaries for unique clusters of trees and other vegetation.
This analysis was completed for each habitat classification identified within the NERC-critical active ROW. For each habitat, an appropriate statistical sampling was performed in the field using GIS-enabled pen-based computers, a very efficient and accurate way of classifying the entire NERC-critical system. Classification was completed within about two months.
This process required Davey Resource Group to digitize roughly 6,000 transmission, tower and structure maps in a process that allowed it to actually line up the transmission line and tower locations with the aerial imagery. From this data, easement lengths on the left and right sides of each line were assigned, and buffers were created to come up with a single service territorywide layer of NERC easements. With this newly created “supereasement” layer, GIS analysis could be performed to answer many of the questions the vegetation management group had.
PG&E used laser range finders for the field sampling portion of the project. This involved roughly 190 sample plots where representative sample trees were measured using the range finders. The scope of this project was to capture a statistically valid inventory of the NERC ROW for trees underneath, adjacent and outside of the ROW. This project supplemented PG&E's annual patrol, which primarily focuses on encroachment, compliance and reliability.
Refining the Deliverables
Combining basic GIS digitizing with the more-sophisticated remote-sensing technology produced data that has been invaluable in managing and planning vegetation management activities in a vast service territory. The deliverables provided to PG&E by the contractor enabled the utility's in-house GIS team to run specialized queries with other GIS data, including the California Natural Diversity Database, coastal zone, population density and more. Spatial queries are a GIS analysis that create new data by combining existing data. Here are some sample questions spatial queries can answer:
How many miles of NERC lines are within the California coastal zone?
How acres of critical U.S. Fish and Wildlife Service habitat occur within the supereasement layer?
How many line miles occur in high-, medium- and low-population densities?
Lessons Learned and Limitations
These types of questions can be answered by overlaying GIS data layers to produce new data layers in formats that can be exported and then consumed by other applications such as Microsoft Access or Excel.
Having been provided shape files that could be integrated easily into its internal GIS systems, the utility was able to quickly put this information to work. Post-processing work to create a seamless service territorywide layer was done for both the supereasement and line-by-line mile layers. Once this process was complete, the data layer could be uploaded to a systemwide server and displayed for PG&E vegetation management employees and contractors. These readily available tools now support project-specific planning and communication efforts by providing information in both tabular and visual formats.
The Five-Year Plan
Using existing technologies in a new framework can sometimes feel like starting from scratch, especially when failure is not an option. And with most technical projects, some complications are encountered along the way, and this project was no exception. Pressure to deliver in a short period of time, communicating complex issues to a range of stakeholders with varied technical backgrounds, and figuring out the possible and practical solutions really explains most of the challenges encountered. Following are some examples of the issues PG&E had to contend with:
- Refining the scope
The scope of this project was to identify all of the trees in the active ROW that would require management within the next five years, but the tools that were being used did not necessarily make this possible. And the variety of habitat types and topography involved made this particularly difficult when the actual height of the transmission lines was also variable, based on the location, weather and load conditions.
Given these facts, the final delivery was a more-conservative inventory of trees requiring management than originally expected. As it turned out, using this data to develop and prioritize a five-year management plan for ROW reclamation did breed productive results. Using this new information combined with the data captured during the routine annual patrols proved useful when analyzing the density of the vegetation compared to the compatibility of the vegetation. In several cases, some of the higher-density habitat types also indicated the highest compatibility of the vegetation involved.
- Analyzing topographic exclusions
The terrain and maximum mature tree height play a very significant role in interpretation of data. If there is a tree in the bottom of a canyon, 200 feet below the transmission lines, it is likely the tree will never need to be managed for reliability. To develop a solution to these additional topographic exclusions, analysis was needed. To accomplish this, a digital elevation model (DEM) was used to create a percent slope raster that helped to identify anywhere the terrain dropped more than 30 meters between structures and excluded the tree inventory where this occurred.
- Determining the number of trees
There were several perceptions of the actual inventory of trees in PG&E's NERC-critical system. The utility knew how many trees where managed each year for compliance and reliability, but that did not always involve every tree in the active ROW. And given the intensity of work completed each year during routine cycles, using aerial imagery up to three years old was a limitation that needed to be acknowledged before any work could begin. What this project did very well was indicate the density of trees in each area and habitat type. Comparing this information across the service territory, using the existing annual patrol data and local expertise, truly helped PG&E develop a prioritized five-year plan.
- Coming up with a cost-effective approach
Surprisingly, given the wide range of options for this type of inventory project, few remote technologies can actually count trees in a cost-effective manner. The contract cost for these deliveries totaled just under $40 per line mile. The time and resources dedicated by PG&E's transmission team and GIS department have added roughly $10 more per line mile to use this data for project planning and creation of a comprehensive five-year plan. So, a conservative estimate of overall cost totals about $50 per line mile. Compared to other methods available, this approach was a logical and cost-effective one, but it did require an existing GIS system to be in place, and a staff ready and willing to work through a value-added process to ensure a complete end product.
The data generated from this project was supplemented with other readily available information to develop a concise five-year plan. This plan serves as a detailed schedule, as well as a financial planning and communications tool. Population density, environmental limitations, local and regional politics, and many other factors needed to be considered. For the ROW reclamation efforts to be successful, PG&E will have to continue focusing on all of these factors. With solid GIS tools and a concise five-year plan, the utility has already begun to use these new tools with positive results.
Tyson McCartney (firstname.lastname@example.org) is a forester with Pacific Gas and Electric Co.'s vegetation management department. He is an ISA-certified arborist and utility specialist with a bachelor's degree in forestry and natural resource management from California Polytechnic University, San Luis Obispo.
Brandon Oberbauer (email@example.com) is a senior geographic information systems analyst with Pacific Gas and Electric Co. With more than 10 years of experience, his current responsibilities include, but are not limited to, converting, querying, analyzing and overlaying GIS vector and raster data. He has been with PG&E since 2005.