In this article I will primarily focus on Transmission Vegetation Management (TVM) Programs (TVMP) as a driver for transmission line operators acquiring LIDAR data. While the data collected for line rerating and TVM are similar, more data must be collected for TVM work.
LIDAR as a base data source for TVM is a concept not well understood by the vegetation management folks within transmission line operators. Data input for TVMP has been historically captured by air patrols (flying over the lines and making visual observations) and ground patrols using visual cues and hand-held range finders. Thus you must be careful not to assume that the line operator will provide you with the detailed requirements for a LIDAR data acquisition. For example, while under NERC rules the transmission operator is only responsible for fall-ins from within the Active Right-of-Way (ROW), they are very definitely interested (as a matter of best practice) in identifying fall-ins from outside the active ROW. Thus a literal interpretation of the requirements may have you collecting data in too narrow a corridor.
It is critically important to note that NERC requirements are dramatically changing the methods that must be used in detecting vegetation hazards. In fact, under NERC Standard FAC-003-02 Technical Reference, it is quite clear that line modeling under full rated conditions as well as wind swing up to fresh gale (winds up to 39 mph) will have to be performed. This means that simple danger tree interference algorithms in software applications such as TerraScan will not be adequate for Minimum Vegetation Clearance Distance (MVCD) computations.
This is because these types of algorithms are static, meaning they do not account for wire sag due to current/temperature nor do they consider how clearances change as the wire moves in the wind. This modeling can be somewhat complex. For example, consider the image accompanying this article (courtesy of Surveying and Mapping Inc. of Austin, Texas). The top wire (phase) will swing at the bottom of the insulators whereas the side phases will swing relative to two pivot points (both top and bottom of the insulators). Thus it should be apparent that the MVCDs will have to be computed with algorithms that provide dynamic modeling. The most popular modeling software for this application (as well as rerating) is PLS-CADD from Power Line Systems (www.powline.com).
This is the fourth installment of our series on applying LIDAR to the disciplines of transmission line rerating and vegetation management. In the three previous articles we laid the foundation for the regulatory drivers as well as the opportunity afforded to LIDAR operators.
In many cases, the LIDAR acquisition company will typically process the LIDAR data to the various classes needed in modeling bare earth, vegetation (perhaps sub-classified an low, medium, high), wires, towers and perhaps other structures. Additional information critical to the process is the delineation of the Right of Way (ROW). This will have to be supplied by the customer since it is a legal, survey definition. The data from LIDAR processing are then passed on to an engineering department (or contractor) who performs the dynamic wire modeling and computes the MVCDs.
Today, most transmission operators do not do integrated line rerating/vegetation management. These functions are typically in separate departments. This implies that, at least in the near term, outputs from engineering products such as PLS-CADD will have to be converted into informational structures that can be used within the existing Transmission Vegetation Management Program (TVMP) of the customer. Remember, the ultimate goal is for the transmission owner to be able to plan vegetation management prescriptions. Thus the format of useful data might be something as simple as textual records in an Excel spreadsheet.
The lurking danger to all of us LIDAR enthusiasts is engaging with a transmission operator without fully understanding exactly the data informational content that the operator will need to make immediate, productive use of the derived information. I think it is safe to say that virtually no transmission operator today could make direct use of a LIDAR data deliverable for their existing TVMP. If we are all very careful and fully understand the clients needs (and each will be different!) then the widespread use of LIDAR for TVM will become a reality. Without this careful matching of deliverables to the clients needs, we could have LIDAR rapidly earning a reputation of an unusable data source.