LIDAR for Electric Transmission Modeling Part V

This is the fifth installment of our series on applying LIDAR to the disciplines of transmission line rerating and vegetation management. In the four previous articles, we laid the foundation for the regulatory drivers as well as the opportunity afforded to LIDAR operators. In the last article I cautioned that we have to ensure that we fully understand the needs of the client and tailor our deliveries to fulfill those needs. I want to devote this installment to the discussion of requirements and the need for education on both sides of the equation – LIDAR derived information provider and data consumer.

We (GeoCue is a tools and training provider but I will speak with your voice, the LIDAR data services/engineering data provider) very much consider ourselves as a profession rather than simply providers of a commoditized product or service. In my opinion, this puts us under a very different set of responsibilities when interacting with our clients. When simply providing a commoditized service to a customer, the onus is on the buyer to ensure that they purchase what they need (caveat emptor). However, when a client is requesting professional services, that client relies upon us to provide domain expertise and guidance, even before the project is awarded.

I was recently reviewing a Request for Proposal (RFP) from a transmission line operator who was seeking services for both line rerating and vegetation management. This was a very thorough RFP that was very specific in providing not only the deliverables but also (and perhaps too much so) many of the methodologies that were expected in producing the deliverables. As I studied this RFP, I noticed several errors which could have resulted in an unhappy client, in the least damaging case and in the worst case, erroneous data.

The client requested LIDAR data with an absolute accuracy of six inches. Furthermore, the client specified how to place control to ensure that compliance with the accuracy requirement could be ascertained. For low altitude LIDAR data acquisition, GPS errors dominate all other error contributors, assuming proper sensor calibration/boresighting. The process specified in the RFP was more suitable for frame imagery block adjustment than for LIDAR. There was also an incorrect assumption (one we see very frequently) that errors would be normally distributed. This is generally not the case.

A more serious error was in the specification of orthos that were to be delivered with the LIDAR data. The specification called for 6 orthos that delineated all phase and guard lines. It was very clear in reading the RPF that the fact that wires and tower tops are generally not in their correct orthographic location in orthos photos was not understood. This phenomenon occurs, of course, because the wires/towers are not in the elevation model used in the rectification process (they are significantly above the model). The displacement is clearly visible in the adjacent image. The LIDAR wire points are in red and tower points in blue. Compare these (which are orthographically correct) to the corresponding features in the ortho image. The wire displacements are as much as 5 feet whereas the top of tower displacement is a whopping 12 feet! There is nothing technically wrong with the orthophotos. This is simply a phenomenon of the orthorectification process that is often not recognized by consumers of these data. Thus it is very important to remind the client that orthos must never be used for collecting location information for any objects above/below the elevation model used in the rectification process.

The final area of concern is that of datums and projections. It is generally not well understood that the definition of datums is often tied to epochs. Simply stated, many datums are not constant over time and thus more than a simple datum must be specified. Consider World Geodetic System 84 (WGS-84). When this datum was first created, it was in general correspondence with North American Datum 1983 (NAD-83). Shortly after the initial determination of WGS-84, an error was found in the origin. Effective with epoch 730, the WGS-84 origin was shifted. This caused a sea level displacement of approximately 3 feet between the two datums. WGS-84 is now defined in terms of the International Terrestrial Reference Frame (ITRF). The net effect of this is that if data are specified in WGS-84 or any other satellite system, the epoch must also be specified. This is particularly critical when data will be conflated with cadastre data that is based on a legal reference system such as NAD-83.

We are often hesitant to provide clarifications via questioning during the RFP process because we think that perhaps our competitors do not understand the issues as well as us so why provide them an advantage? While I certainly understand how fierce competition is these days, we really need to advance the best practices of our industry. A free, open and honest flow of communication between the transmission line industry and the service providers to that industry will, in the end, elevate the opportunities for everyone.